Appendicular myological reconstruction of the forelimb of the giant titanosaurian sauropod dinosaur Dreadnoughtus schrani

Abstract

Soft tissues are variably preserved in the fossil record with external tissues, such as skin and feathers, more frequently preserved than internal tissues (e.g. muscles). More commonly, soft tissues leave traces of their locations on bones and, for muscles, these clues can be used to reconstruct the musculature of extinct vertebrates, thereby enhancing our understanding of how these organisms moved and the evolution of their locomotor patterns. Herein we reconstruct the forelimb and shoulder girdle musculature of the giant titanosaurian sauropod Dreadnoughtus schrani based on observations of osteological correlates and dissections of taxa comprising the Extant Phylogenetic Bracket of non‐avian dinosaurs (crocodilians and birds). Fossils of Dreadnoughtus exhibit remarkably well‐preserved, well‐developed, and extensive muscle scars. Furthermore, this taxon is significantly larger‐bodied than any titanosaurian for which a myological reconstruction has previously been attempted, rendering this myological study highly informative for the clade. In total, 28 muscles were investigated in this study, for which 46 osteological correlates were identified; these osteological correlates allowed the reconstruction of 16 muscles on the basis of Level I or Level II inferences (i.e. not Level I' or Level II' inferences). Comparisons with other titanosaurians suggest widespread myological variation in the clade, although potential phylogenetic patterns are often obscured by fragmentary preservation, infrequent myological studies, and lack of consensus on the systematic position of many taxa. By identifying myological variations within the clade, we can begin to address specific evolutionary and biomechanical questions related to the locomotor evolution in these sauropods.

We reconstruct the forelimb and shoulder girdle musculature of the giant titanosaurian sauropod Dreadnoughtus schrani. In total, 28 muscles were investigated in this study, for which 46 osteological correlates were identified. Comparisons with other titanosaurians suggest widespread myological variation among the clade.

1. INTRODUCTION

All sauropod dinosaurs, including titanosaurs, exhibited a similar bauplan with small heads, long necks, massive bodies, ‘columnar’ limbs, and long tails (Upchurch et al., 2004; Rauhut et al., 2011). Over 70 species of titanosaurian sauropods have been described to date (see Martínez et al., 2016; Wilson et al., 2016, and references therein), and they ranged in size from dwarf, island‐dwelling taxa to 60+ ton giants (Benson et al., 2014; Lacovara et al., 2014; González Riga et al., 2016; Carballido et al., 2017). Despite their high diversity, titanosaurian remains are frequently fragmentary (Upchurch et al., 2004).

One of the major events in titanosauriform evolution was the development of a unique limb stance termed ‘wide‐gauge posture’ (Wilson and Carrano, 1999; Carrano, 2005). This posture is characterized by the footfalls being placed well away from the body midline, despite the retention of a columnar limb, by the widening of the pectoral and pelvic girdles. While wide‐gauge posture is most often discussed concerning the hind limb (Wilson and Carrano, 1999), the forelimb of titanosauriforms also exhibits increased gauge width (Henderson, 2006) owing, in part, to dorsomedial protrusion of the humeral head (Ullmann et al., 2017). Although the evolutionary driver(s) to increase gauge width during stance have not been definitively established, Wilson and Carrano (1999) suggested that this adaptation allowed titanosaurs to reach gigantic sizes, with the clade ultimately producing the largest terrestrial organisms yet discovered. However, the relative expression of anatomical features thought to be associated with gauge width is not tightly correlated with body size in sauropod phylogeny (Sander et al., 2011; Ullmann and Lacovara, 2016).

Biomechanically, the acquisition of wide‐gauge posture in titanosauriforms involved an incompletely understood tradeoff between locomotor efficiency and stability. In this tradeoff, wide stance may have enhanced lateral stability during locomotion (Mannion and Upchurch, 2010; Ullmann et al., 2017), but it would also have increased the rotational force, or moment, about the shoulder and hip joints (Wilson and Carrano, 1999). For the animal to have remained upright, this force—which would have been greatly increased in giant titanosaurs due to their immense mass—would have had to be counteracted by muscle force(s) (Wilson and Carrano, 1999). Thus, to better understand wide‐gauge posture, its evolution, and any potential selective advantages it may have conferred to titanosaurs, the relationship between appendicular anatomy and muscular function in these sauropods must be better understood.

In general, the lack of quantity and quality of preservation has made it difficult to reconstruct the appendicular musculature of most titanosaurs, and indeed most sauropodomorph dinosaurs. Although some sauropods are known from relatively complete skeletons with excellent appendicular representation, most osteological descriptions only include occasional mentions of probable osteological correlates for muscle attachment (e.g. Harris, 2007; Curry Rogers, 2009; Gallina and Apesteguía, 2015; Poropat et al., 2015). A recent study made generalizations about forelimb myology among sauropodomorph dinosaurs (Otero, 2018), but there remain few species‐level myological studies to date on titanosaurs (Diamantinasaurus, Klinkhamer et al., 2018, 2019; Epachthosaurus, Ibiricu et al., 2018; Neuquensaurus, Otero and Vizcaíno, 2008; Opisthocoelicaudia, Borsuk‐Bialynicka, 1977).

The appendicular skeleton of Dreadnoughtus schrani is conducive to detailed myological study because at least one of every stylopodial and zeugopodial element is preserved and the surface texture of most bones is beautifully preserved, including numerous osteological correlates (Lacovara et al., 2014; Ullmann and Lacovara, 2016). Our detailed myological reconstruction is only the third species‐level myological study of a titanosaurian forelimb to date, making it important for elucidating soft tissue anatomy and muscle function in this clade. In addition, Dreadnoughtus is significantly larger‐bodied than all other titanosaur taxa for which myology has been previously reconstructed, and it is generally found in cladistic analyses to occupy a moderately derived phylogenetic position ‘intermediate’ to the positions of these other taxa (e.g. González Riga et al., 2016; Tykoski and Fiorillo 2016; Carballido et al., 2017).

2. MATERIALS AND METHODS

Preserved forelimb material of the holotypic individual of Dreadnoughtus schrani (Museo Padre Molina [MPM]‐PV 1,156) includes the left scapula and coracoid, both sternal plates, and the left humerus, radius, and ulna (Lacovara et al., 2014). To reconstruct the muscles of Dreadnoughtus, extant phylogenetic bracket (EPB; Witmer, 1995) comparisons were drawn with crocodilians and birds (i.e. avian dinosaurs), the closest living relatives of non‐avian dinosaurs (hereafter, ‘dinosaurs’), and extended to include lepidosaurs and turtles. Dissections of an American alligator (Alligator mississippiensis) and two avian taxa (preserved pigeon, Columba livia; fresh turkey, Meleagris gallopavo) were completed to confirm attachment locations reported in the literature and were in agreement with previous reports. Further support for EPB attachment sites in Dreadnoughtus was possible via the identification of abundant osteological correlates—evidence of muscle attachments on bone in the form of textured surfaces (e.g. pitting or striations as in Figure 1; Bryant and Seymour, 1990; Dilkes et al., 2012; Rothschild et al., 2016).

Figure 1.

Examples of common types of scarring on pectoral girdle and forelimb bones of Dreadnoughtus schrani. (a) Origin of the M. subcoracoscapularis on the medial face of the scapula (MPM‐PV 1156‐48), observed as striations on a raised knob (white arrow). (b) Origin of the M. biceps brachii on the dorsolateral aspect of the coracoid (MPM‐PV 1156‐47), observed as pitting on a raised ridge (white arrow). (c) Insertion of the M. coracobrachialis brevis on the anterior face of the proximal end of the humerus (MPM‐PV 1156‐49), observed as adjoining patches of striations (white arrows). (d) Origin of the M. brachialis inferior on the anterior face of the humerus (MPM‐PV 1156‐49), observed as pitting (white arrow). (e) Insertion of the M. biceps brachii on the anterolateral aspect of the proximal end of the ulna (MPM‐PV 1156‐50), observed as striations on a raised ridge (white arrow). (f) Insertion of the M. extensor carpi ulnaris on the posterolateral aspect of the distal end of the ulna (MPM‐PV 1156‐50), observed as pitting (white arrow)

Herein, birds are described in Nomina Anatomica Avium terminology (Baumel et al., 1993) and all clades that do not have a specified nomenclature are described in Romerian terminology (Romer, 1956; Wilson, 2006). When the name of an avian muscle differed from that of its crocodilian homolog, the crocodilian name was used. Differences in limb posture among EPB taxa and sauropod dinosaurs result in different orientations of homologous features within the body (e.g. the deltopectoral crest is on the anterior face of the humerus of a sauropod, whereas it is on the ventral face in crocodilians). Therefore, in our descriptions, terminology for each taxon is based on neutral limb orientation during stance. To facilitate comparisons with other studies of sauropodomorph pectoral girdles, we describe the scapulae of sauropods in anatomical orientation (with the scapular blade extending posterodorsally at a ~ 45° angle from the anteroventrally facing glenoid), following Ullmann and Lacovara (2016) and Schwarz et al. (2007a).

Osteological correlates are produced through remodeling as a response to stress applied by a muscle (Frankel and Nordin, 2001), but visible texturing is not always produced, nor do they always create surface texturing over their entire area of attachment (Bryant and Seymour, 1990, and references therein; Dilkes et al., 2012). Therefore, in Dreadnoughtus, muscles were reconstructed only over the area of each corresponding osteological correlate. As such, it is acknowledged that the total area reconstructed for an attachment may be incomplete. For consistency, several muscles do not appear in our figures, as there is no evidence of their exact attachment sites beyond EPB comparisons. In our text, we discuss the likelihood of those attachment areas. Overall, these factors can result in bones appearing to be sparsely covered by muscle attachments, even if this was not the condition in life. Our highly conservative myological reconstruction provides data specifically on the areas encompassed by osteological correlates, which is useful for understanding and comparing the stress regimens imparted by muscles among related taxa. Table 1 summarizes each muscle investigated, its attachment sites and number of divisions, and whether each of these are equivocal or unequivocal in dinosaurs. In addition, levels of inference are reported (Table 2) to characterize the confidence placed in each reconstruction (Witmer, 1995, 1997). Supporting Information Table S1 documents in detail the EPB myology used herein, and further discussion of EPB considerations are also provided in the Supporting Information Data S1.

Table 1.

Summary of (un)equivocal states of the muscles investigated in this study in the order they appear in the text. Bold font indicates muscles with an equivocal number of divisions

Presence Unequivocal; Origin & Insertion Unequivocal	Presence Unequivocal; Origin Equivocal	Presence Unequivocal; Insertion Equivocal	Presence Unequivocal; Origin & Insertion Equivocal	Presence Equivocal
M. rhomboideus	M. serratus profundus	M. biceps brachii	Mm. costocoracoideus	M. levator scapulae
M. serratus superficialis	M. deltoideus scapularis	M. extensor carpi radialis	M. deltoideus clavicularis	M. teres major
M. latissimus dorsi	M. supracoracoideus	M. flexor carpi ulnaris	M. brachialis inferior	‘M. subcoracoideus’
Mm. coracobrachialis	M. scapulohumeralis posterior	M. flexor carpi radialis		M. scapulohumeralis anterior
‘M. subscapularis’	M. pectoralis	M. flexor digitorum longus		M. humeroradialis
Mm. triceps		M. pronator quadratus		M. transversus palmaris
M. extensor carpi ulnaris
M. pronator teres
M. extensor digitorum longus

Table 2.

Level of Inference, attachment texture, and reconstruction condition for each muscle. Level I – both bounding extant taxa support the reconstruction; Level II – only one bounding taxon supports the reconstruction; Level III – neither bounding taxon supports the reconstruction; apostrophe (') denotes lack of osteological correlate texturing

Muscle	Level of inference (presence)	Origin Scar	Level of inference (origin)	Insertion scar	Level of inference (insertion)	At least one attachment reconstructed?
M. levator scapulae	II?	Not examined	N/A	Faint striations	II	Yes
M. rhomboideus	I?	Not examined	N/A	Striations*	I'	No
M. serratus superficialis	I?	Not examined	N/A	Faint striations	I	Yes
M. serratus profundus	I?	Not examined	N/A	Striations	I	Yes
Mm. costocoracoideus	I?	Not examined	N/A	Striations	II	Yes
M. latissimus dorsi	I?	Not examined	N/A	Striations	I	Yes
M. teres major	II	Faint striations*	II	Striations*	II	No
M. deltoideus scapularis	I	Pitting^	II	Indeterminate	I'	Yes
M. deltoideus clavicularis	I	Striations^	II	Pitting	II	Yes
M. supracoracoideus	I	Striations	Coracoid‐ I; scapula‐ II	Striations	I	Yes
M. coracobrachialis brevis ventralis	I	Striations	I	Striations^	I	Yes
‘M. subscapularis’	I	Striations^	I	Striations	I	Yes
‘M. subcoracoideus’	II'	Indeterminate	II'	Striations*	I	No
M. scapulohumeralis anterior	II	Pitting^	II	Pitting^	II	Yes
M. scapulohumeralis posterior	I	Striations^	II	Striations*	I	Yes
M. pectoralis	I	Striations	Sternum‐ I	Pitting	I	Yes
Mm. triceps	I'	Humerus striations; scapula & coracoid‐smooth	Humerus‐ II; scapula‐ I; coracoid‐ I	Striations	I	Yes
M. biceps brachii	I	Pitting^	I	Striations (ulna^)	Radius‐ I; ulna‐ II	Yes
M. brachialis inferior	I	Pitting	II	Striations*	II	Yes
M. humeroradialis	I or II (see text)	Striations^	II	Pitting	II	Yes
M. extensor carpi radialis	I	Striations	I	Indeterminate	II	Yes
M. extensor carpi ulnaris	I	Striations	I	Pitting^	I	Yes
M. flexor carpi ulnaris	I	Striations	I	Indeterminate	II	Yes
M. flexor carpi radialis	I'	Indeterminate	I'	Smooth	II'	Yes
M. pronator teres	I'	Indeterminate	I'	Striations	I	Yes
M. extensor digitorum longus	I'	Striations	I	Indeterminate	I'	Yes
M. flexor digitorum longus	I'	Humerus‐ indeterminate; ulna‐ striations^	Humerus‐ I'; ulna‐ I; carpus‐ II'	Indeterminate	II'	Yes
M. pronator quadratus	I	Striations^	I	Striations^	II	Yes
M. transversus palmaris	I' or II' (see text)	Indeterminate	II'	Indeterminate	II'	No

^, raised attachment area; *, shares attachment area with at least one other muscle; ?, one attachment site not investigated, therefore exact level of inference could not be determined; N/A, level of inference not given as osteological correlate was not examined.

3. RESULTS

3.1. Axial to limb

3.1.1. M. levator scapulae

The presence of the M. levator scapulae is equivocal in dinosaurs (Supporting Information Table S1; Dilkes, 2000; Burch, 2014; Otero, 2018). The scapula of Dreadnoughtus exhibits potential scarring (in the form of faint, parallel striations) on the distal two‐thirds of the dorsal margin of the blade that could correspond to the M. levator scapulae. This region is also possibly adjacent to the potential insertion site of the M. rhomboideus (see below). Thus, it is difficult to determine whether only one or both of these muscles was present. The M. levator scapulae, therefore, is tentatively reconstructed in Dreadnoughtus to insert along the dorsal margin of the scapular blade (Figure 2), since striations corresponding to the M. rhomboideus would be predicted to be more medially positioned. The M. levator scapulae are also reconstructed at this location in Opisthocoelicaudia (Borsuk‐Bialynicka, 1977). Though the M. trapezius, if present, would have likely inserted with the M. levator scapulae, unlike this muscle, the M. trapezius does not have an osteological origin in archosaurs (Jasinoski et al., 2006; Burch, 2014). Therefore, no unique osteological correlates can be identified for the M. trapezius in Dreadnoughtus, and thus it is not reconstructed herein.

Figure 2.

Lateral view of left pectoral girdle and forelimb elements of Dreadnoughtus schrani. Black outlines indicate muscle attachments exhibiting scarring texture. Red outlines indicate attachments reconstructed with a degree of uncertainty (either from EPB comparisons, lack of osteological correlate, and/or taphonomic deformation). Blue outlines indicate muscles for which presence would be equivocal by EPB comparisons alone, but osteological correlates are present at expected attachment location. Uppercase labels indicate muscle origins; lowercase labels indicate insertions. B., b., biceps brachii; Cbr. B., coracobrachialis brevis; Delt. C., delt. c., deltoideus clavicularis; Delt. S., deltoideus scapularis; E. C. R., extensor carpi radialis; E. C. U., extensor carpi ulnaris; E. D. L., extensor digitorum longus; F. D. L., flexor digitorum longus; H., humeroradialis; lev. s., levator scapulae; Pr. Q., pronator quadratus; Sc., supracoracoideus; ser. s., serratus superficialis; Sh. A., scapulohumeralis anterior; Sh. P., scapulohumeralis posterior; Tr., triceps. Scale bar: 1 m

3.1.2. M. rhomboideus

The presence of the M. rhomboideus is unequivocal, though the number of divisions in dinosaurs is equivocal (Table S1; Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006; Burch, 2014; Otero, 2018). Generally, the M. rhomboideus would have inserted distally on the scapular blade, on its dorsal edge or its medial side (Table S1; Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006; Burch, 2014; Otero, 2018); however, the scapula of Dreadnoughtus exhibits no visible osteological correlates on the medial side of the distal blade. As mentioned above, the striations on the dorsal margin of the distal scapular blade may correspond to the insertion of this muscle, resulting in an avian‐like condition (cf. Table S1; Dilkes, 2000; Jasinoski et al., 2006; Burch, 2014; Otero, 2018), but we consider it more likely that the M. levator scapulae inserted in this region instead. There is also a small area of slight swelling with anteroposteriorly directed striations on the medial aspect of the dorsal scapular blade approximately one‐third of the distance from the distal end. It is possible that this could correspond to a portion of the attachment of the M. rhomboideus, but it could also represent the attachment of the M. serratus profundus (see below). Alternatively, the M. rhomboideus may have attached to the cartilaginous suprascapula. Since there is no definitive evidence regarding the location of the insertion of this muscle, it is not reconstructed in Dreadnoughtus, though it probably was present.

3.1.3. M. serratus superficialis

EPB analyses support the presence of this muscle in dinosaurs, but it remains equivocal whether one or two divisions were present (Table S1; Dilkes, 2000; Jasinoski et al., 2006; Burch, 2014; Otero, 2018). Since both divisions insert near the same location (Table S1; Dilkes, 2000; Jasinoski et al., 2006; Burch, 2014; Otero, 2018), and at least one was likely present, Dreadnoughtus is conservatively reconstructed with only one division (Figures 2 and 3), the M. serratus superficialis, as evidence for two divisions is not obvious. The ventral border of the middle and distal portions of the scapular blade of Dreadnoughtus preserves faint striations that are better developed along their proximal‐most extent. These are a potential osteological correlate for either the M. serratus superficialis or the M. scapulohumeralis posterior (see below); these attachments may also abut one another. Borsuk‐Bialynicka (1977) similarly reconstructed the attachment of the M. serratus superficialis along tuberosities on the middle third of the ventral border of the scapula in Opisthocoelicaudia, though this area is more proximal than the area exhibiting striations on the ventral border of the scapula of Dreadnoughtus. The area of attachment for the M. serratus superficialis in Dreadnoughtus may have extended proximodorsally to the distal aspect of the autapomorphic anteroventral‐posterodorsal ridge on the medial face of the scapular blade described by Ullmann and Lacovara (2016), but texturing indicative of an osteological correlate is not present over this entire region.

Figure 3.

Medial view of left pectoral girdle and forelimb elements of Dreadnoughtus schrani. See Figure 2 caption for explanation of outlines. Uppercase labels indicate muscle origins; lowercase labels indicate insertions. b., biceps brachii; br., brachialis inferior; csc., costocoracoideus; F. C. R., flexor carpi radialis; F. C. U., flexor carpi ulnaris; F. D. L., flexor digitorum longus; Pr. T., pronator teres; Sbs., subcoracoscapularis; ser. p., serratus profundus; ser. s., serratus superficialis; tr., triceps. Scale bar: 1 m

3.1.4. M. serratus profundus

The M. serratus profundus is unequivocally present in dinosaurs (Table S1; Dilkes, 2000; Jasinoski et al., 2006; Burch, 2014; Otero, 2018). On the medial side of the dorsal edge of the distal scapular blade of Dreadnoughtus (approximately one‐third the length of the blade from the distal end), there is a small, slightly swollen area with anteroposteriorly directed striations that could correspond to the insertion of the M. serratus profundus (Figure 3), as it is proximal to the expected location of the M. rhomboideus insertion. As stated above, Figure 3 depicts the area represented by the osteological correlate, not the entire area of probable attachment.

3.1.5. Mm. costocoracoideus

The homology of the Mm. costocoracoideus of crocodilians and the M. sternocoracoideus of birds is debated (Table S1; Jasinoski et al., 2006 and references therein; see discussion by Otero, 2018). Although the number of divisions of the Mm. costocoracoideus present in dinosaurs is equivocal, it is likely that dinosaurs had at least one part of this muscle (Table S1; Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006). A small region of the medial surface of the coracoid of Dreadnoughtus exhibits striations, which could represent the insertion of one part of the Mm. costocoracoideus, potentially the pars profundus (Figure 3). Borsuk‐Bialynicka (1977) suggested that if the M. costocoracoideus was present in Opisthocoelicaudia, it would have attached to the concave medial surface of the coracoid. This muscle was also reconstructed in a similar region as in Dreadnoughtus (though more ventrally placed) in the early‐diverging sauropodomorph Saturnalia (Langer et al., 2007).

3.1.6. M. latissimus dorsi

The presence of the M. latissimus dorsi is unequivocal in dinosaurs, whereas the number of divisions is equivocal (Table S1; Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006; Abdala and Diogo, 2010; Maidment and Barrett, 2011; Burch, 2014; Otero, 2018). If two divisions were present, as in birds, the insertions of these divisions would likely have been adjacent, making them difficult to differentiate. Therefore, Dreadnoughtus is herein reconstructed with only one area for insertion of this muscle, as in crocodilians (Figure 4).

Figure 4.

Posterior view of left forelimb elements of Dreadnoughtus schrani. Brown outline encircles an osteological correlate for which a specific soft tissue could not be assigned. See Figure 2 caption for explanation of other outlines. Uppercase labels indicate muscle origins; lowercase labels indicate insertions. delt. s., deltoideus scapularis; E. C. U., e. c. u., extensor carpi ulnaris; l. d., latissimus dorsi; pr. q., pronator quadratus; sh. a., scapulohumeralis anterior; Tr., triceps. Scale bar equals 1 m

Previous myological reconstructions of sauropodomorphs have been inconsistent with regard to how proximal or lateral the insertion of the M. latissimus dorsi has been placed. Borsuk‐Bialynicka (1977), Otero (2010, 2018), and Poropat et al. (2015) all placed this insertion on the posterior side of the humeral shaft at a low, rough bulge in various titanosaurs. By contrast, in the basal sauropodomorph Saturnalia and the titanosaur Elaltitan, respectively, Langer et al. (2007, Figure 3c–d) and Mannion and Otero (2012) placed this insertion on the lateral aspect of the deltopectoral crest, posterolateral to its anterior apex (similar to where the insertion of the M. scapulohumeralis anterior is herein reconstructed in Dreadnoughtus; Figure 4). Because the humerus of Dreadnoughtus exhibits a concentrated area of striations along the posterior face of the shaft just distal to the apex of the deltopectoral crest, the insertion of the M. latissimus dorsi is herein reconstructed at that location (Figure 4). This concurs with the hypotheses of Borsuk‐Bialynicka (1977), Otero (2010, 2018), and Poropat et al. (2015), and is also consistent with the condition in crocodilians and birds, as the reconstructed insertion is medial to the deltoid muscles and distal to the M. scapulohumeralis anterior. A rugose bulge on the posterior face of the humerus at about midshaft is also described in Lirainosaurus (Sanz et al., 1999; Díez Díaz et al., 2013); this is likely an osteological correlate of the M. latissimus dorsi in that Spanish titanosaur. An osteological correlate of the ‘brachial muscle’ reported by Curry Rogers (2009: 1,072) on the posterior humerus of Rapetosaurus just proximal to midshaft also likely represents the insertion of the M. latissimus dorsi (Otero, 2018).

3.1.7. M. teres major

The presence of the M. teres major is equivocal in dinosaurs (Table S1; Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006; Otero, 2018). As summarized in Table S1, it is unlikely that there are distinct, confidently identifiable osteological correlates of this muscle in dinosaurs. No discernible osteological correlates for the M. teres major are preserved on the scapula or humerus of Dreadnoughtus; therefore, this equivocal muscle is not reconstructed in this titanosaur. In contrast, Klinkhamer et al. (2019) reconstructed the M. teres major in Diamantinasaurus based on purported osteological correlates for both its origin and insertion; however, the locations of these osteological correlates (ventral edge of scapular blade, posterior margin of humeral head) do not correspond to those for the origin and insertion of this muscle in crocodilians (dorsal edge of the scapular blade, posterolateral humeral shaft; Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006).

3.2. Girdle to distal

3.2.1. M. deltoideus scapularis

EPB comparisons suggest that the M. deltoideus scapularis was present in dinosaurs, but its number of heads is equivocal (Table S1; Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006; Maidment and Barrett, 2011; Burch, 2014; Otero, 2018). On the dorsal edge of the lateral face of the scapula of Dreadnoughtus, near the base of the blade, there is a small, elevated area exhibiting small pits that may represent an osteological correlate of a muscle (Figure 2). This could possibly represent the origin of the M. deltoideus scapularis. Considering that in the majority of EPB taxa this muscle originates over a large area (Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006; Maidment and Barrett, 2011; Burch, 2014; Otero, 2018), it is possible the M. deltoideus scapularis attached over a larger area in Dreadnoughtus as well. If this muscle did attach over a larger area than occupied by its osteological correlate, its origin would likely extend distally to cover much of the lateral face of the distal blade, including ventral to the insertion of the M. levator scapulae (e.g. as in extant crocodilians; Dilkes, 2000; Meers, 2003). The origin of the M. deltoideus scapularis was also reconstructed at the base of the scapular blade in Opisthocoelicaudia (Borsuk‐Bialynicka, 1977). Klinkhamer et al. (2019) reconstructed the origin of this muscle more distally on the scapular blade in Diamantinasaurus. Curry Rogers (2009) and Otero (2010) both reported a muscle scar posterior to the acromial ridge in Rapetosaurus and Neuquensaurus, respectively; because these scars are in a similar location, it is likely that they also correspond to the origin of the M. deltoideus scapularis. The posterior surface of the deltopectoral crest of the humerus of Dreadnoughtus is damaged by compression fractures and offsets caused by taphonomic deformation. Despite the resulting lack of support from osteological correlates, based on EPB comparisons (Table S1; Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006; Maidment and Barrett, 2011; Burch, 2014; Otero, 2018) the M. deltoideus scapularis likely attached at this location, just lateral to the origin of the humeral head of the Mm. triceps (Figure 4). Borsuk‐Bialynicka (1977) described a rough surface and a potential osteological correlate at this location in Opisthocoelicaudia, proximal to her reconstructed insertion of the M. scapulohumeralis anterior, and identified it as the insertion of the M. deltoideus scapularis. The insertion of the M. deltoideus scapularis in Diamantinasaurus was reconstructed on the proximolateral humeral head (Klinkhamer et al., 2019), potentially more on the lateral (rather than posterior) aspect than in Dreadnoughtus, but anteroposterior compression of the humerus of Dreadnoughtus makes this potential anatomical difference uncertain.

3.2.2. M. deltoideus clavicularis

The attachments of the M. deltoideus clavicularis are equivocal in dinosaurs (Table S1; Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006; Abdala and Diogo, 2010; Maidment and Barrett, 2011; Burch, 2014; Otero, 2018). EPB analysis supports the presence of at least one head of the M. deltoideus clavicularis in dinosaurs, and osteological correlates observable in Dreadnoughtus are in accordance with this hypothesis (see Data S1). Because distinct osteological correlates are lacking for the equivocal second head, we did not reconstruct the second head in this titanosaur.

The acromial region of the scapula of Dreadnoughtus preserves a striated area that may be an osteological correlate for the origin of the M. deltoideus clavicularis (Figure 2). These striations are more prominent along the acromial ridge and the shallowly concave fossa that borders this ridge distally. Langer et al. (2007) also suggested that the deltoid muscles of Saturnalia, which would include the M. deltoideus clavicularis, originated from the acromial region of the scapula. Additionally, in Diamantinasaurus, Klinkhamer et al. (2019: table 1) reconstructed the M. deltoideus clavicularis as originating on the lateral aspect of the scapula. Unlike Dreadnoughtus and many other titanosaurians (Upchurch et al., 2004), the scapula of Lirainosaurus is described as not exhibiting an acromial ridge (Díez Díaz et al., 2013). On the lateral side of the humerus of Dreadnoughtus there is a distinct region of circular pitting that likely represents the insertion of the M. deltoideus clavicularis (Figure 2). Given that the only known humerus of this titanosaur has been anteroposteriorly compressed, it is probable this region did not face as purely laterally in life as is currently observed. This proposed insertion site retains the association of this muscle inserting anterior to the M. deltoideus scapularis, as seen in EPB taxa (Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006), and in a similar location to the insertion reconstructed in Diamantinasaurus (Klinkhamer et al., 2019). Langer et al. (2007) also reconstructed the insertion of this muscle and that of the M. deltoideus scapularis as being on the posterolateral side of the deltopectoral crest in Saturnalia.

3.2.3. M. supracoracoideus

The number of divisions and location of the origin(s) of the M. supracoracoideus is equivocal in dinosaurs (see Data S1 and Table S1; Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006; Maidment and Barrett, 2011; Burch, 2014; Otero, 2018). Striations located posterodorsal, posterior, and ventral to the coracoid foramen of Dreadnoughtus likely represent at least part of the origin of this muscle (Figure 2). It is possible, however, that this attachment area was larger than what is suggested by this preserved osteological correlate, and that the M. supracoracoideus may have additionally attached anteroventral to the coracoid foramen, to a rounded area below the origin of the M. biceps brachii. On the lateral face of the scapula of Dreadnoughtus, near the anterior margin, striations are present through approximately the ventral half of the coracoid articulation. These may be an osteological correlate for an expanded origin of the M. supracoracoideus (Figure 2), in agreement with Otero (2018). Nevertheless, as the holotypic individual of Dreadnoughtus is immature (Lacovara et al., 2014; Ullmann and Lacovara, 2016), it is also possible that these striations could be from cartilage (cf. Tsai and Holliday, 2015; Tsai et al., 2018) that would have connected the coracoid and scapula until their fusion in adulthood. Similar striations are present on this region of the scapula of a juvenile brachiosaurid (Sauriermuseum Aatal [SMA] 0,009; Carballido et al., 2012) described by Schwarz et al. (2007b); however, these authors do not assign these surface modifications to any specific soft tissue attachment. Thus, it is not known whether, in Dreadnoughtus, the M. supracoracoideus originated in part from the scapula. As such, the scapular origin is only tentatively reconstructed.

Langer et al. (2007) reconstructed the origin of the M. supracoracoideus on both the scapula and coracoid in the basal sauropodomorph Saturnalia. Alternatively, the origin of this muscle was reconstructed in the same general location on only the coracoid in Opisthocoelicaudia and Neuquensaurus (Borsuk‐Bialynicka, 1977; Otero, 2010) or more anteriorly on only the coracoid in Diamantinasaurus (Klinkhamer et al., 2019). The humerus of Dreadnoughtus has a flat area on the anteroproximal portion of the deltopectoral crest that exhibits a combination of striations and potential pitting that probably corresponds to the insertion of the M. supracoracoideus. The site is distinguishable from other surrounding muscle attachments due to differences in the orientation of its striations. Similarly, Saturnalia is reconstructed with the insertion of this muscle along the entire anterolateral margin of the deltopectoral crest (Langer et al., 2007). Klinkhamer et al. (2019) also reconstructed the insertion of the M. supracoracoideus in Diamantinasaurus in this general location. Borsuk‐Bialynicka (1977) reconstructed the insertion of this muscle on the proximolateral corner of the humerus of Opisthocoelicaudia. This is nonetheless an unlikely location for this attachment, as it does not accord with EPB comparisons (see Table S1). It is more likely that the insertion of this muscle in this derived Mongolian titanosaur was ventral to this location on the anterior face of the proximal portion of the deltopectoral crest, as is herein reconstructed in Dreadnoughtus. Therefore, naming the tuberosity on the proximolateral corner of the humerus that is present in Opisthocoelicaudia and other titanosaurs (e.g. Upchurch, 1998) after the M. supracoracoideus may be inaccurate.

3.2.4. Mm. coracobrachialis

There was likely a minimum of one branch of the Mm. coracobrachialis present in dinosaurs (see Data S1 and Table S1; Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006; Maidment and Barrett, 2011; Burch, 2014; Otero, 2018), and as Dreadnoughtus only exhibits osteological correlates of the M. coracobrachialis brevis ventralis, it is the sole branch reconstructed herein. The origin of this muscle corresponds to a region of small striations on the anteroventral portion of the lateral face of the coracoid, anterior to the infraglenoid lip (Figure 2). In Opisthocoelicaudia (Borsuk‐Bialynicka, 1977) and Saturnalia (Langer et al., 2007), the origin of this muscle was reconstructed across the same general region as in Dreadnoughtus. In Diamantinasaurus (Klinkhamer et al., 2019, Figure 3b), the origin of this muscle appears to have been reconstructed more dorsally.

On the proximomedial part of the anterior face of the humerus of Dreadnoughtus, there is a ridge of bone bearing large, deep striations. Proximomedial to this ridge is an additional, small patch of striations that are oriented parallel to those on the ridge (Figures 1c and 5). Though they likely only represent a portion of its area of attachment, these osteological correlates are consistent with the placement of the hypothesized insertion of the M. coracobrachialis brevis ventralis in numerous titanosaurians (e.g. Elaltitan, Neuquensaurus, Notocolossus, Opisthocoelicaudia, and Saltasaurus [Borsuk‐Bialynicka, 1977; Powell, 2003; Otero, 2010; Mannion and Otero, 2012; González Riga et al., 2016]; note that in these forms this muscle is often referred to simply as the M. coracobrachialis). These taxa, as well as Gondwanatitan (Kellner and de Azevedo, 1999) and Paralititan (Smith et al., 2001), possess a proximolaterally–distomedially oriented ‘shelf’ within a fossa on the anterior face of the proximal humerus, as in Dreadnoughtus, which represents an osteological correlate for the insertion of this muscle. Only Langer et al. (2007) termed the muscle that inserted at this location the M. coracobrachialis brevis ventralis. Of the taxa possessing a shelf for the insertion of this muscle, only Dreadnoughtus, Opisthocoelicaudia (Borsuk‐Bialynicka, 1977), and Saturnalia (Langer et al., 2007) have been reported to possess additional, discernible osteological correlates (e.g. striations) on this shelf. The humeri of the Australian titanosaur Diamantinasaurus exhibit a modified shelf in the form of a more distally located ‘strongly curving ridge’ that Poropat et al. (2015: 1,006) regarded as an attachment site of the M. coracobrachialis brevis. A depression for muscular attachment reported at this location in Malawisaurus (Gomani, 2005) and Atacamatitan (Kellner et al., 2011) also probably corresponds to the insertion of the M. coracobrachialis brevis ventralis.

Figure 5.

Anterior view of left forelimb elements of Dreadnoughtus schrani. Brown outline encircles an osteological correlate for which a specific soft tissue could not be assigned. See Figure 2 caption for explanation of other outlines. Uppercase labels indicate muscle origins; lowercase labels indicate insertions. Abbreviations: b., biceps brachii; Br., brachialis inferior; cbr. b., coracobrachialis brevis; F. D. L., flexor digitorum longus; h., humeroradialis; p., pectoralis; pr. q., pronator quadratus; sc., supracoracoideus. Scale bar: 1 m

3.2.5. M. subcoracoscapularis (Mm. subscapularis and subcoracoideus)

Following Abdala and Diogo (2010) and Otero (2018), the Mm. subscapularis and subcoracoideus will be considered divisions of the M. subcoracoscapularis herein. Since at least one division (usually the ‘M. subscapularis’) of the M. subcoracoscapularis occurs in all investigated EPB taxa, this muscle was likely present in dinosaurs (see Data S1 and Table S1; Dilkes, 2000; Meers, 2003; Abdala and Diogo, 2010; Maidment and Barrett, 2011; Burch, 2014; Otero, 2018). However, since Dreadnoughtus does not exhibit any osteological correlates that can be definitively attributed to the ‘M. subcoracoideus’ division (inadequate preservation of the medial surface of the coracoid impedes identification of any potential osteological correlate in this area), it is not reconstructed. Dreadnoughtus shares a striated, raised knob on the medial side of the scapula for origination of the ‘M. subscapularis’ division of the M. subcoracoscapularis (Figures 1a and 3) with several other titanosaurians (Aeolosaurus, Lirainosaurus, Neuquensaurus, Paralititan, Pitekunsaurus, Saltasaurus; Filippi and Garrido, 2008; Ullmann and Lacovara, 2016), and a ‘low, rough eminence’ is present at this location in the diplodocoid Suuwassea (Harris, 2007: 502). Opisthocoelicaudia lacks any raised features on the medial side of the scapula, and instead exhibits a triangular concavity that spans this region for the origin of the M. subcoracoscapularis (Borsuk‐Bialynicka, 1977). In Saturnalia, Langer et al. (2007) reconstructed this attachment as being positioned more distally on the medial side of the scapular blade. Klinkhamer et al. (2019) alternatively reconstructed the origin of the ‘M. subscapularis’ in Diamantinasaurus on the proximolateral scapula, which is unlikely as this attachment occurs on the opposite face of the scapula in all examined EPB taxa (see Table S1).

In Dreadnoughtus, there is a region of striations just distal to the proximomedial corner of the humerus that may correspond to the insertion of the M. subcoracoscapularis (Figure 3). This area also potentially encompasses the insertion of the M. scapulohumeralis posterior (see below). As a result of anteroposterior taphonomic deformation, this region of bone faces anteromedially, but it likely would have faced more medially in life. Borsuk‐Bialynicka (1977) and Klinkhamer et al. (2019) reconstructed the insertion of the M. subcoracoscapularis (M. subscapularis sensu Klinkhamer et al., 2019) on the medial corner of the proximal surface of the humerus, comparable to the condition in crocodilians (Meers, 2003). It is also possible this insertion may have been located more distally on the medial side of the proximal humerus, as has been reconstructed in Saturnalia (Langer et al., 2007).

3.2.6. M. scapulohumeralis anterior

The presence of the M. scapulohumeralis anterior in dinosaurs is equivocal (Dilkes, 2000; Jasinoski et al., 2006; Burch, 2014; Otero, 2018), but it is possible that crocodilians and turtles apomorphically lost this muscle, as they are hypothesized to exhibit many derived features relative to the basal archosauromorph condition (e.g. Sereno, 1991; Brusatte et al., 2010; Crawford et al., 2015). Jasinoski et al. (2006) and Otero (2018) reconstructed the origin of the M. scapulohumeralis anterior in dinosaurs on the proximal ventral margin of the scapular blade, similar to the condition in birds. Alternatively, if Dreadnoughtus exhibited a condition more like that of lepidosaurs, the M. scapulohumeralis anterior would be predicted to originate on the lateral face of the proximal part of the scapula distal to the origin of the M. supracoracoideus (cf. Table S1; Dilkes, 2000). No osteological correlate is apparent in Dreadnoughtus at the location predicted by the avian condition of this muscle, but there is a low swelling with small, faint, radiating, directional pitting within the posterior portion of the broad acromial fossa, paralleling the acromial ridge. We infer that this swelled region likely corresponds to the origin of the M. scapulohumeralis anterior, based on the condition in lepidosaurs (Figure 2; cf. Dilkes, 2000, Figure 5). Borsuk‐Bialynicka (1977) also reconstructed the origin of this muscle in Opisthocoelicaudia as being in a concavity on the lateral scapula just proximal to the acromial ridge. Otero (2010) instead reconstructed the M. deltoideus scapularis at this location in Neuquensaurus and suggested that this muscle was equivalent to the M. scapulohumeralis anterior of Borsuk‐Bialynicka (1977). As discussed above, EPB comparisons for dinosaurs suggest the M. deltoideus scapularis would have originated from the scapular blade rather than the acromion (Table S1; Dilkes, 2000; Jasinoski et al., 2006; Maidment and Barrett, 2011; Burch, 2014; Otero, 2018); therefore, in our view, the hypothesized M. deltoideus scapularis of Otero (2010) should be regarded as the M. scapulohumeralis anterior. Unlike in Dreadnoughtus, in Saturnalia the M. scapulohumeralis anterior was reconstructed as originating on the acromial ridge with the M. deltoideus clavicularis, instead of anterior to the origin of the latter muscle (Langer et al., 2007).

The humerus of Dreadnoughtus exhibits pitting on a prominent bulge that projects laterally from the posterolateral edge of the deltopectoral crest; this likely corresponds to the insertion of the M. scapulohumeralis anterior (Figure 4). This location is at the same proximodistal level as the anterior apex of the deltopectoral crest. Though Mannion and Otero (2012) reported this osteological correlate as absent in the Patagonian titanosaur Argyrosaurus, Borsuk‐Bialynicka (1977) also reconstructed the insertion of this muscle on the posterolateral side of the deltopectoral crest of Opisthocoelicaudia. Additionally, a rugosity that may correspond to the M. scapulohumeralis anterior insertion was also described at this location on the humerus of Lirainosaurus (Sanz et al., 1999). Langer et al. (2007) reconstructed the insertion of this muscle on the medial side of the posterior face of the proximal humerus in Saturnalia, a hypothesis that is not supported by EPB comparisons (Table S1).

3.2.7. M. scapulohumeralis posterior

The M. scapulohumeralis posterior occurs in all investigated EPB taxa except turtles (Dilkes, 2000), making the presence of this muscle unequivocal in dinosaurs. The placement of its origin, however, varies among EPB taxa, rendering the precise location of this origin in dinosaurs equivocal (see Data S1 and Table S1; Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006; Maidment and Barrett, 2011; Burch, 2014; Otero, 2018). Herein, the M. serratus superficialis, the scapular head of the Mm. triceps, and the M. scapulohumeralis posterior are all discussed as possibly corresponding to striations on the posteroventral tubercle of the scapula of Dreadnoughtus. As the scapular head of the Mm. triceps originates just distal to the glenoid in birds and crocodilians (see below; Table S1; Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006; Maidment and Barrett, 2011; Burch, 2014), it is most likely that it attached on the supraglenoid buttress in Dreadnoughtus (in agreement with the placement of this muscular head by Otero, 2018). In Sphenodon, crocodilians, and ratites, the M. scapulohumeralis posterior originates between the scapular head of the Mm. triceps and the M. serratus superficialis (Table S1; Dilkes, 2000: figs. 5 and 6; Meers, 2003, Figure 3; Jasinoski et al., 2006, Figures 3 and 5). Additionally, the majority of the M. serratus superficialis inserts distal to the Mm. scapulohumeralis posterior and triceps in all EPB taxa examined (excluding turtles; Table S1; Dilkes, 2000: figs. 5 and 6; Meers, 2003 Figure 3; Jasinoski et al., 2006 Figures 3, 4, 5), meaning that its insertion would likely be the most distal of the three attachments along the ventrolateral margin of the scapula. Thus, it is most likely that the M. scapulohumeralis posterior attached on the posteroventral tubercle of the scapula in Dreadnoughtus, proximodistally between the origin of the Mm. triceps and the insertion of the M. serratus superficialis (Figure 3).

In Diamantinasaurus, the posteroventral tubercle was described as the attachment site of an unspecified muscle (Poropat et al., 2015), which, based on the above discussion, was probably the M. scapulohumeralis posterior. However, Klinkhamer et al. (2019) did not reconstruct any muscle attachment on this tubercle in Diamantinasaurus and instead reconstructed the M. scapulohumeralis to originate more posterodorsally on the scapular blade. Since they placed this attachment on the blade, we assume they are reconstructing the M. scapulohumeralis posterior. If this is the case, we suggest this is a less likely attachment site for the origin of the M. scapulohumeralis posterior than the posteroventral tubercle, as discussed above. Although some titanosaurians (e.g. Lirainosaurus; Díez Díaz et al., 2013) lack this tubercle, this muscle probably still originated in this general location in these taxa. Similarly, in the basal sauropodomorph Saturnalia, this muscle was reconstructed (as the M. scapulohumeralis caudalis) as originating from the medial side of the scapular blade (Langer et al., 2007).

In Dreadnoughtus, the insertion of the M. scapulohumeralis posterior may have been in the area where striations are present on the medial side of the proximal humerus, adjacent to the scar for the M. subcoracoscapularis (Figure 3). Taphonomic deformation has altered this area of bone such that it faces anteromedially. Conversely, in Opisthocoelicaudia (Borsuk‐Bialynicka, 1977: 24) and Saturnalia (Langer et al., 2007) this muscle was reconstructed as inserting on the medial side of the humeral head just distal to the insertion of the M. subcoracoscapularis. The M. scapulohumeralis of Klinkhamer et al. (2019: table 1) was also reconstructed to insert on the ‘proximoposterior humerus’.

3.2.8. M. pectoralis

In extant archosaurs, the origin and insertion of the M. pectoralis are unequivocal; however, the number of divisions of this muscle is equivocal (Table S1; Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006; Burch, 2014; Otero, 2018). In Dreadnoughtus, the M. pectoralis likely originated at least from the ventral side of the sternum. The surface of the right sternal plate of the holotype of this titanosaur (MPM‐PV 1156‐46) is too altered by taphonomic deformation and technical preparation to discern any potential osteological correlates. In contrast, the ventral surface of the left sternal plate (MPM‐PV 1156‐45) is mostly smooth, with a few striations radiating from near the anterolateral ridge; as such, we consider this area to represent the origin of the M. pectoralis. The origin of this muscle is also reconstructed at this location in Opisthocoelicaudia and Neuquensaurus, although in the former this area was reported as smooth and in the latter a crest was identified as the site of origin (Borsuk‐Bialynicka, 1977; Otero, 2010). Alternatively, Klinkhamer et al. (2019: table 1) tentatively reconstructed the origin of the M. pectoralis as on the ‘lateral sternal’, as no osteological correlate was identified for its origin.

EPB analysis supports the insertion of the M. pectoralis of Dreadnoughtus as being on the deltopectoral crest of the humerus (Table S1; Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006; Maidment and Barrett, 2011; Burch, 2014; Otero, 2018) around its anterior apex. This area exhibits directional pitting indicative of muscle attachment, especially on the lateral side of the apex (Figure 5). The insertion of the M. pectoralis has also been reconstructed on the deltopectoral crest in Diamantinasaurus, Neuquensaurus, Notocolossus, and Opisthocoelicaudia (Borsuk‐Bialynicka, 1977; Otero, 2010; González Riga et al., 2016; Klinkhamer et al., 2019). In Neuquensaurus and Notocolossus, the insertion of the M. pectoralis was described only as on the deltopectoral crest (Otero, 2010; González Riga et al., 2016), whereas it was described as attaching specifically to the distomedial part of the deltopectoral crest in Diamantinasaurus. Thus, based on the conditions in Dreadnoughtus, Diamantinasaurus, and EPB taxa, this muscle likely attached to a large area surrounding the anterior apex of the deltopectoral crest in titanosaurs. In Lirainosaurus, this muscle has been reconstructed as inserting on the posterior side of the proximal humerus (Sanz et al., 1999); EPB comparisons do not support this hypothesized location (Table S1; Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006; Maidment and Barrett, 2011; Burch, 2014; Otero, 2018). Langer et al. (2007) reconstructed this muscle as inserting anteromedially on the deltopectoral crest of the sauropodomorph Saturnalia.

3.3. Brachium to distal

3.3.1. Mm. triceps

The number of divisions of the Mm. triceps is equivocal in dinosaurs (Table S1; Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006; Abdala and Diogo, 2010; Maidment and Barrett, 2011; Burch, 2014; Otero, 2018). Based on EPB comparisons, it is likely that a scapular and at least one humeral head of the Mm. triceps were present in dinosaurs (Table S1; Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006; Abdala and Diogo, 2010; Maidment and Barrett, 2011; Burch, 2014; Otero, 2018). Concerning the humeral head(s), Dreadnoughtus has two areas (situated proximolaterally and proximomedially, respectively) on the posterior side of the humerus that exhibit small striations that may be osteological correlates for two humeral origins of the Mm. triceps (Figure 4). Again, only the areas encompassing each osteological correlate are indicated in Figure 4, though it is likely that each attachment extended over a larger area, potentially occupying the majority of the posterior face of the proximal humerus. There are also two depressions on the posterior side of the proximal humerus of Neuquensaurus that probably represent origin sites for this muscle (M. anconeus sensu Otero, 2010). A ridge on the posterior side of the humerus in Diamantinasaurus (Poropat et al., 2015) and Opisthocoelicaudia (Borsuk‐Bialynicka, 1977) may divide two similarly positioned depressions, also for attachment of the Mm. triceps. Otero (2018) reported a ridge on the humeral shaft that extends distally in two basal sauropodomorphs, Plateosaurus and Massospondylus, which he associated with the humeral origin of the Mm. triceps.

The scapular head of the Mm. triceps has been proposed to originate from the ventral end of the supraglenoid buttress of the scapula in other titanosaurians (Borsuk‐Bialynicka, 1977; Poropat et al., 2015) and slightly more distally on the supraglenoid buttress in the early‐diverging sauropodomorph Saturnalia (Langer et al., 2007). The proposed location in Saturnalia is more distal than is commonly reconstructed in dinosaurs (cf. Borsuk‐Bialynicka, 1977; Dilkes, 2000; Jasinoski et al., 2006; Maidment and Barrett, 2011; Poropat et al., 2015; Otero, 2018). Diamantinasaurus (Poropat et al., 2015) and Dreadnoughtus do not exhibit any indication of an osteological correlate at this location; therefore, the origin of the scapular head of the Mm. triceps may have migrated to the posteroventral tubercle, which is present in both of these taxa. However, though medially directed striations on the posteroventral tubercle could pertain to the Mm. triceps, EPB comparisons indicate it is more likely that the M. scapulohumeralis posterior attached on that structure (see M. scapulohumeralis posterior section above; Table S1 and references therein). According to EPB comparisons (Table S1; Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006; Maidment and Barrett, 2011; Burch, 2014), the scapular head of the Mm. triceps originated just distal to the glenoid; thus, this muscle likely originated on the supraglenoid buttress in Dreadnoughtus (Figure 2). Based on its described similarities to Dreadnoughtus, Diamantinasaurus is predicted to have had a similar condition. However, Klinkhamer et al. (2019) reconstructed the scapular head of the Mm. triceps as originating more posteriorly on the scapular blade.

The proposed plesiomorphic condition of four heads of the Mm. triceps (Abdala and Diogo, 2010) suggests the likely presence of an additional head originating from the coracoid in Dreadnoughtus. Nevertheless, the coracoid of this dinosaur exhibits few areas of potential osteological correlates, and it does not show any clear indication of an osteological correlate for an additional head of the Mm. triceps. One possibility is that a coracoid head of the Mm. triceps was present but did not leave an osteological correlate. In Saturnalia, there is an oval pit on the medial side of the coracoid that was suggested to correspond to the coracoid head of the Mm. triceps (Langer et al., 2007), indicating the potential presence of a coracoid head in the sauropodomorph lineage. However, because a coracoid head is variably present in birds (Dilkes, 2000; Jasinoski et al., 2006; Burch, 2014), it may also be possible that this head of the Mm. triceps was variably present among sauropodomorphs. Alternatively, as tentatively suggested by Otero (2018), the coracoid head of the Mm. triceps may have had a second origin on the scapula. Because this extra attachment is only known in crocodilians (Meers, 2003; Otero, 2018), making it equivocal in dinosaurs, we did not reconstruct it herein. Therefore, Dreadnoughtus is reconstructed with only two humeral heads and a scapular head of the Mm. triceps based on preserved osteological correlates at EPB‐supported locations (cf. Table S1).

The olecranon process of the ulna preserves striations in Dreadnoughtus that likely represent the insertion of the Mm. triceps (Figure 4). These striations are most visible on the posterolateral side of this process, although this may be a result of taphonomic alteration to the medial side. The insertion of this muscle is also reconstructed on the posterior aspect of the olecranon process in Diamantinasaurus, Neuquensaurus, Opisthocoelicaudia, and Saturnalia (Borsuk‐Bialynicka, 1977; Langer et al., 2007; M. anconeus sensu Otero, 2010).

3.3.2. M. biceps brachii

It is equivocal whether the M. biceps brachii inserted on the ulna and how many divisions were present in dinosaurs (Table S1; Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006; Maidment and Barrett, 2011; Burch, 2014; Otero, 2018). In Dreadnoughtus, there is a ridge on the lateral face of the coracoid that extends ventrally from the dorsal margin and divides the bone into approximate anterior and posterior halves. The dorsal portion of this ridge of bone exhibits a pitted texture, indicating it is likely an osteological correlate for the origin of the M. biceps brachii (Figures 1b and 2). The origin of this muscle was also reconstructed at this location in Opisthocoelicaudia (Borsuk‐Bialynicka, 1977). Otero (2010) reported a faint ridge perpendicular to the dorsal margin of the coracoid in Neuquensaurus that he regarded as possibly representing the origin of the M. coracobrachialis. Huene (1929) additionally attributed this ridge in this titanosaur (then referred to Titanosaurus) to the attachment of pectoral musculature. Based on EPB comparisons (Table S1; Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006; Burch, 2014; Otero, 2018), this ridge likely corresponds to the origin of the M. biceps brachii. Giraffatitan (Otero, 2018), Saltasaurus (Otero, 2018), Rapetosaurus (Curry Rogers, 2009), and the diplodocoid Suuwassea (Harris, 2007) also exhibit a similar dorsoventral ridge on the coracoid that likely corresponds to the origin of this muscle. However, many titanosaurians (e.g. Diamantinasaurus; Poropat et al., 2015) do not have a ridge or tuberosity on the lateral aspect of the coracoid. In Diamantinasaurus, Klinkhamer et al. (2019) reconstructed the origin of the M. biceps brachii as from the posteroventral area of the coracoid, though they noted that no osteological correlate was preserved. Langer et al. (2007) reconstructed the origin of the M. biceps brachii as being positioned more anteroventrally on the coracoid of Saturnalia.

Although the insertion of the M. biceps brachii on the ulna is equivocal in Dreadnoughtus, this muscle likely at least inserted on the proximal end of the radius in this titanosaur. Indeed, there is a concavity on the medial face of the proximal radius, at least a portion of which probably corresponds to the insertion of the M. biceps brachii (Figure 3). Striations are present on the anterior side of this medial concavity. If the M. biceps brachii of Dreadnoughtus also inserted on the ulna, it either did not leave an osteological correlate or it attached distal to the radial insertion on a deeply striated tuberosity. Although this tuberosity is positioned more distally than the hypothesized radial insertion, an ulnar insertion of the M. biceps brachii is tentatively reconstructed here because this location is directly posterodistal to the M. biceps brachii insertion on the radius (Figures 1e and 5). Langer et al. (2007) reconstructed the insertion of this muscle on both the ulna (at the same location as in Dreadnoughtus) and radius (at a similar location but more distally than in Dreadnoughtus) of Saturnalia. In contrast, Borsuk‐Bialynicka (1977) reconstructed the M. biceps brachii as inserting only on the radius in Opisthocoelicaudia, and suggested that the ulnar insertion may have been lost when the elbow turned posteriorly during the course of sauropodomorph evolutionary history. Otero (2018) also reported an osteological correlate for the radial insertion of the M. biceps brachii in several additional titanosauriforms (Elaltitan, Giraffatitan, and Neuquensaurus). Additionally, in Diamantinasaurus, Klinkhamer et al. (2019) reconstructed this muscle as inserting with the M. brachialis inferior, at a similar location as in Dreadnoughtus but on the proximoanterior instead of the proximomedial radius.

3.3.3. M. brachialis inferior

Although the precise attachment locations for the M. brachialis inferior are equivocal in dinosaurs, this muscle was likely present in these archosaurs (Table S1; Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006; Maidment and Barrett, 2011; Burch, 2014; Otero, 2018). In Dreadnoughtus, there is no fossa between the distal condyles of the humerus that might indicate an avian‐like origin. Instead, there is a sub‐oval area of pitted texture just distal to the apex of the deltopectoral crest that may correspond to the origin of this muscle (Figures 1d and 5). If this muscle did attach here, it is possible that only the dorsal portion of this attachment left an osteological correlate, and that this muscle continued to attach more distally along the shaft, as in lepidosaurs and crocodilians (Table S1; Dilkes, 2000; Meers, 2003; Jasinoski et al., 2006; Burch, 2014). Since, in Dreadnoughtus, the hypothesized origin of the M. brachialis inferior follows the reptilian condition, the insertion is also reconstructed in accordance with that condition, but it is possible this muscle also or instead attached to the ulna. Thus, the insertion of this muscle is herein reconstructed as being with that of the M. biceps brachii on the anterior portion of the medial side of the proximal end of the radius, where the bone exhibits striations (Figure 3). Klinkhamer et al. (2019) and Borsuk‐Bialynicka (1977), respectively, reconstructed the origin and insertion of the M. brachialis inferior in the titanosaurians Diamantinasaurus and Opisthocoelicaudia to similar locations as reconstructed herein in Dreadnoughtus.

3.3.4. M. humeroradialis

Whether the M. humeroradialis is equivocally or unequivocally present in dinosaurs is debated (Table S1; Meers, 2003; Jasinoski et al., 2006; Abdala and Diogo, 2010; Burch, 2014; Otero, 2018). Based primarily on crocodilian myology (Table S1; Meers, 2003; Jasinoski et al., 2006; Burch, 2014; Otero, 2018), Dreadnoughtus preserves what are likely osteological correlates of the origin and insertion of the M. humeroradialis. Posterodistal to the ventral end of the deltopectoral crest, there is a slightly raised and striated area on the lateral side of the humerus that likely corresponds to the origin of the M. humeroradialis (Figure 2). This reconstructed origin is in keeping with the close association of this muscle and the M. brachialis inferior in crocodilians (Table S1; Meers, 2003; Jasinoski et al., 2006; Burch, 2014; Otero, 2018). The insertion of the M. humeroradialis was likely on a ridge with pitted texture on the anteromedial face of the radius (Figure 5), just anterior to the insertion of the M. pronator teres. The M. humeroradialis has also previously been reconstructed in other saurischians based on the presence of potentially corresponding osteological correlates (e.g. in maniraptoran theropods [Jasinoski et al., 2006], a basal theropod [Burch, 2014], and a basal sauropodomorph [Langer et al., 2007]). In Diamantinasaurus, this muscle was reconstructed as originating proximally on the lateral side of the posterior face of the humerus (Klinkhamer et al., 2019), in a similar location to where the origin of the lateral humeral head of the M. triceps is reconstructed for Dreadnoughtus herein (and for Neuquensaurus by Otero, 2010). The insertion of the M. humeroradialis in Diamantinasaurus was reconstructed on the proximolateral aspect of the anterior face of the radius (Klinkhamer et al., 2019), thus in a similar, but more lateral position than that reconstructed in Dreadnoughtus.

3.3.5. M. extensor carpi radialis

The M. extensor carpi radialis was likely present in dinosaurs; however, its number of divisions is debated (Table S1; Dilkes, 2000; Meers, 2003; Abdala and Diogo, 2010; Burch, 2014; Otero, 2018). As only a humeral origin is reconstructed herein (see below), for brevity we refer to this attachment simply as the M. extensor carpi radialis. Striations on the lateral epicondyle of the humerus of Dreadnoughtus approximately 25 cm proximal to the articular surface of the lateral condyle likely correspond to the attachment of this muscle (Figure 2). Borsuk‐Bialynicka (1977) also reconstructed this muscle as originating on the lateral epicondyle of the humerus in Opisthocoelicaudia, proximal to the origins of the M. extensor digitorum longus (M. extensor digitorum communis sensu Borsuk‐Bialynicka, 1977) and anterior to that of the M. extensor carpi ulnaris. The origin of the M. extensor carpi radialis is also reconstructed on the lateral epicondyle in Diamantinasaurus and Saturnalia (Langer et al., 2007; Klinkhamer et al., 2019).

With the exception of Argyrosaurus (Mannion and Otero, 2012), no titanosaur is known to preserve ossified carpal elements; therefore, it is likely that, in Dreadnoughtus, the M. extensor carpi radialis did not insert on the carpus. Nevertheless, the radius of this titanosaur does not preserve any apparent osteological correlate for the insertion of this muscle. Langer et al. (2007) reconstructed the insertion of the M. extensor carpi radialis as being situated anterolaterally on the distal radius of Saturnalia. Conversely, in Opisthocoelicaudia, this muscle is reconstructed with its insertion on metacarpal I and possibly metacarpal II (Borsuk‐Bialynicka, 1977). Curry Rogers (2009) also reconstructed the insertion of this muscle on metacarpal I. In Diamantinasaurus, Klinkhamer et al. (2019: table 1) reconstructed the insertion of the M. extensor carpi radialis as on the ‘lateral radius into metacarpals’. It is possible that in Dreadnoughtus this muscle inserted on the radius but did not leave an osteological correlate, or that it may have inserted more distally on the forelimb, on elements that are not preserved in this titanosaur. Thus, the insertion of the M. extensor carpi radialis is not reconstructed herein.

3.3.6. M. extensor carpi ulnaris

The M. extensor carpi ulnaris is unequivocally present in dinosaurs, but confusion with other forearm muscles has resulted in different names and attachment locations having been reported (see Data S1 and Table S1 for further details; Dilkes, 2000; Meers, 2003; Abdala and Diogo, 2010; Burch, 2014; Otero, 2018). The distal end of the humerus of Dreadnoughtus preserves striations (visible despite minor damage to the surface of the bone at this location) on its posterolateral corner that likely correspond to the origin of the M. extensor carpi ulnaris (Figures 1f and 2). As preserved, this area is demarcated from the probable origin site of the M. extensor digitorum longus by a faint ridge that is likely an artifact of taphonomic deformation. Although these attachment areas may have been contiguous in life, EPB comparisons would suggest that the two muscles originated in close proximity to one another, making it likely that this expanse of osteological correlate(s) represents both muscle origins (Table S1; Dilkes, 2000; Burch, 2014; Otero, 2018). In accordance with this hypothesis, Langer et al. (2007) reconstructed the origin of the M. extensor carpi ulnaris in Saturnalia at the same location as proposed herein for Dreadnoughtus, but did not differentiate it from the origin of the M. extensor digitorum longus (M. extensor digitorum communis sensu Langer et al., 2007). In Diamantinasaurus, Klinkhamer et al. (2019) reconstructed the M. extensor carpi ulnaris as originating posteriorly on the humeral lateral condyle. Borsuk‐Bialynicka (1977) also reconstructed the origin of the M. extensor carpi ulnaris at this location in Opisthocoelicaudia, but also proposed an additional attachment of this muscle situated laterally on the posterior face of the distal end of the ulna. This accords with the observations by Dilkes (2000) and Burch (2014) that, in crocodilians, this muscle has an additional insertion positioned laterally on the distal ulna. The ulna of Dreadnoughtus preserves a raised, distinct, pitted area at this location (Figure 4). Since this location is on the distal rather than proximal ulna, it is herein reconstructed as an insertion of the M. extensor carpi ulnaris, similar to the reconstruction of Borsuk‐Bialynicka (1977) (as opposed to a second origin of this muscle on the proximal end of the ulna as is present in birds; Dilkes, 2000). As mentioned above, most titanosaurs are not thought to have had ossified carpals, making it likely that any ancestral insertion of the M. extensor carpi ulnaris on the carpals shifted either distally to the metacarpals (Borsuk‐Bialynicka, 1977) and/or more proximally to the distal ulna. In Diamantinasaurus, Klinkhamer et al. (2019: table 1) reconstructed the insertion of this muscle as on the ‘lateral ulna into metacarpals’.

3.3.7. M. flexor carpi ulnaris

The presence and origin of the M. flexor carpi ulnaris are unequivocal in dinosaurs, but the insertion of this muscle is equivocal (Table S1; Dilkes, 2000; Meers, 2003; Burch, 2014; Otero, 2018). As noted above, since most titanosaurs are thought not to have had ossified carpals, it is possible that the insertion of this muscle shifted proximally to the ulna (as in Apteryx) or distally to the metacarpals in these sauropods.

Striations are visible on the posterior part of the medial epicondyle of the humerus of Dreadnoughtus (Figure 3). This osteological correlate likely relates to the origin of the M. flexor carpi ulnaris, in agreement with reconstructions of this attachment in Diamantinasaurus, Opisthocoelicaudia, and Saturnalia (Borsuk‐Bialynicka, 1977; Langer et al., 2007; Klinkhamer et al., 2019) as well as the conditions in crocodilians and birds. In Opisthocoelicaudia, an additional origin of this muscle is reconstructed as arising from the medial face of the proximal ulna, and is suggested to join with that from the medial epicondyle of the humerus (Borsuk‐Bialynicka, 1977); however, this is not supported by EPB comparisons. Conversely, in Saturnalia, this muscle is reconstructed with an insertion at this location on the proximal ulna (Langer et al., 2007). This portion of the ulna of Dreadnoughtus has been taphonomically crushed, preventing identification of potential osteological correlates. Additionally, no metacarpals are preserved in this titanosaur. As there are no known osteological correlates for the insertion of the M. flexor carpi ulnaris in Dreadnoughtus, it is not reconstructed herein.

3.3.8. M. flexor carpi radialis

This muscle is unequivocally present in dinosaurs (Table S1; Dilkes, 2000; Meers, 2003; Abdala and Diogo, 2010). The anterior face of the medial condyle of the humerus of Dreadnoughtus does not preserve intact surface texture, precluding the definitive identification of osteological correlates. As a result, the exact location of the origin of the M. flexor carpi radialis cannot be determined; nevertheless, this muscle likely attached on this structure as it does in extant archosaurs. Therefore, the M. flexor carpi radialis is tentatively reconstructed as originating from the anterior face of the medial epicondyle, from the same region as the Mm. pronator teres and flexor digitorum longus (discussed below; Figure 3), similar to the reconstruction of these muscle origins in Diamantinasaurus (Klinkhamer et al., 2019). Regarding the insertion of the M. flexor carpi radialis, in Opisthocoelicaudia, Borsuk‐Bialynicka (1977) identified this insertion as being located anteriorly on the medial face of the distal radius. However, the medial face of the radius of Dreadnoughtus does not exhibit any distinct osteological correlates distal to the probable insertion of the M. pronator teres (discussed below); instead, this region of the bone is smooth. Although the M. flexor carpi radialis likely inserted somewhere on this region of the radius in Dreadnoughtus (which would be a Level I’ inference), we chose not to reconstruct this insertion, as its exact location remains uncertain. In Diamantinasaurus, Klinkhamer et al. (2019) reconstructed the insertion of the M. flexor carpi radialis as potentially continuing more distally to the metacarpals.

3.3.9. M. pronator teres

The M. pronator teres is unequivocally present in dinosaurs (Table S1; Dilkes, 2000; Meers, 2003; Abdala and Diogo, 2010; Burch, 2014; Otero, 2018). According to EPB analyses, in Dreadnoughtus, the Mm. pronator teres, flexor carpi radialis, and flexor digitorum longus would all have originated from the medial epicondyle of the humerus (Table S1; Dilkes, 2000; Meers, 2003; Abdala and Diogo, 2010; Burch, 2014; Otero, 2018), but this region does not preserve osteological correlates in this titanosaur (Figure 3). As such, the combined possible area of origin of these muscles may be only tentatively reconstructed and, within this area, the specific site of each is unknown. This reconstruction generally agrees with that of other titanosaurs (e.g. Diamantinasaurus; Klinkhamer et al., 2019). Concerning the insertion of the M. pronator teres, the radius of Dreadnoughtus exhibits a broadly rounded ridge that extends distally from the anteromedial corner of the proximal end; striations on the posteromedial side of this ridge likely represent an osteological correlate for this insertion (Figure 3). Although the surface is slightly damaged proximal to this area, this muscle probably attached more proximally as well. The insertion of the M. humeroradialis is located on the anterior side of this same ridge (Figure 5). Borsuk‐Bialynicka (1977) also reconstructed the M. pronator teres as inserting proximally along a medial crest on the radius of Opisthocoelicaudia. In Neuquensaurus, however, this muscle was reconstructed as inserting more distally, on a ridge medial to the interosseous ridge (Otero, 2010). Additionally, Otero (2018) reported a ridge possibly corresponding to a linea intermuscularis between the Mm. pronator teres and pronator quadratus in several titanosaurs (Diamantinasaurus, Elaltitan, Neuquensaurus, Opisthocoelicaudia, Rapetosaurus). Klinkhamer et al. (2019) also reconstructed the insertion of the M. pronator teres as on the anteromedial radius, but, contrary to the conditions in EPB taxa, also reconstructed an insertion on the metacarpals (Table S1; Dilkes, 2000; Abdala and Diogo, 2010; Burch, 2014; Otero, 2018).

3.3.10. M. extensor digitorum longus

The M. extensor digitorum longus is unequivocally present in dinosaurs, but there is disagreement on the location of its insertion (Table S1; Dilkes, 2000: fig. 8; Meers, 2003; Burch, 2014; Otero, 2018). Striations on the lateral summit of the lateral epicondyle of the Dreadnoughtus humerus likely pertain to the origin of the M. extensor carpi radialis. At the distal end of this attachment, the striations give way to a relatively smooth surface, approximately 5 cm in proximodistal extent, which, in turn, grades into a second, more distally positioned area of striations (Figure 2). Though these two striated patches could both pertain to the M. extensor carpi radialis, the hiatus between them suggests that they represent the attachment of two distinct muscles, which are herein interpreted as the origins of the M. extensor carpi radialis (proximally) and M. extensor digitorum longus (distally). The origin of the M. extensor digitorum longus is also reconstructed at this more distal location in Opisthocoelicaudia (M. extensor digitorum communis sensu Borsuk‐Bialynicka, 1977). Klinkhamer et al. (2019) alternatively reconstructed the origin of this muscle as anterior (rather than distal; Table S1; Dilkes, 2000: fig. 8; Meers, 2003; Otero, 2018) to that of the M. extensor carpi radialis in Diamantinasaurus. In Saturnalia, Langer et al. (2007) did not differentiate between the origins of the Mm. extensor carpi ulnaris and extensor digitorum longus (their M. extensor digitorum communis), but placed both distal to that of the M. extensor carpi radialis on the lateral epicondyle of the humerus. Since no carpals or metacarpals are preserved in Dreadnoughtus, the insertion of the M. extensor digitorum longus is not reconstructed herein. In other titanosaurs, however, the insertion of this muscle has been reconstructed as attaching to either metacarpal III (Diamantinasaurus, Klinkhamer et al., 2019) or metacarpals III through V (Opisthocoelicaudia, M. extensor digitorum communis sensu Borsuk‐Bialynicka, 1977).

3.3.11. M. flexor digitorum longus

It is agreed that the presence and origin of the M. flexor digitorum longus are unequivocal in dinosaurs, whereas its number of divisions and insertion are debated (Table S1; M. palmaris communis sensu Dilkes, 2000; Meers, 2003; Abdala and Diogo, 2010; Burch, 2014; Otero, 2018). The anterior portion of the medial epicondyle of the Dreadnoughtus humerus is not sufficiently preserved so as to identify osteological correlates and distinguish between the origins of the Mm. flexor carpi radialis, flexor digitorum longus, and pronator teres. Because EPB comparisons clearly support the origination of the M. flexor digitorum longus from this region (Table S1; Dilkes, 2000; Meers, 2003; Abdala and Diogo, 2010; Burch, 2014; Otero, 2018), we tentatively reconstruct its origin on the medial epicondyle of the humerus along with those of the Mm. flexor carpi radialis and pronator teres (Figure 3). Langer et al. (2007) and Klinkhamer et al. (2019) similarly reconstructed the humeral origin of the M. flexor digitorum longus anteriorly on the medial epicondyle of the humeri of Saturnalia and Diamantinasaurus, respectively. Klinkhamer et al. (2019) additionally reconstructed the ulnar origin in Diamantinasaurus as being from the proximomedial face of the posterior ulna. Borsuk‐Bialynicka (1977) reconstructed the ulnar origin of the M. flexor digitorum longus (as the M. flexor digitorum communis) in Opisthocoelicaudia on a distinct, raised osteological correlate positioned medially on the anterior face of the distal ulna. The medial aspect of the distal ulna of Dreadnoughtus preserves no osteological correlates; however, a medially positioned striated bulge is present on its anterior face, as in Opisthocoelicaudia. Because of this similarity, we follow Borsuk‐Bialynicka (1977) in reconstructing the origin of the ulnar head of the M. flexor digitorum longus at this location, but note that this assignment is tentative because this osteological correlate is positioned more anteriorly on the distal ulna than in most studied EPB taxa (Dilkes, 2000; Abdala and Diogo, 2010 and references therein; Otero, 2018). Alternatively, the M. flexor digitorum longus could have attached to the posteromedial distal ulna and not left an osteological correlate, in which case the osteological correlate on the anterior face of the ulna would correspond to another (unidentified) soft tissue structure. Again, as there are no manual elements preserved in Dreadnoughtus, the insertion of this muscle is not reconstructed herein.

3.4. Antebrachium to distal

3.4.1. M. pronator quadratus

Present in crocodilians and birds, the origin of the M. pronator quadratus is also unequivocal in dinosaurs, whereas its insertion is equivocal (Table S1; Meers, 2003; Abdala and Diogo, 2010; Burch, 2014; Otero, 2018). In Dreadnoughtus, a low ridge that extends proximodistally along the middle of the anterior face of the ulna (i.e. the side that faces the radius) exhibits intermittent rough striations (Figure 2). These striations are more pronounced at the proximal and distal ends of the ridge, where the striated areas expand into wider, subtriangular regions. A corresponding ridge on the posterior edge of the radius (the interosseous ridge) exhibits slight texturing (Figure 4). Because these ridges would have faced one another in life, they may represent the osteological correlates for the origin and insertion, respectively, of the M. pronator quadratus of Dreadnoughtus.

In several titanosaurians, the interosseous ridge of the radius has been regarded as an attachment site for a muscle, membrane or ligament (Borsuk‐Bialynicka, 1977; Curry Rogers, 2009; Zaher et al., 2011; Poropat et al., 2015), and more than one of these soft tissue structures may have attached here. EPB comparisons support the attachment of at least a muscle (Table S1; Meers, 2003; Abdala and Diogo, 2010; Burch, 2014; Otero, 2018). As in Dreadnoughtus, in Rapetosaurus the insertion of the M. pronator quadratus is reconstructed along the most of the length of the radius (Curry Rogers, 2009), and in Opisthocoelicaudia, most of an anterior ridge on the ulna is also attributed to the origin of the M. pronator quadratus (Borsuk‐Bialynicka, 1977). Klinkhamer et al. (2019) reconstructed the origin and insertion of this muscle in Diamantinasaurus at roughly the same locations on the ulna and radius, respectively, but each over a smaller extent. A pair of longitudinal ridges on the posterior face of the radius and the anterior face of the ulna of Argyrosaurus (Mannion and Otero, 2012) also likely correspond to the origin and insertion of this muscle in that taxon. The origin of the M. pronator quadratus is reconstructed opposite this location on the ulna in Elaltitan, Neuquensaurus, Rapetosaurus, and Saturnalia (Langer et al., 2007; Curry Rogers, 2009; Otero, 2010; Mannion and Otero, 2012). Otero (2018) additionally reported a longitudinal ridge, usually at the distal end of the ulna, attributed to the origin of the M. pronator quadratus in several other macronarians (Bonitasaura, Camarasaurus, Giraffatitan, and Narambuenatitan).

3.4.2. M. transversus palmaris

According to Abdala and Diogo (2010, and references therein), this muscle is consistently present in reptiles and probably also occurs in birds, although its homologies in the latter are currently unresolved (also see Table S1). In Dreadnoughtus, the medial surface of the distal radius is damaged, preventing the identification of a possible osteological correlate for the origin of the M. transversus palmaris. Similarly, no manual elements of Dreadnoughtus are preserved, precluding the assessment of an osteological correlate for the insertion of this muscle. The origin site of the M. transversus palmaris is reported as a rugose bulge on the radius of the early‐branching sauropodomorph Saturnalia (Langer et al., 2007). Therefore, it is possible that this muscle was present in Dreadnoughtus, although it is not reconstructed herein due to the lack of robust information on its likely attachment sites. A scar reported on the posteromedial surface of the distal radius of Rapetosaurus (Curry Rogers, 2009) may correspond to the origin of this muscle.

3.4.3. Osteological correlates with unknown affinities

There are two osteological correlates on the distal radius of Dreadnoughtus that could not be confidently associated with specific muscles. The first is located medially on the anterior face (Figure 5). This region exhibits striations, suggesting it is an osteological correlate for some soft tissue attachment. At this time, however, it is unknown whether this area represents the attachment of a ligament or a muscle (e.g. an additional head of a muscle, a muscle that was not investigated in this study, or a migrated attachment for a muscle that was inferred to not have a visible osteological correlate). The second location is a subtriangular region that occupies the majority of the posterior face of the distal radius (Figure 4). Entirely covered in deep striations, this region is clearly an osteological correlate for the attachment of soft tissue that created significant stress, resulting in extensive remodeling. It is possible that this area corresponds to the origin of the M. transversus palmaris that has migrated posteriorly as a result of the autapomorphic shape of the distal radius of Dreadnoughtus. However, it is also possible that this scar: (1) is a distal continuation of the insertion of the M. pronator quadratus; (2) corresponds to a muscle that was not investigated in this study; or (3) represents a ligamentous rather than muscular attachment. Thus, the identity of the soft tissue(s) that attached here remains uncertain.

4. DISCUSSION

Several of the muscles reconstructed herein and the nature of their osteological correlate(s) among sauropod dinosaurs warrant further discussion. Although the observations discussed above do not identify any clear trends in myology that set Dreadnoughtus schrani apart from other titanosaurian sauropods based on body size, phylogeny, or probable gauge width, there is, as yet, no clear reason to expect significant differences among these closely related and functionally similar taxa. However, the dataset for comparison remains small, so these variations may yet be related to some or all of the above factors. Thus, the conclusions presented below are provisional, and it is acknowledged that our understanding of these and other myological patterns in titanosaurians will become clearer as the soft tissue anatomy of these dinosaurs receives further study.

Most studies reconstructing the myology of extinct animals outline the presumed area of a muscle attachment based on a combination of observed osteological correlate texturing along with a (presumed) surrounding, additional area of attachment inferred from extant relatives (e.g. Maidment and Barrett, 2011; Burch, 2014; Ibiricu et al., 2018). Although this approach is logical and useful for conveying some information, outlining of muscle attachment sites based solely on osteological correlates can also be informative and offers unique merits. In particular, our more conservative method makes it possible to capture detailed information regarding stress regimens imparted by the muscles of an extinct organism that would be obscured were muscle attachment sites speculated to occupy their entire potential area of EPB‐inferred attachment. The position and size of an area of high stress that resulted in the formation of an osteological correlate are valuable for both comparative and functional studies, and these attributes can provide insights into variations muscle use/function among closely related taxa (e.g. Cundall, 1986; Bryant and Seymour, 1990; Meers, 2003). Given the large body size of many titanosauriforms, these animals are known to often exhibit well‐defined osteological correlates (e.g. Borsuk‐Bialynicka, 1977; Otero and Vizcaíno, 2008; Ibiricu et al., 2014, 2018; this study), which, if considered in such a detailed comparative framework, could facilitate understanding of the biomechanical significance of variations in muscular anatomy among members of the clade. Therefore, to help future workers in drawing detailed comparisons with other dinosaurs, we elected solely to diagram muscle attachment sites as the areas occupied by preserved osteological correlates. Further, if our EPB comparisons indicated that a muscle would be present at a location but no osteological correlate texturing was preserved, we did not reconstruct the muscle, even though it may have been present in life. We acknowledge that this may result in bone surfaces in our reconstruction appearing somewhat sparsely covered. However, as discussed above, our figures portray minimum attachment areas, and we reiterate that many of the muscles investigated herein would likely have attached to a larger area (of uncertain size and shape) than that evidenced by osteological correlate texturing.

4.1. Comparative myology of sauropod dinosaurs

As additional titanosaur myological reconstructions are completed, it will become easier to identify the differential development of osteological correlates among taxa. Ideally, variations in the expression of an osteological correlate for muscle attachment among closely related taxa could be linked to a functional significance. Unfortunately, this is not always possible due to factors of preservation, the dearth of comparative data on related taxa, and/or uncertainties in phylogenetic relationships. For instance, although greater development of osteological correlates is associated with increased stress, no trends in phylogeny, body size or gauge width can, as yet, adequately explain the varying development (noted above) of the osteological correlate for the origin of the M. pectoralis observed among Diamantinasaurus, Dreadnoughtus, Neuquensaurus, and Opisthocoelicaudia. Similarly, differential development of osteological correlates for the origin of the M. biceps brachii and insertions of the Mm. scapulohumeralis anterior and supracoracoideus among various titanosaurian taxa (present in Dreadnoughtus and Lirainosaurus but absent in Argyrosaurus) suggest different levels of stress applied by this muscle among taxa, which could in turn imply some type of biomechanical distinction(s) among these sauropods, but the specifics remain unknown. We are hopeful that myological investigations of additional sauropodomorphs will clarify such ambiguities.

Fortunately, for some muscles such as the M. subcoracoscapularis, there exists sufficient evidence to infer a possible functional difference between sauropodomorph taxa. The osteological correlate for the origin of the M. subcoracoscapularis exhibits variable development at the base of the scapular blade. Otero (2018) placed the origin of the scapular part of the M. subcoracoscapularis in basal sauropodomorphs on a ventromedial ridge extending along the blade from its base and reported that sauropods lack this ridge, suggesting that the M. subcoracoscapularis was reduced in these latter taxa. However, this might not be the case in several titanosaurs that exhibit a raised knob (for this origin) at the base of the scapular blade (e.g. Aeolosaurus, Dreadnoughtus, Lirainosaurus, Neuquensaurus, Paralititan, Pitekunsaurus, Saltasaurus; Filippi and Garrido, 2008; Ullmann and Lacovara, 2016).

Meers (2003) suggested that, in crocodilians, based on its attachment locations (Table S1), the M. subcoracoscapularis was likely a stabilizer of the humeral joint and potentially an adductor of the limb. The geometry of this muscle in titanosaurians suggests that it may have had a similar function in these dinosaurs. Increased development of the origin of this muscle in select lithostrotians (listed above) likely reflects an increase in applied stress that could be related to counteracting torque on the glenoid from wide‐gauge posture in these taxa (cf. Wilson and Carrano, 1999); however, the variably reported presence of this feature among titanosaurs precludes confirmation of such a phylogenetically based inference. That the origin of the M. subcoracoscapularis is less well‐developed in a narrow‐gauge diplodocoid (Suuwassea) supports this hypothesis, but further investigation is needed (e.g. the considerable difference in body size between Suuwassea and taxa such as Dreadnoughtus and Paralititan could also be a confounding factor).

It is also possible to comment on potential reasons for variation in expression of the osteological correlate for the insertion of the M. coracobrachialis brevis ventralis on the humerus. Although this insertion exhibits highly variable morphology among titanosaurians, with some exhibiting a depression (e.g. Atacamatitan, Malawisaurus) and others a shelf either with (e.g. Dreadnoughtus, Opisthocoelicaudia) or without (e.g. Elaltitan, Gondwanatitan, Neuquensaurus, Notocolossus, Paralititan, Saltasaurus) striated texture, the functional and/or phylogenetic explanation(s) for this variability cannot yet be definitively identified. It is possible that the M. coracobrachialis brevis ventralis attached over a wider area which encompassed the entire anterior fossa (e.g. Gorscak et al., 2014; González Riga et al., 2016), yet stress sufficient to leave scarring was only applied at a specific location(s) in some species. Although there is a slight indication from the observations above that more derived titanosaurians exhibit a shelf‐like morphology (possibly related to increasingly wide‐gauge posture as this muscle is a humeral adductor), variability in the morphology of this structure among lithostrotians precludes the identification of a clear phylogenetic pattern at this time. Investigation of more taxa and clarification of the phylogenetic relationships of basal titanosaurians may elucidate this potential relationship. However, the osteological correlate for this insertion is variably present in humeri of Neuquensaurus (Mannion and Otero, 2012), indicating that individual, sexual, and/or ontogenetic variation may also factor into its degree of development. As discussed in association with other osteological correlates, where greater development of this insertion occurs, it at least suggests greater stress applied at the attachment site, potentially indicating a more frequent or forceful use of this muscle.

The origin of the M. scapulohumeralis posterior on the posteroventral tubercle of the scapula also exhibits variable morphology among titanosaurs. The vertical orientation of the humerus in sauropods would likely have resulted in this muscle functioning as an adductor in these animals. As such, the muscle probably had a role in counteracting added torque on the shoulder joint arising from wide‐gauge posture. It is therefore possible that the M. scapulohumeralis posterior applied more force, and consequently more stress, to the posteroventral tubercle, which would have resulted in more strain on and deformation of this structure (potentially leading to the development of medially directed striations). Indeed, in some derived titanosaurians with extremely wide‐gauge posture, there are multiple processes at this location (Alamosaurus, specimen MCS‐7 of Neuquensaurus; D’Emic et al., 2011). Nevertheless, the posteroventral tubercle exhibits varied morphologies among Titanosauriformes (D’Emic, 2012; Mannion et al., 2013), so potential correlation of increasingly wide‐gauge posture with greater strain on the posteroventral tubercle cannot presently be confirmed.

5. CONCLUSIONS

Twenty‐eight muscles were investigated in this appendicular myological study of Dreadnoughtus schrani, only four of which could not be reconstructed based on available evidence. Among titanosaurians, few appendicular myological studies have been conducted, and Dreadnoughtus is significantly larger‐bodied than the taxa examined in these studies. Although it exhibited wide‐gauge posture, as a moderately derived lithostrotian (González Riga et al., 2016), Dreadnoughtus had a stance that was narrower than the extreme wide‐gauge posture (with lateral inclination of the femur) exhibited by Neuquensaurus (Otero and Vizcaíno, 2008) and Opisthocoelicaudia (Borsuk‐Bialynicka, 1977). During the evolution of wide‐gauge posture, modifications to the appendicular skeleton occurred, some of which altered the attachment location(s) or line(s) of action of appendicular muscles (Wilson and Carrano, 1999; Ullmann et al., 2017). Basic physics suggests that adductor muscles such as the Mm. subcoracoscapularis may have been of increased importance in larger and wider‐gauge taxa, in order to counteract the elevated torque at the shoulder and hip joints as a result of greater weight and/or further spacing of the footfalls from the body midline. For some muscles, such as the M. subcoracoscapularis, there is evidence of increased stress (i.e. better developed osteological correlate, raised process, etc.) at attachment sites that are potentially correlated with titanosaurian evolution, whereas for other muscles (e.g. the M. scapulohumeralis posterior) there is no clear pattern among the observed variation related to phylogeny or body size. Phylogenetic patterns may also be obscured by body size variation, ontogeny, functional differences, taphonomy, and/or other biologic and abiotic factors. With more appendicular reconstructions, we can better understand how changes in osteology and myology may have influenced one another in sauropod dinosaurs, and use this information to build a stronger foundation for study of the locomotor implications of wide‐gauge posture in sauropod titanosaurs.

AUTHOR CONTRIBUTIONS

K.K.V., P.V.U., M.C.L., and K.J.L. designed the study. K.K.V. and P.V.U. acquired the data. K.K.V., P.V.U., and M.C.L. analysed the data. The first draft of the manuscript was prepared by K.K.V., which then received input from all coauthors.

Supporting information

 There should be a supplementary table and supplementary text.

 

Voegele KK, Ullmann PV, Lamanna MC, Lacovara KJ. Appendicular myological reconstruction of the forelimb of the giant titanosaurian sauropod dinosaur Dreadnoughtus schrani . J. Anat. 2020;237:133–154. 10.1111/joa.13176

DATA AVAILABILITY STATEMENT

All data generated by this study are included in the manuscript and accompanying supplementary material.