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. 2024 Oct 15;246(2):171–189. doi: 10.1111/joa.14152

Ontogeny and associated changes of the extinct sloth Simomylodon uccasamamensis (Xenarthra, Mylodontidae) from the Pliocene of the eastern Puna, Argentina

Sofía I Quiñones 1,✉, Carlos A Luna 1, Ángel R Miño‐Boilini 1, Adriana M Candela 2, Alfredo E Zurita 1
PMCID: PMC11737310  PMID: 39404197

Abstract

Within Xenarthra (Eocene‐Recent), Folivora developed (late Eocene‐Recent) a remarkable diversity with respect to ecology and taxonomy over its evolutionary history. Knowledge of the diversity achieved by members of this clade in high‐altitude areas of South America (i.e., Altiplano and Puna regions of Peru, Bolivia, and northwestern Argentina) has been improved in recent decades. A particular example involves the late Neogene Mylodontidae Simomylodon uccasamamensis, known mostly from multiple specimens recovered from the Bolivian Altiplano. Although several anatomical descriptions of this ground sloth have been published, almost nothing is known about its ontogenetic development and the associated morphological changes. Here we describe and compare new specimens of S. uccasamamensis from the upper level of the Tafna Formation (Pliocene) in the eastern Puna (ca. 3800 masl), Argentina, representing the southernmost record of this species. The new material is represented by specimens showing different ontogenetic stages, from infant to adult. One subadult specimen reached an estimated body mass of ca. 232 kg. The comparative study of external and internal morphology (the latter obtained from CT scans and radiography) shows remarkable changes in the mandible and molariforms associated with ontogeny; in addition, evidence suggests that the mfs2‐3 are the first functional teeth, followed by mf1 and cf1. Based on our body mass estimates (ca. 232 kg.), we inferred an average lifespan of 14 years, 9‐month gestation time, and sexual maturation at 4.1 years, quite similar to the values we obtained based on estimated body masses of adult specimens from Bolivia published by previous authors. Along its latitudinal distribution (ca. 14° S‐21° S) S. uccasamamensis co‐occurred with other ground sloths (e.g., Megatheriinae, Thalassocninae, and Scelidotheriinae), suggesting niche partitioning. The presence of this medium‐sized ground sloth is consistent with the similarity between the faunas of eastern Puna and the Bolivian Altiplano during the Pliocene, which is also concordant with what was observed in other clades, such as Rodentia and Notoungulata.

Keywords: anatomy, computed tomography, Folivora, juvenile, paleohistology

The Mylodontidae are one of the most characteristic groups of sloths of the Neogene of South America. Although they are widely distributed in the continent, in some areas, such as the Altiplano‐Puna Plateau, knowledge of their distribution and characteristics is still scarce. In the present work, we analyze the morphology and histology of different individuals of Simomylodon ucassamensis, an endemic sloth to this region. Its morphological and ontogenetic changes are analyzed for the first time, constituting a basic study for the analysis of these variations within this family of xenarthrans.

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1. INTRODUCTION

Within Placentalia, Xenarthra represents an endemic South American clade that probably originated and diversified as a result of the “splendid isolation” (see Simpson, 1980) of South America during most of the Cenozoic Era (O'Leary et al., 2013; Vizcaíno & Bargo, 2014). Although the oldest xenarthran fossils belong to the early Eocene (ca. 55 Ma) (Bergqvist et al., 2004, 2019), molecular evidence suggests that the origin of this clade could be significantly older, ca. 68 Ma, during the latest Cretaceous (Gibb et al., 2016). In South America, Xenarthra developed a remarkable diversity, encompassing two large clades, Cingulata (“armored xenarthrans”) and Pilosa, the latter including Vermilingua and Folivora (Casali et al., 2022; Gibb et al., 2016).

Within Xenartha, Folivora developed a large morphological, ecological and taxonomic diversity during the Paleogene, but especially during the Neogene and Quaternary interval, including more than 100 genera (Pujos et al., 2016), with some terminal Pleistocene forms reaching between 2 and 5 tons, (e.g., the Megatheriidae Eremotherium laurillardi, Megatherium americanum, and the Mylodontidae Lestodon armatus; Christiansen & Fariña, 2003; McDonald, 2023). This large diversity is currently arranged in four families: Scelidotheriidae, Megatheriidae, Megalonychidae, and Mylodontidae (Casali et al., 2022), mostly known from southern South America and North America (Pujos et al., 2016). Molecular evidence inferred from DNA and protein sequences indicates that Bradypus (extant three‐toed sloths) is included within Megatherioidea (sensu Gaudin, 2004) and Choloepus (two‐toed sloths) as a member of Mylodontoidea (sensu Gaudin, 2004) (see Delsuc et al., 2018; Presslee et al., 2019). However, morphological phylogenies place the genus Choloepus within Megalonychidae whereas Bradypus appears as the sister taxon of the remaining fossil and extant sloth diversity (see Gaudin, 2004).

The Mylodontidae range temporally from the Oligocene to the Late Pleistocene. Although the Pleistocene diversity of Mylodontidae is well characterized, with genera and species strongly supported by morphology, less is known about their Paleogene and Neogene diversity (Gaudin & Croft, 2015). This is especially true for Neogene lineages of ground sloths living in Andean and sub‐Andean areas of South America, between ca. 2500 and 4000 masl, even though recent findings in high‐altitude regions of Peru and Bolivia have shed some light on their evolutionary histories (see, among others, Saint‐André et al., 2010; Boscaini, Gaudin, Mamani Quispe, et al., 2019; Boscaini et al., 2021).

Simomylodon uccasamamensis (Mylodontidae, Mylodontini; see Boscaini, Gaudin, Mamani Quispe, et al., 2019) is a medium‐sized late Neogene ground sloth most likely endemic to the Altiplano of Bolivia (see Boscaini et al., 2021; Boscaini, Gaudin, Mamani Quispe, et al., 2019; Boscaini, Gaudin, Toledo, et al., 2019; Saint‐André et al., 2010). It was also recently reported from the eastern Puna in Argentina (Quiñones et al., 2019), and represents so far the only Neogene Mylodontinae Mylodontini adapted to high altitude. This species was originally recognized and described in detail by Saint‐André et al. (2010), and later by Boscaini, Gaudin, Mamani Quispe, et al. (2019), Boscaini, Gaudin, Toledo, et al. (2019), Boscaini et al. (2021), including cranial and postcranial elements from several localities in Bolivia, making this taxon one of the best characterized mylodontines. Moreover, Boscaini, Gaudin, Mamani Quispe, et al. (2019) suggested sexual dimorphism in this species, with the larger specimens referred to males and the smaller to females, which is proposed in several other taxa of ground sloths, such as Megatheriinae, Lestodontinae, and Scelidotheriinae (Cartelle & Bohórquez, 1982; Czerwonogora & Fariña, 2012; De Iuliis, 1996; De Iuliis & Cartelle, 1999; Miño‐Boilini & Zurita, 2015; Varela et al., 2021), and Megalonychinae (McAfee & Beery, 2021).

Although S. uccasamamensis is well characterized by several specimens from Bolivia (see Boscaini, Gaudin, Mamani Quispe, et al., 2019; Boscaini et al., 2021), almost nothing is known about the ontogenetic development and the associated morphological changes, a subject very poorly known in fossil Pilosa in general (for Scelidotherium see Rusconi, 1938; for Nothrotheriops see Naples, 1990; for Eremotherium see Cartelle & De Iuliis, 2006; for Proscelidodon see Montalvo et al., 2020; for Nothrotherium see Pujos et al., 2023). On the other hand, in recent years, xenarthran bone microstructure has received more substantial attention as part of the search for useful phylogenetic and ecological characters. In sloths, and in particular in Mylodontidae, some contributions have analyzed the histology of the dermal ossicles (Brambilla et al., 2019; de Ricqlès et al., 2009; Hill, 2006; Toledo et al., 2021), in addition to those of Company et al. (2011), Straehl et al. (2013), and Amson et al. (2014), who analyzed the long bone microanatomy for ontogenetic, phylogenetic, and ecological characters.

In this framework, this contribution provides a detailed descriptive and comparative study of the external and internal morphology of new cranial and postcranial remains of S. uccasamamensis coming from the upper levels of the Tafna Formation, eastern Puna, to determine their taxonomic assignment, identify morphological features related to ontogenetic variations, and infer biological traits such as age and body masses. Although previous descriptions of S. uccasamamensis have made it one of the best‐known Neogene ground sloths, this study provides the first comprehensive analysis of ontogenetic variation in this species.

2. MATERIALS AND METHODS

The materials are housed in the Paleontological Collection of the Museo de Geología, Mineralogía y Paleontología, Instituto de Geología y Minería, Universidad Nacional de Jujuy, San Salvador de Jujuy, Argentina, under the acronym JUY‐P. The comparative study to morphologically characterize the specimens and assess their taxonomic status was performed, whenever possible, directly with the materials, but in some cases, we relied on high‐quality photographs or bibliographic sources.

2.1. CT scanning and digital reconstruction

The mandibles referred to S. uccasamamensis, JUY‐P‐0084, JUY‐P‐0085, JUY‐P‐0087, JUY‐P‐0089, and JUY‐P‐0399 were scanned using Computer Tomographic (CT) acquisition methods. The cranium and postcranial skeleton were excluded because they were not preserved in all specimens. The CT data were acquired on a 64‐channel medical axial tomography multi‐slicer, General Electric Optima in the Hospital Tránsito Cáceres de Allende (Cordoba, Argentina) and a medical 64‐channel Phillips Multislice CT scanner at the Instituto de Cardiología de Corrientes (Corrientes, Argentina). The DICOM file specifications and associated data obtained for each specimen are detailed in Table 1. Analysis of the images and 3D reconstructions was developed with the open‐source software 3D Slicer v4.1.1 (Fedorov et al., 2012).

TABLE 1.

Specification of DICOM series for each specimen of S. uccasamamensis tomographed.

Specimen #slices ST (mm) FOV (mm) Power (KV) I‐XRay (mA) ExpT (mseg)
JUY‐P‐0399 134 0.62 320 120 121 1460
JUY‐P‐0085 605 0.6 250 120 50 1000
JUY‐P‐0089 665 0.6 250 120 50 1000
JUY‐P‐0087 393 0.62 320 120 121 1460
JUY‐P‐0084 563 1 250 120 50 1000
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Abbreviations: # Slicec, number of slices; ExpT, exposure time; FOV, field of view; I‐XRay, X‐Ray tube; KV, kilovoltage; mA, Amperes; mm, millimeters; msec, millisecond; ST, slice thickness.

2.2. Radiographs

Radiographs of the mandibles referred to S. uccasamamensis JUY‐P‐0084, JUY‐P‐0085, JUY‐P‐0087, JUY‐P‐0089, and JUY‐P‐0399 were produced using a Dinar PAF‐100 X‐Ray apparatus (1.2 mA s–1; 60 kV) with the aim of analyzing the internal characteristics of the mandible.

2.3. Paleobiology

Body mass estimation follows the equation of Toledo et al. (2014). According to the value obtained, lifespan, gestation time, and age at sexual maturation were determined using the formulae proposed by Blueweiss et al. (1978) in a comparative framework to infer the relationship between body mass and the aforementioned variables.

2.4. Histology

Histological analyses of some S. uccasamamensis specimens were carried out to determine the skeletochronology of those remains associated with cranial materials. For this purpose, dermal ossicles belonging to specimens JUY‐P‐0089 and JUY‐P‐0400 were used. The section planes of these elements were oriented along their major axis. Similarly, thin sections were performed on the clavicle of specimen JUY‐P‐0085, transverse to the longitudinal axis of this bone. Thin sections were prepared at the Laboratorio de Preparación de Vertebrados Fósiles (CECOAL‐CONICET‐UNNE), Corrientes, Argentina, following the methodology of Chinsamy and Raath (1992) and Cerda et al. (2020). The bone microstructure of the thin sections was studied under optical microscopy using normal and polarized light. Histological descriptions follow Francillon‐Vieillot et al. (1990) and Hill (2006). The degree of compaction (expressed as the percentage occupied by bone tissue in a given area) and the osteon population density (OPD, percentage occupied by osteons in a 1 mm2 area) of the clavicle were measured using the ImageJ program (Rasband, 2003).

2.5. Ontogenetic ages

Four ontogenetic stages of S. uccasamamensis were identified based on size differences of the mandible. This followed the proposal of Holte (2012) who used the terms infant, juvenile, subadult, and adult to describe four ontogenetic developmental stages of Megalonyx specimens. The recognition of juvenile individuals was also based on small size, the presence of immature bone, molariforms with pyramidal morphology, and the presence of cusps (see Esteban, 1996; Montalvo et al., 2020; Rusconi, 1938). To plot morphological changes in the mandibles of the specimens assigned to the different ontogenetic stages, we plotted the measurements of the length of the molariform series and the height of the mandible at the level of the mf2 to produce a bivariate graph.

2.6. Institutional abbreviations

JUY‐P: Museo de Geología, Mineralogía y Paleontología, Instituto de Geología y Minería, Universidad Nacional de Jujuy, San Salvador de Jujuy, Argentina; MNHN‐BOL V: Museo Nacional de Historia Natural de Bolivia, División vertebrados. La Paz, Bolivia.

2.7. Abbreviations

Cf/cf, upper and lower caniniform teeth, respectively; LAGs, lines of arrested growth; masl., meters above sea level; Mf/mf, upper and lower molariform tooth, respectively; OPD, osteon population density; RCA, relative cortical area; Tl, total length.

2.8. Geological setting and stratigraphy

The Simomylodon remains under study come from the Upper Member of the Tafna Formation (Turner, 1964; Zurita et al., 2017) exposed near Casira, Jujuy Province, Argentina (Figure 1). Geologically, this region is included within the Puna Province, in the Central Andes at >3650 masl. The sedimentary succession is mainly sandstones, with some interspersed conglomerate, and various levels of tuffs and tuffites, indicating a distal alluvial fan environment with interdigitation of eolian and fluvial deposits. The fossiliferous levels are highly bioturbated, fine sands with silty matrix, interpreted by Camacho et al. (2015) as mainly eolian deposits. The mammalian fauna recovered so far comprises mainly ground sloths such as Simomylodon uccasamamensis and Thalassocnus sp., the native Notoungulata Posnanskytherium desaguaderoi, and some caviomorph rodents that are currently under study (Escamilla et al., 2023; Ferrero et al., 2024; Quiñones et al., 2019, 2022). Although the age of the sequence has not been determined by radiometric dating, the presence of S. uccasamamensis suggests a Pliocene age (Boscaini, Gaudin, Mamani Quispe, et al., 2019; Quiñones et al., 2019). Recently Escamilla et al. (2023) reported that the presence of some Ctenomyidae rodents indicates a Pliocene age for the sequences exposed in Casira.

FIGURE 1.

FIGURE 1

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Geographic, geological, and stratigraphic provenance of S. uccasamamensis in the eastern Puna, Argentina. (a). Geographic provenance of different records in Peru, Bolivia, and northwestern Argentina; (b) geological context of the Casira locality; (c) stratigraphic section of the Upper Member of Tafna Formation in the Casira locality showing the provenance of specimens here studied.

3. RESULTS

3.1. Systematic paleontology

XENARTHRA Cope, 1889.

Suborder Folivora Delsuc et al., 2001.

Superfamily Mylodontoidea Gill, 1872.

Family Mylodontidae Gill, 1872.

Subfamily Mylodontinae Gill, 1872.

Tribe Mylodontini Gill, 1872.

Genus Simomylodon Saint‐André et al., 2010.

Simomylodon uccasamamensis Saint‐André et al., 2010.

Figures 2–8 and Tables 2–4.

Holotype: MNHN‐Bol V 11731 (ex GB 078), anterior part of the skull without dentition (Saint‐André et al., 2010).

Paratype: MNHN‐Bol V 3321, premaxillae and fragments of the left maxilla with Mf1‐Mf3 and the right maxilla with Mf2‐Mf4 (Saint‐André et al., 2010).

Included species: monospecific genus.

Type locality and Age: Ayo Ayo‐Viscachani (Department of La Paz, Bolivia); Umala Formation, Upper Pliocene (Saint‐André et al., 2010).

3.1.1. Geographic and chronologic distribution

This species was recorded (Figure 1a) in the Upper Miocene‐Lower Pliocene of Choquecota (Saint‐André, 1994; Saint‐André et al., 2010); Lower Pliocene of Pomata‐Ayte, Casira and Inchasi (Departments of Oruro and Potosí, Bolivia) (Anaya & MacFadden, 1995; Boscaini et al., 2021; Boscaini, Gaudin, Mamani Quispe, et al., 2019; Saint‐André, 1994; Saint‐André et al., 2010); and Upper Pliocene of Ayo Ayo‐Viscachani (Department of La Paz, Bolivia) (Saint‐André, 1994; Saint‐André et al., 2010). Quiñones et al. (2019) reported the first record for Argentina, in the Upper Member of the Tafna Formation (Pliocene). Recently, Salas‐Gismondi et al. (2023) reported this species from the early Pliocene in the vicinity of Cuzco, Peru.

Referred materials

JUY‐P‐0081, distal fragment of the right femur; JUY‐P‐0082, proximal fragment of right femur; JUY‐P‐0084, skull, mandible, atlas and axis, caudal vertebrae, distal fragment of femur; JUY‐P‐0085, skull fragment, mandible, fragment of clavicle, fragment of left ulna, two ungueal phalanges, distal end of tibia and two caudal vertebrae; JUY‐P‐0086, fragment of hemimandible; JUY‐P‐0087, mandible; JUY‐P‐0089 palatal fragment, mandible, axis, left astragalus and one dorsal vertebra, one dermal ossicle; JUY‐P‐0177, right astragalus, fragment of vertebrae and proximal fragment of left calcaneus; JUY‐P‐0185, right femur; JUY‐P‐0399, right hemimandible; JUY‐P‐0400, axis, two cervical vertebrae and several dermal ossicles.

Stratigraphic and geographic provenance

Upper section of the Tafna Formation (sensu Zurita et al., 2017), Casira locality, eastern Puna, Jujuy Province, Argentina (Figure 1b,c).

3.1.2. Description and comparisons

Skull

The skull JUY‐P‐0084 (adult; Figure 2a) has a mediolaterally wide braincase in relation to the total cranial length. In the lateral view (Figure 2a.2 and b.1), the dorsal profile is flat with a marked prefrontal depression at the level of the anterior margin of the orbit, in front of which the nasals are slightly elevated, showing an almost identical morphology to S. uccasamamensis described by Saint‐André et al. (2010) and Boscaini, Gaudin, Mamani Quispe, et al. (2019) on the basis of specimens from Bolivia (MNHN‐BOL V 3348, MNHN‐Bol V 3726, MNHN‐Bol V 3717). In the same way, as mentioned by the aforementioned authors, the lacrimal is posterior to the maxilla, with the lacrimal foramen antero‐posteriorly extended; below the lacrimal foramen, the jugal portion of the zygomatic arch is present in two specimens (JUY‐P‐0084 and juvenile JUY‐P‐0085), with a subtriangular area where it joins the maxilla. The posterior side of the zygomatic process is curved, forming the arc that continues posteriorly giving origin to the orbital notch. The morphology of JUY‐P‐0084 (Figure 2a.2) and JUY‐P‐0085 (Figure 2b.1) also coincides with that described for S. uccasamamensis by Boscaini, Gaudin, Mamani Quispe, et al. (2019) in that the jugal process extends antero‐posteriorly forming a wide lamina with three projections, the ascending process, the intermediate process, and the descending process; the ascending process is subrectangular and long, with a wide base that narrows posteriorly becoming rounded; the intermediate process is shorter and wider than the ascending one; its area is subtriangular with an acute apex directed ventrally, near the zygomatic process of the squamosal; and the descending process is subquadrangular, wider than long. The zygomatic process of the squamosal, in both adult and juvenile specimens, bears a long digitiform structure, subrectangular in cross‐section with a planar external face, and a slightly convex dorsal margin, as in S. uccasamamensis (Boscaini, Gaudin, Mamani Quispe, et al., 2019). In the occipital region, the articular surface of the occipital condyles is semicircular without a defined neck, (as in MNHN‐Bol V 3711 and MNHN‐Bol V 3718) and the foramen magnum has a detached notch on its dorsal border (see Boscaini, Gaudin, Mamani Quispe, et al., 2019).

FIGURE 2.

FIGURE 2

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Skulls of S. uccasamamensis (a) JUY‐P‐0084 in (1) dorsal, (2) lateral, and (3) ventral views; (b) JUY‐P‐0085 in (1) lateral and (2) ventral views. Bo, basioccipital; Cf, caniniform; ec, ectotympanic; ecc, exoccipital crest; fr, frontal; jf, jugal foramen; jp, jugal process; lf, lacrimal foramen; magf, foramen magnum; md, mastoid depression; Mf, molariform; mx, maxilla; nas, nasal; nc, nuchal crest; oc, occipital condyle; pal, palatine; par, parietal; pmx, pre‐maxillary; pp, paracondylar process; pt, pterygoid; sf, stylohyal fossa; spc, supraoccipital; szp, squamosal zygomatic process; tfs, temporal fossa. Scale: 50 mm.

In the ventral view (Figure 2a.3) of the adult specimens JUY‐P‐0084 and subadult JUY‐P‐0089, the morphology coincides with that described by Boscaini, Gaudin, Mamani Quispe, et al. (2019), as the palate is subtriangular, with their margins transversely concave because of the protrusion of the alveolar ridges, becoming strongly divergent anteriorly and the palatine surface is rugose with numerous vascular perforations. In both adult JUY‐P‐0084 and juvenile JUY‐P‐0085 (Figure 2b.2) individuals, it is observed that the jugal apophysis of the maxilla projects perpendicular to the longitudinal plane of the skull; on the ventral region of the jugal apophysis, the infraorbital foramen is preserved; it has a narrow base and is located at the level of the posterior margin of the Mf2; the zygomatic process is further back, between the Mf3 and the middle axis of Mf4; and posterolaterally, the squamosal portion of the zygomatic arch diverges obliquely from the wall of the skull. The different measurements of the skull analyzed are summarized in Table 2.

TABLE 2.

Measurements (in mm) of the skull of Simomylodon uccasamamensis.

Skull
Specimen TL PL LDS LMS MPW PPW MSW MPW CH SH IBD EBD
JUY‐P‐0084 350 130 121 94 86 31 118 75 103 86 33 91
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Abbreviations: APW, anterior palatal width; CH, skull height; EBD, external bicondyloid diameter; IBD, internal bicondyloid diameter; LDS, Length of dentary series; LMS, Length of molariform series; MPW, minimum postorbital width; MSW, maximum snout width; PL, palatal length; PPW, posterior palatal width; SH, snout height; TL, Total length.

The above descriptions allow us to indicate that, at least at the level of the preserved cranial parts, there are no significant differences between adult and subadult specimens.

Upper dental series

The upper dental series of JUY‐P‐0084 consists of one anterior caniniform and four molariforms; the series diverges anteriorly and has no diastema between Cf1 and Mf1, similar to that described in S. uccasamamensis (see Boscaini, Gaudin, Mamani Quispe, et al., 2019). Likewise, the particular morphology of each of the dental elements is similar to that previously described by other authors for this species (see Boscaini, Gaudin, Mamani Quispe, et al., 2019; Saint‐André et al., 2010).

Mandible

The preservation of a larger number of jaws has allowed us to observe variations in their morphology throughout ontogeny. In lateral view (Figure 3a.2–e.2), the ventral mandibular margin of JUY‐P‐0089 (Figure 3c.2), JUY‐P‐0087 (Figure 3d.2) and JUY‐P‐0084 (Figure 3e.2) is convex, while that of JUY‐P‐0399 (Figure 3a.2) and JUY‐P‐0085 (Figure 3b.2) is slightly convex. Toward the angular process, there is a slight concavity in JUY‐P‐0084 and JUY‐P 0085. The profile of the symphysis varies from straight in JUY‐P‐0399 (infant) to sigmoid in JUY‐P‐0084 and JUY‐P‐0087 (adults), convex at the level of the mental foramina and concave toward the symphysial margin, as described by Boscaini, Gaudin, Mamani Quispe, et al. (2019) in S. uccasamamensis. The anterior margin of the coronoid process rises, ontogenetically, at the level of the second lobe (JUY‐P‐0399) or behind the posterior lobe (JUY‐P‐0084, JUY‐P‐0085, JUY‐P‐0087, and JUY‐P‐0089) of the mf3, in agreement with that observed in S. uccasamamensis. In all the studied specimens, the posterior opening of the mandibular canal (which can be seen in the lateral view) is situated below the occlusal plane and close to the anterior margin of the coronoid process. The mandibular canal extends to the mental foramina of the symphysial region, which is positioned anteriorly in JUY‐P‐0084 and JUY‐P‐0087 and more towards the lateral margins in JUY‐P‐0399 and JUY‐P‐0089 (Figure 3). The mental foramina vary in size and number in the different specimens (see Ontogeny).

FIGURE 3.

FIGURE 3

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Mandibles of S. uccasamamensis. (a) JUY‐P‐0399; (b) JUY‐P‐0085; (c) JUY‐P‐0089; (d) JUY‐P‐0087; (e) JUY‐P‐0084. In (1) anterior, (2) lateral, and (3) occlusal views. ap, angular process; cf, caniniform; cp, condylar process; crp, coronoid process; mf, molariform; mfr, mandibular foramen; ms, mandibular symphysis; pomc, posterior opening of the mandibular canal. Scale: 50 mm.

In occlusal view (Figure 3a–e.3) (JUY‐P‐0084, JUY‐P‐0087, and JUY‐P‐0089) and at the level of the anterior margin of the mandible, the symphysis is wide and flat, with a constriction in front of the caniniforms, acquiring a typical shovel‐shape, as in S. uccasamamensis. The posterior margin of the mandibular symphysis begins at the level of the middle axis of the caniniform and varies from a smooth U‐shape to a V‐shape (see Ontogeny). The different measurements of the mandibles analyzed are compiled in Table 3, and several of the morphological and size changes described above can be observed in Figure 9, where the specimens assigned to the different ontogenetic stages are plotted (based on the length of the molariform series and the height of the mandible at the mf2 level).

TABLE 3.

Measurements (in mm) of the mandible of Simomylodon uccasamamensis.

Mandible
Specimen HMR TL pTL LSD LSM WpT
JUY‐P‐0399 31 86 22 46 36 ‐
JUY‐P‐0084 65 233 38 110 81 76
JUY‐P‐0085 53 145 ‐ 63 63 ‐
JUY‐P‐0087 66 233 37 95 75 60
JUY‐P‐0089 53 178 30 83 68 52
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Abbreviations: HMR, height of mandibular ramus; LDS, length of dentary series; LMS, length of molariform series; pTL, pre‐tooth length; TL, total length; WpT, width of pre‐tooth margin.

FIGURE 9.

FIGURE 9

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Graph showing the increase of cf‐mf3 series length in relation to mandibular height at mf2 level during ontogeny of Simomylodon uccasamamensis.

Lower dental series

The mandibular teeth are heterodont, with one pair of caniniforms and three pairs of molariforms. In JUY‐P‐0399 (Figure 3a), JUY‐P‐0089 (Figure 3c), and JUY‐P‐0084 (Figure 3e), the caniniform (cf) and the three molariforms are preserved, whereas in JUY‐P‐0087 (Figure 3d) only the alveolus of the caniniform and mf1‐mf3 are preserved. There is a marked anterior divergence of the dental series as in S. uccasamamensis (see Boscaini, Gaudin, Mamani Quispe, et al., 2019).

In lateral view, the caniniform is dorsoventrally larger than the molariforms. Because of the mandibular divergence, the cf1 is slightly labially displaced, but remains on the same axis as the rest of the dental series, as in S. uccasamamensis. Caniniforms vary in cross‐section from subcircular to subtriangular in the different specimens, with a wide and straight lingual surface and a smooth concavity. As in S. uccasamamensis, the occlusal surface is beveled with two clear wear facets in JUY‐P‐0084 and there is no diastema between cf1 and mf1.

The mf1 is elliptical to kidney‐shaped in cross‐section, with a diagonal axis‐oriented anterolabial to posterolingually. In JUY‐P‐0084, mf1 is more transversely inclined with respect to the axis of the dental series, unlike JUY‐P‐0399, JUY‐P‐0085, JUY‐P‐0087, and JUY‐P‐0089, in which the mandibular axis is perpendicular. In this molariform, a change has been observed from the elliptical morphology present in juveniles to a more kidney‐shaped morphology in adult individuals (JUY‐P‐0084 and JUY‐P‐0087, respectively) determined by the presence of a slightly concave anterolingual face and a convex posterior face.

The mf2 is subrectangular in cross‐section and its main axis is oriented from anterolabial to posterolingual in all the individuals analyzed, a characteristic already observed in S. uccasamamensis.

The mf3 is bilobed and anteroposteriorly elongated. On both labial and lingual sides there are well‐defined apicobasal sulci limiting a marked narrowing in the middle of its length where the furrow of the labial side is shallow, and that of the lingual side is deeper. This isthmus defines the characteristic lobes of mf3. The anterior lobe is subtriangular in cross‐section, with the anterior margin planar or slightly concave. The posterior lobe is subtriangular with rounded margins and a clear inclination toward the lingual side. These characteristics are similar to those shared by all known specimens of the species.

Clavicle

(JUY‐P‐0085). This is the first record and description of the clavicle for S. uccasamamensis (Figure 4a). It preserves the sternal epiphysis and part of the diaphysis. The diaphysis is oval in cross‐section with a slight dorsoventral flattening. The rugose sternal epiphysis is proximodistally expanded, increasing the extent of its surface. The clavicle has a slight sigmoid curve between the epiphysis and diaphysis, as in Glossotherium (see Owen, 1842), but less developed than in Nothrotheridae and Megatheriidae (see Amson et al., 2015; Stock, 1925).

FIGURE 4.

FIGURE 4

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Clavicle portion of Simomylodon uccasamamensis (JUY‐P‐085) sampled in this study, in (a.1) anterior and (a.2) posterior views; (b) microanatomy and (c) histological section of the clavicle.

The thin section of the clavicle is semicircular, with a planar surface at one end. The degree of compactness of this section is 84.06%. Histologically, the cortical tissue is dominated by dense Haversian bone. The diameter of the secondary osteons varies from 146 to 572 μm, with an average value of 326 μm. In most of the sections, they are oriented perpendicular to the plane of the section with the exception of the lateral sector where they are parallel to the section. The mean OPD, measured in different sectors of the clavicle cortical tissue, is 81.68 ± 7.52 (mean ± standard deviation). The medullary cavity is composed of cancellous bone with trabeculae covered by lamellar endosteal tissue (Figure 4b,c).

Femur

JUY‐P‐0185 (Figure 5b) is an almost complete right femur, lacking only the head; JUY‐P‐0082 (Figure 5c) preserves only the posterior side, JUY‐P‐0084 (Figure 5d) the distal epiphysis, and JUY‐P‐0081 part of the right diaphysis with its distal portion. In the anterior view, the distal end is narrower than the proximal end, as in all Mylodontinae. The greater trochanter of JUY‐P‐0185 (Figure 5b.1) is wide both mediolaterally and anteroposteriorly, as in S. uccasamamensis. Between the greater trochanter and the femoral head, two well‐defined parallel crests extend from the proximal margin to part of the diaphysis, as in S. uccasamamensis. Distally, the diaphysis has a marked convexity below the greater trochanter, on the lateral margin, as in S. uccasamamensis. The femur has a slight twist, so the femoral head is inclined medial to the longitudinal axis. On the medial margin, 23–24 mm below the femoral head of JUY‐P‐0082, is the trochanter minor, with an elliptical and convex surface, and vertically oriented major axis. Distal to the trochanter minor, the medial margin of the diaphysis becomes concave. In posterior view, the femoral head of JUY‐P‐0082 (Figure 5c) is poorly preserved. In JUY‐P‐0082 and JUY‐P‐0185 (Figure 5b.2), the trochanter major is subtriangular as in S. uccasamamensis. The different measurements of the materials analyzed are compiled in Table 4.

FIGURE 5.

FIGURE 5

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(a) Femur (JUY‐P‐0185) in (1) anterior and (2) posterior views; (b) femur (JUY‐P‐0082) in posterior view; (c) distal portion of the femur (JUY‐P‐0084) in (1) anterior and (2) posterior views. fh, femoral head; gt, greater trochanter; lc, lateral condyle; lep, lateral epicondyle; lt, lesser trochanter; mc, medial condyle; ptr, patellar trochlea; trfs, trochanteric fossa.

TABLE 4.

Measurements (in mm) of the postcranial of Simomylodon uccasamamensis.

Femur
Specimen MLF PWF DWF Wtroc TDF FHL FCW MCL PGW
JUY‐P‐0185 266 103 a 107 123 81 45 84 66 60
JUY‐P‐0082 268 112 ‐ 122 79 ‐ ‐ ‐ ‐
JUY‐P‐0084 ‐ ‐ 130 ‐ ‐ ‐ ‐ 88 64
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Abbreviations: DWF, Distal width of femur; FCW, femoral condylar width; FHL, femoral head length; MCL, medial condyle length; MLF, maximum length of femur; PGW, patellar groove width; PWF, proximal width of femur; TDF, transverse diameter of femur—at midshaft; Wtroc, width across trochanters.

a

Approximal.

Dermal ossicles

Dermal ossicles have been found associated with specimens JUY‐P‐0084 and JUY‐P‐0400 (1 and 137, respectively). These structures have different morphologies (spherical, subspherical, platyform, and irregular), with mean maximum diameters ranging from 3.5 mm to 17.4 mm and an overall average value of 8.7 mm (Figure 6). The outer surfaces of the ossicles are rugose, with smooth or slightly irregular edges, with small grooves surrounding and inserted into some of the ossicles. All the ossicles have been found associated with cervical vertebrae, which indicates they originated from the neck. The general morphology is similar to those described for the specimen of S.uccasamamensis from Casira (Bolivia) by Boscaini et al. (2021), as well as for other members of the family Mylodontidae such as Paramylodon garbanii (=Glossotherium chapadmalense; see Morgan, 2008; McDonald & Morgan, 2011) (see Hill, 2006), My. darwinii (Haro et al., 2017; Toledo et al., 2021) and Pa. harlani (Sinclair, 1910).

FIGURE 6.

FIGURE 6

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Dermal ossicle of Simomylodon uccasamamensis (JUY‐P‐400) sampled in this study; (a) superficial, basal, and lateral views of the dermal ossicle; (b) transverse section; (c) detail of the histological features of this element; (d) general view of the cortex showing the distribution of growth marks (white arrows). lsf: longitudinally sectioned fibers; ol: osteocyte lacunae; po, primary osteons; vc: vascular cavity.

Microstructurally, dermal ossicles are compact structures. They consist of densely packed bundles of mineralized structural fibers of variable orientation. Although most are diffuse bundles, two directions are predominant, oriented at 35° with respect to the basal surface. Some primary osteons and vascular canals are also present. In the cortical region, at least eleven LAGs parallel to the outer surface are present, along with bundles of Sharpey fibers.

3.2. Ontogeny and associated morphological changes

In all the mandibles, the structures present in adult specimens (JUY‐P‐0084 and JUY‐P‐0087) are also present in juveniles (JUY‐P‐0399, JUY‐P‐0085) and subadults (JUY‐P‐0089), although the relative size and shape of some structures and their position change according to ontogenetic stages (see Figures 7, 8, 9).

FIGURE 7.

FIGURE 7

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CT reconstruction of the mandible of S. uccasamamensis (a) JUY‐P‐0399; (b) JUY‐P‐0085; (c) JUY‐P‐0089; (d) JUY‐P‐0087; (e) JUY‐P‐0084. In (1) lateral and (2) occlusal views. cf, caniniform; mf, molariform.

FIGURE 8.

FIGURE 8

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Radiograph of the mandible of S. uccasamamensis (a) JUY‐P‐0399; (b) JUY‐P‐0085; (c) JUY‐P‐0089; (d) JUY‐P‐0087; (e) JUY‐P‐0084. avnc, alveolar vascular‐nervous canal.

In occlusal view, the anterior margin of the predental area of all the specimens (JUY‐P‐0399, JUY‐P‐008, JUY‐P‐0089, JUY‐P‐0084, and JUY‐P‐0087) is shovel‐shaped. However, in the subadult (JUY‐P‐0089), this structure is narrower than in adults (JUY‐P‐0084 and JUY‐P‐0087), indicating that it increases in transverse diameter with ontogeny. In medial and occlusal views of the smallest specimen (infant, JUY‐P‐0399 Figure 7a), it can be inferred that the symphysis was not fused, although only one of the hemimandibles is preserved. In the remaining specimens (JUY‐P‐0089, JUY‐P‐0087, and JUY‐P‐0084 Figure 7c–e) the contact area of the symphysis is dorsoventrally deep, extending anterodorsally from its posterior margin. In adults (JUY‐P‐0084 and JUY‐P‐0087), the contour of the symphysis is sigmoidal in lateral view, because of an anterodorsal concavity and an anteroventral convexity, both of which are absent in infant and subadult specimens (JUY‐P‐0399 and JUY‐P‐0089), resulting in a straight anterior profile of the symphysis (see Figure 7a,c–e.1). These ontogenetic changes deepen the predental spout accompanied by a modification of the posterior margin of the mandibular symphysis, from a smooth U‐shape in juveniles to a distinctive V‐shape in adults (Figure 3(a–e.3)), as was reported by Naples (1990) for Nothrotheriops. These changes demonstrate a significant reshaping of the front of the mandibular body with ontogeny.

In lateral view, the ventral margin of the mandibular ramus of infant and juvenile specimens (JUY‐P‐0399 and JUY‐P‐0085) exhibits a slight convexity, unlike subadult and adults (JUY‐P‐0089, JUY‐P‐0084, and JUY‐P‐0087) in which the convexity is more pronounced, diminishing at the level of mf3 (lateral view from Figure 3a–e). In X‐Rays (Figure 8) of the smallest mandibles (JUY‐P‐0399, JUY‐P‐0085, and JUY‐P‐0089), the alveolar vascular nervous canal is ventral to the molariforms. However, in adult specimens, this canal is lingually and dorsally displaced by ventral and labial growth of the mandibular ramus (Figure 8d,e). The alveolar canal ends anteriorly in the mental foramina, which varies in shape, number, and position. In infants and juveniles (JUY‐P‐0399 and JUY‐P‐0089) they are numerous, small, and situated towards the lateral margin, whereas in adults (JUY‐P‐0084 and JUY‐P‐0087) there is only one large foramen, in an anterior position, next to the mandibular symphysis. In X‐Rays (Figure 8) and lateral CT reconstructions (Figure 7), the vertical development of the mandible is driven by the growth at the alveolar level, adding height and thickness to the mandibular body (Figure 9); this process is similar to that observed in Nothrotheriops shastensis (Naples, 1990). Molariforms of the infant and juvenile specimens (JUY‐P‐0399 and JUY‐P‐0085) do not reach the ventral margin of the mandibular body and are anterodorsally inclined in the alveoli (Figures 7a,b and Figure 8a,b). In the subadult (JUY‐P‐0089) molariforms are somewhat more dorsoventrally developed and vertical, and the mandibular ramus has begun to deepen and widen (Figures 7c and Figure 8c), whereas in adults (JUY‐P‐0084 and JUY‐P‐0087) molariforms reach the ventral margin and sometimes are slightly inclined posteriorly (Figure 7d,e and Figure 8d,e).

As noted above, the shape and size of the posterior region of the mandibles are similar in juveniles and adults. However, the coronoid process in juveniles (JUY‐P‐0085) is shorter and less robust than in adults. The position of the anterior margin of the coronoid process varies with respect to molariforms with ontogeny. Thus, in the smallest specimens JUY‐P‐0399 and JUY‐P‐0085, the coronoid process begins behind the mf3, while in JUY‐P‐0089, it begins at the middle of the second lobe of mf3, and in JUY‐P‐0084 and JUY‐P‐0087, at the middle constriction of the mf3. The mandibular notch, between the coronoid process and the condyle, is shallower in juveniles (JUY‐P‐0085), revealing a shorter and wider coronoid process than in the adult (JUY‐P‐0084), and a shorter condylar process, as observed in juveniles of Nothrotheriops (see Naples, 1987, 1990). Ventrally, the angular process in juveniles is relatively shorter and less developed anteroposteriorly and dorsoventrally than in adults.

3.2.1. Lower dentition

Juvenile specimens (JUY‐P‐0399 and JUY‐P‐0085) exhibit a greater development of the extra‐alveolar portion of each tooth. Additionally, the molariforms do not reach the ventral surface of the mandibular ramus. Therefore, in juvenile stages, the mandibular ramus is not as dorsoventrally developed as in adults. As molariforms grew, they reached the ventral margin and consequently, the mandibular ramus expanded dorsoventrally and labiolingually. Taking into account the development of the molariforms and their expansion within the ramus, Naples (1990) stated that the ventral expansion of the molariforms occurred from anterior to posterior during growth.

The cf1 in the infant (JUY‐P‐0399) is well developed and is the most fully erupted tooth. In lateral view, it is rectangular with rounded margins, blunt tip, and scarce wear on the posterolingual face of the external dentine layer. As the caniniform grew, the wear changed, forming two well‐marked wear facets (anterior and posterior), obtaining a chisel shape that was pointed in adults (Figure 7).

The mf1 is the smallest molariform in all specimens (Figure 7). In the youngest specimen (JUY‐P‐0399 see Figure 7a), it is the second‐most erupted tooth. In lateral view, mf1 is rectangular with rounded margins, with the occlusal end worn on the anterior and posterior margins. It lacks labial and lingual furrows. With growth (JUY‐P‐0085 and JUY‐P‐0089), wear increases on the posterior margin and the tooth acquires an even wear surface in adults (JUY‐P‐0084). CT reconstructions show that in juvenile molariforms the labial and lingual furrows began to form below the exposed apical section. In contrast, in adults the apicobasal sulci in adults are well developed, defining two lobes (anterior and posterior) on each side of the molariform.

The mf2 is subrectangular in occlusal view, with rounded margins. In all specimens (JUY‐P‐0399, JUY‐P‐0085, JUY‐P‐0089, JUY‐P‐0084, and JUY‐P‐0087), the wear of this molariform is similar with two sharp crests, one at the lingual end of the anterior lobe and another one at the labial end of the posterior lobe. In all specimens, the soft dentine is exposed, forming the beginning of the central basin.

The mf3 is bilobed. In occlusal view, both lobes are similar in size in the smallest specimen (JUY‐P‐0399); with growth, the anterior lobe became larger than the posterior lobe. In the juvenile and subadult (JUY‐P‐0085 and JUY‐P‐0089), the entire surface is strongly worn, except for the anterolingual margin where a small cusp can be seen, whereas in the adult (JUY‐P‐0084) wear is similar over the entire surface (Figure 7).

During growth, the angle formed by the dorsoventral axis of the molariforms and the mandibular ramus changed gradually. In adults (JUY‐P‐0084 and JUY‐P‐0087), molariforms display a labiolingual curve across the width of the mandible, whereas in juveniles (JUY‐P‐0399, JUY‐P‐0085) they remain straight. A similar change in the vertical angle (already explained) occurred with respect to the anteroposterior axis of mandibular molariforms. These changes during development would explain the wear on the surface of the molariforms.

4. DISCUSSION AND CONCLUSIONS

4.1. Taxonomy

From a morphological and taxonomic perspective, the comparative study performed in this paper revealed that these specimens under study belong to S. uccasamamensis (Figure 10) due to the following combination of diagnostic characters: (1) wide braincase in relationship to the total cranial length; (2) in lateral view, the dorsal profile is planar with a marked prefrontal depression at the level of the anterior margin of the orbit, in front of which, the nasals are slightly elevated; (3) long zygomatic process of the squamosal antero‐laterally oriented; (4) wide V‐shaped palate, becoming strongly divergent anteriorly; (5) the diastema between Cf1 and Mf1 is absent; (6) Mf2 and Mf3 possess a marked lingual sulcus; and (7) the symphyseal spout is anteriorly flat. Taking into account the relative size of the mandibles and molariforms, as well as the lack of tooth wear, mandibles JUY‐P‐0399, JUY‐P‐0085, and JUY‐P‐0089 are interpreted as belonging to infant, juvenile, and subadult specimens, respectively (sensu Holte, 2012). It is also interesting to note that the absence of diastema between cf1 and mf1, and the reduction of the labial furrow of mf3, considered diagnostic characters of S. uccasamamensis, are already present in these individuals, indicating that they are not affected by ontogenetic development and highlighting their taxonomic validity.

FIGURE 10.

FIGURE 10

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Reconstruction of S. uccasamamensis from the Pliocene of Casira, Jujuy Province, Argentina. Design by P. Cuaranta.

4.2. Ontogeny

Ontogenetic studies in fossil vertebrates have proven to be useful in supplying information with systematic and taxonomic implications (Sánchez‐Villagra, 2010). Among Xenarthra, such studies have been important when assessing the diversity achieved by Cingulata during some periods (González Ruiz et al., 2020; Luna et al., 2018; Zurita et al., 2011). One central aspect for testing hypotheses in these studies involves the presence of several specimens of different ontogenetic stages having the same geographic and stratigraphic provenance (Sánchez‐Villagra, 2010). This point highlights the importance of the present contribution for Folivora, since this is one of the first contributions to have co‐occurring specimens (i.e., Casira, upper level of the Tafna Formation) belonging to different ontogenetic stages.

The material studied in this paper includes skulls, mandibles, and several postcranial elements. The comparative study of the mandibles, in particular, has yielded new anatomical information with respect to the three ontogenetic patterns: (1) the anteroposterior and dorsoventral diameter of the horizontal ramus of the mandible; (2) the anterodorsal–posteroventral and mediolateral length of the pre‐dental area; and (3) the increasing size of coronoid and angular processes. However, as stated by Naples (1990), although changes are known, the order in which they occur or their rate cannot be inferred.

Observations about the type of tooth wear are based on the following criteria: (1) the angle at which the teeth lie within the alveoli; (2) the alignment of the teeth in the dental series; (3) the asymmetric occlusal contact of the teeth; and (4) the continuous growth of the teeth, which tend to form macrowear occlusal facets (see Naples, 1990; Ungar, 2015). In JUY‐P‐0399 (infant), the cf1 wear is almost absent, unlike the adult (Figure 3a and Figure 7a), but wear on mf1 begins to be evident through two small facets, forming a slightly beveled occlusal surface. In more advanced stages (juvenile, subadult, and adult) wear is greater at the posterior margin (Figure 3 and Figure 7). Wear of the mf2 of JUY‐P 0399 is formed by two sharp crests, lingual on the anterior lobe and labial on the posterior lobe; and on mf3, there is strong wear of the entire surface, except for the anterolingual margin, in which there is a small cusp (Figure 3). The wear pattern of the two last molariforms is the same in all specimens, without major differences. Accordingly, mf2‐mf3 are the first to become functional because, as in adult specimens, the occlusal surfaces are occupied by wear facets. Hence, these teeth erupted first, followed by the progressive forward incorporation of mf1 and cf1. These observations partially coincide with what has been recorded in other groups of xenarthrans. In the case of Glyptodontidae, Ameghino (1891, 1920) suggested that the mf1 was the last to erupt or become functional. Recently, González Ruiz et al. (2020) observed in a juvenile specimen of cf. Eucinepeltus Ameghino, 1891 (Propalaehoplophorinae) that the two first molariforms have no evidence of wear. Likewise, Ciancio et al. (2012) observed in the extant armadillo genus Dasypus that the dental eruption occurs posteroanteriorly. For the Folivora, Boscaini, Gaudin, Mamani Quispe, et al. (2019) in Simomylodon and Varela et al. (2021) in Lestodon inferred that the scarce wear of cf1 is due to its “scarce functionality” for chewing, a feature that may be more probably related to sexual dimorphism. The development process postulated here is different from that seen in Nothrotheriops, in which juveniles show wear surfaces on the anterior molariforms, suggesting that wear facets begin anteriorly and progress posteriorly (Naples, 1990). However, it should be noted that Nothrotheriops lacks a caniniform tooth, so may not be directly comparable to Simomylodon or Lestodon.

As discussed above, mandibular morphology and tooth wear are characteristics that are useful for the recognition of different ontogenetic stages, but these stages do not allow the determination of the numerical chronological age of individuals. Growth marks have been observed in ground sloths in both long bones and dermal ossicles (de Ricqlès et al., 2009; Hill, 2006). Unfortunately, not all of the individuals from the ontogenetic sequence utilized in this work have associated long bones that would allow detailed studies of microanatomical characteristics in relation to cranial development. In those specimens that do have long bone fragments, preservation is poor for obtaining optimal thin sections. In this context, the presence of dermal ossicles in some of the specimens is interesting. The dermal ossicles consist mainly of structural fiber bundles, with little or no remodeling in which there is a good record of growth marks that allow the chronological age of the individuals to be determined. As indicated above, specimen JUY‐P‐400 has 11 LAGs. This individual, based on the size of the associated cervical vertebrae, is similar to individual JUY‐P‐0089 which was characterized as a subadult. In consequence, the number of LAGs could be interpreted as typical of that ontogenetic stage. As indicated by de Ricqlès et al. (2009) these growth cycles in the periphery of the osteoderms closely resemble the rings and zones described in the bones of many extant ectotherms, in which they have been shown to form annually. Therefore, we propose an age of at least 11 years for a subadult individual of Simomylodon.

As mentioned above, most of the materials used for skeletochronology of other mammals derive from long bones. However, other postcranial elements, such as the clavicle, have also been used to determine age in humans (Inthasan & Mahakkanukrauh, 2017; Lee et al., 2014; Sobol et al., 2015; Stout & Paine, 1992). This has not yet been used in megamammals. The characteristics observed in this work constitute a first approach to the use of this methodology for age determination in ground sloths.

Lee et al. (2014) observed that with increasing age of individual humans, a decrease of the relative cortical area (RCA) and an increase in osteon population density (OPD), the latter consistent with increased bone remodeling. The only clavicle recovered from S. uccasamamensis corresponds to individual JUY‐P‐0085 identified as a juvenile. The values obtained for RCA (or compactness) of the section performed is 84.06, while the OPD, measured in different sectors of the clavicle cortical tissue and understood as the percentage occupied by osteons in an area of 1 mm, is 81.68 ± 7.52 (mean ± standard deviation). The OPD value is consistent with the histological description of this element, characterized by a dense Haversian bone. Straehl et al. (2013) observed that this type of tissue characterizes the long bones of adult specimens of different Xenarthra Technician. However, the microanatomical and histological features observed in the clavicle of this specimen do not correspond to the ontogenetic stage assigned to this specimen based on its cranial characteristics. The clavicle ossifies late in xenarthrans in general (Hautier et al., 2011); but in sloths, for example, it is well developed in adult individuals of Choloepus but completely absent in Bradypus (Hautier et al., 2010). Although more work is needed to elucidate the characteristics of its development, the microstructure of the clavicle is of interest for future analyses.

Based on measurements of the femur of individual JUY‐P‐185 (the only one with a complete element), a preliminary estimate of the body mass of S. uccasamamensis was calculated using the allometric equations of Toledo et al. (2014), resulting in a value of 232.49 kg. In turn, Boscaini et al. (2021) following Fariña et al. (1998) and Toledo et al. (2014), obtained a mean body mass estimate of c. 370 kg. This difference between these values may be related to the fact that the individual analyzed in this work is probably a subadult (30% smaller than the adult individual JUY‐P‐0084). Based on these values, we used the formula of Blueweiss et al. (1978), who proposed that patterns of biological cycle can be demonstrated from intervals of change in body mass because there is a relationship between size, life expectancy, and interval between births and gestation periods. To understand the parameters of the biological cycle of S. uccasamamensis, we used the formulae developed by Blueweiss et al. (1978) and a body mass of 232.49 kg, to obtain a 14 years average lifespan, with 9‐month gestation time, and sexual maturation at 4.1 years. Meanwhile, using the mean body mass obtained by Boscaini et al. (2021) of ca. 370 kg, the estimated lifespan is 15.3 years, with a 10.1‐month gestation time, and sexual maturation at 4.6 years. In extant sloth species such as Choloepus didactylus (ca. 4–8 kg) and C. hoffmanni (ca. 7–9 kg), longevity in the wild is unknown, although ages of 25–30 years have been observed in captive specimens. Gestation time is 10.5 months; sexual maturity is reached 2–3 years in C. hoffmanni and 3–4 years in C. didactylus. The tridactyl sloth, Bradypus tridactylus (ca. 3.2–6 kg), achieves a longevity of 12–15 years, with a gestation period of 6 months, and sexual maturity reached 3–6 years (Avey‐Arroyo, 2002; Benirschke, 2008; Bezerra et al., 2007; Gilmore et al., 2008; Taube et al., 2001). Although there is no evidence for the number of offspring in Simomylodon, Pujos et al. (2023) suggested that Nothrotherium maquinense gave birth to a single offspring at a time, as typically occurs in living sloths Bradypus and Choloepus. These observations suggest that some biological variables (e.g., gestation time and sexual maturation) seem to be independent of the body mass and can be interpreted as conservative characters within the evolutionary history of Folivora. On the other hand, the formula of Blueweiss et al. (1978) depends on body mass and therefore could be useful for studying adult individuals. However, it is necessary to indicate that, based on the age obtained from the growth lines observed in the dermal ossicles of the juvenile individual, the values obtained for longevity using this formula may be lower than the true values.

Finally, the linear measurements of the skull of the adult specimen (JUY‐P‐0084; Tl = 350 mm) indicate that it is somewhat larger (ca. 13%) compared to the largest specimen from Bolivia (MNHN‐Bol V 3717; see Boscaini, Gaudin, Mamani Quispe, et al., 2019, Figure 7a–d, Tl = 306 mm), suggesting that our specimen was a male (see Boscaini, Gaudin, Mamani Quispe, et al., 2019). Its larger size could be due to latitudinal and/or temporal variables, but further evidence is necessary to test these hypotheses.

4.3. Paleoecology and biogeography aspects of Simomylodon uccasamamensis

The records here analyzed, coming from the Pliocene levels of the Upper section of the Tafna Formation, represent the southernmost record for this species and the first in Argentina, and show that S. uccasamamensis inhabited two different geomorphostructural systems in its southern geographic distribution, the Bolivian Altiplano, and the eastern Puna in Argentina. This ground sloth had a latitudinal distribution between Cuzco, Peru (14° S) and Casira, Argentina (21° S), showing a geographic pattern very similar to that of other South American endemic clades such as the Notoungulata Posnakytherium desaguaredoi (Ferrero et al., 2024). All localities from Argentina, Bolivia, and Peru with records of S. uccasamamensis lack evidence of immigrant species, suggesting that this ground sloth was not in contact with Holarctic taxa, which are first recorded in the Humahuaca basin, Maimará Formation, at a window interval from 6.6 Ma to 5.8 Ma (Candela et al., 2023).

Within a paleoecological framework, it is interesting to note the different ground sloths that co‐occurred with S. uccasamamensis along its latitudinal distribution, which reached more than 1200 km from north to south. In its northernmost distribution (Pliocene of Peru) this ground sloth was associated with the Megatheriidae Megatheriops rectidens and the Scelidotheriinae Proscelidodon (Salas‐Gismondi et al., 2023), while in the latest Miocene and latest Pliocene interval of the Bolivian Altiplano, it appears associated with Proscelidodon patrius in Inchasi, Megatherium altiplanicum in Casira, and Aymaratherium jeani and Megatherium altiplanicum in Pomata‐Ayte (Anaya & MacFadden, 1995; Cerdeño et al., 2012; Pujos et al., 2016). Finally, in its southernmost distribution, in Casira, eastern Puna, this ground sloth co‐occurred with the enigmatic Thalassocninae Thalassocnus sp., the first evidence of this aquatic ground sloth in a fully continental sequence (see Quiñones et al., 2022).

The morphology of the premaxillae and maxillae gives valuable information about the potential diet of ground sloths. Bargo et al. (2006) inferred that the wide‐muzzled ground sloths like Glossotherium and Lestodon, which share a very similar morphology with S. uccasamamensis, can be interpreted as bulk feeders. In turn, the evidence of the enigmatic Thalassocnus suggests, according to De Esteban‐Trivigno (2011), that it may have been omnivorous/frugivorous (see also Quiñones et al., 2022). On the other hand, and even though Thalassocnus was not included in the analysis of Bargo et al. (2006), the morphology of the muzzle seems to be quite similar to those of Mylodon, Megaterium, and Scelidotherium (i.e., narrow‐muzzled), taxa interpreted as mixed or selective feeders. In consequence, and in agreement with that inferred for the ground sloths of Peru by Salas‐Gismondi et al. (2023), a niche partitioning is expected for sloths in the eastern Puna according to the anatomical evidence. Despite a remarkable sampling effort, we have so far found no evidence of another lineage of ground sloths co‐occurring in Casira. The phytoliths from the upper section of the Tafna Formation (Pliocene) show an abundance of dicotyledons and C3 grasses. Moreover, in most samples, elements of Podostemaceae and a few palm globular phytoliths and non‐phytoliths are found (Contreras et al., 2019). This Pliocene flora supported the co‐occurrence in eastern Puna of at least two species of Pilosa belonging to different lineages, Thalassocnus sp. and S. uccasamamensis.

AUTHOR CONTRIBUTIONS

Sofía I. Quiñones: conceptualization, methodology, investigation, computed tomography analysis, preparation of figures and tables, writing—original draft preparation and funding acquisition. Carlos A. Luna: methodology, investigation, histological analysis, preparation of figures and tables, writing—original draft preparation. Alfredo E. Zurita: writing—original draft preparation and funding acquisition. Ángel R. Miño‐Boilini: writing—review and editing. Adriana M. Candela: writing—review and editing.

ACKNOWLEDGMENTS

The authors of this contribution wish to thank the authorities and people of Cieneguillas, Calahoyo, Casira, and Santa Catalina for their hospitality during the fieldwork, as well as the Secretaría de Cultura and IdGyM‐UNJu (Instituto de Geología y Minería, Universidad Nacional de Jujuy) of the province of Jujuy for authorizing the development of the research project and facilitating access to the paleontological material (resol. 978 MCyT/23). Thanks are also due to Carolina Repetto (Hospital Escuela Veterinario, Facultad de Veterinaria, Universidad Nacional del Nordeste) for the X‐Rays analysis; Pablo Calderon (Technician of the Hospital Tránsito Cáceres de Allende, Cordoba, Argentina); and Dr. Peloso y Technician Vera (Instituto de Cardiología de Corrientes, Argentina). Thank C. Deschamps and D. Gillette who helped with the English and P. Cuaranta for the paleoart. We also thank both reviewers (A. Boscaini and an anonymous reviewer) and editor Dr. Philip Cox, whose comments and suggestions greatly improved the manuscript. This contribution was supported by grants PI Q21/02 (SGCyT‐UNNE) and PICT‐2021‐I‐INVI‐00638.

Quiñones, S.I. , Luna, C.A. , Miño‐Boilini, Á.R. , Candela, A.M. & Zurita, A.E. (2025) Ontogeny and associated changes of the extinct sloth Simomylodon uccasamamensis (Xenarthra, Mylodontidae) from the Pliocene of the eastern Puna, Argentina. Journal of Anatomy, 246, 171–189. Available from: 10.1111/joa.14152

DATA AVAILABILITY STATEMENT

All data from this work is available for sharing.

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