Table 5. Hypothesized transformations in the course of cranial ontogeny in sauropods.
| Hypothesized transformation | Taxa providing evidence | Sources | Comments |
|---|---|---|---|
| Extreme decrease in relative skull size | Sauropodomorpha | Ikejiri (2004), Rauhut et al. (2011), this study | While relative skull size decreases through ontogeny in many vertebrates, the exceptional diminution of skull size on the evolutionary line leading to Eusauropoda (Rauhut et al., 2011) indicates that negative allometry of the skull with respect to the postcranial skeleton in sauropods was especially extreme |
| Increasingly fused interfrontal and interparietal sutures | Diplodocoidea | Salgado (1999), Whitlock, Wilson & Lamanna (2010), Tschopp & Mateus (2013) and Tschopp, Mateus & Benson (2015) | – |
| Decrease in relative anteroposterior width of nasal process of premaxilla | Camarasaurus; Europasaurus | Britt & Naylor (1994) and Marpmann et al. (2014) | – |
| Stepped margin of the muzzle becomes more distinct | Camarasaurus; Shunosaurus | Britt & Naylor (1994) and Wilson & Sereno (1998) | This hypothesized transformation does not apply to taxa lacking a stepped muzzle (e.g., diplodocids) |
| Muzzle increases in relative width | cf. Diplodocus; Camarasaurus; Europasaurus | Whitlock, Wilson & Lamanna (2010), Whitlock (2011b) and Marpmann et al. (2014) | – |
| Muzzle transitions from rounded and narrow to square and blunt | Diplodocinae | Whitlock, Wilson & Lamanna (2010) and Tschopp & Mateus (2013) | In addition to general widening of the muzzle, the anterior, tooth-bearing region of diplodocids transitions from transversely narrow and rounded to square and blunt. This transition has been suggested to signify niche partitioning between juveniles and adults, and is thus far hypothesized only for diplodocids, or possibly Diplodocoidea generally |
| Elongation of the snout | Theropoda; Massospondylus; cf. Diplodocus; Titanosauria | Varricchio (1997), Salgado, Coria & Chiappe (2005), Whitlock, Wilson & Lamanna (2010), García et al. (2010) and K. E. Chapelle, 2016, unpublished data | Such a transformation is typical of theropods and basal sauropodomorphs. However, rostral elongation was diminished in eusauropods, followed by subsequent elongation of the rostrum in diplodocoids and some advanced titanosauriforms (see Discussion) |
| Posterodorsal retraction of the external nares | Titanosauria | Salgado, Coria & Chiappe (2005) and García & Cerda (2010) | – |
| Reduction of the maxillary process of the jugal | Titanosauria | Salgado, Coria & Chiappe (2005) | – |
| Decreasing participation of jugal in ventral margin of the skull | Europasaurus; Titanosauria | Marpmann et al. (2014), Salgado, Coria & Chiappe (2005) and García et al. (2010) | The presence of a free ventral edge of the jugal in adult Europasaurus, though reduced from the juvenile condition, is hypothesized to reflect paedomorphosis. The jugal has a large participation in the ventral margin of the skull in embryonic titanosaurians, a character otherwise absent in sauropods more derived than Shunosaurus, with the exception of Europasaurus, Giraffatitan, and Malawisaurus (Royo-Torres & Upchurch, 2012) |
| Frontal becomes relatively wider and less elongate | Diplodocidae; Europasaurus; Titanosauria | García et al. (2010), Tschopp & Mateus (2013), this study | The observation that the frontal is relatively longer anteroposteriorly in the paedomorphic Europasaurus, embryonic titanosaurians, and some juvenile sauropods (Bellusaurus, Daanosaurus) than is typical for sauropods suggests that this feature may vary ontogenetically |
| Decrease in depth of orbital rim penetration of the frontal (in dorsal view), perhaps with concomitant increase in width of frontal, including articulations with prefrontal and nasal | Europasaurus; Bellusaurus; Daanosaurus; Titanosauria; possibly cf. Diplodocus | Marpmann et al. (2014), Chiappe et al. (1998), García et al. (2010), Whitlock, Wilson & Lamanna (2010), this study | See Discussion |
| Decrease in distance separating the supratemporal fenestrae | Europasaurus; Camarasaurus | Marpmann et al. (2014), this study | – |
| Decrease in relative size of supratemporal fenestrae | Titanosauria | Salgado, Coria & Chiappe (2005) | The supratemporal fenestra is well-developed in titanosaurian embryos but is reduced in more adult titanosaurian skulls, suggesting ontogenetic reduction of this aperture in titanosaurians |
| Opening or closure of frontal/frontoparietal/parietal fenestra? | Camarasaurus; Europasaurus; Apatosaurus; Titanosauria | Salgado, Coria & Chiappe (2005), Balanoff, Bever & Ikejiri (2010), Marpmann et al. (2014) and Woodruff & Foster (2017) | See Discussion |
| Increasingly large separation between squamosal and quadratojugal | Flagellicaudata | Whitlock, Wilson & Lamanna (2010) and Tschopp & Mateus (2013) | Flagellicaudatans lack contact between the squamosal and quadratojugal, while these two elements are nearly touching in a juvenile diplodocid (CM 11255), suggesting this feature may vary ontogenetically. Notably, embryonic titanosaurians lack the squamosal-quadratojugal articulation present in more adult titanosaurians (Salgado, Coria & Chiappe, 2005), possibly indicating lineage-specific ontogenetic trajectories |
| Increasing depth of quadrate fossa | Europasaurus | Marpmann et al. (2014) | This mirrors a phylogenetic increase in the depth of this fossa in Sauropodomorpha (reversed in Flagellicaudata) |
| Increasing development of tooth-like process on body of pterygoid | Europasaurus | Marpmann et al. (2014) | – |
| Increasing depth of basipterygoid fossa of the pterygoid | Europasaurus | Marpmann et al. (2014) | – |
| Crista interfenestralis ossifies | Apatosaurus; Nebulasaurus; Bellusaurus | Balanoff, Bever & Ikejiri (2010), Xing et al. (2015a), this study | – |
| Ossification of bony septum dividing the optic (CN II) foramen into separate foramina | Europasaurus; basal sauropodomorphs | Marpmann et al. (2014) | The presence of a single optic foramen in basal sauropodomorphs and apparent reversion to the plesiomorphic state in Europasaurus suggests ontogenetic ossification of this septum |
| Closure of external mandibular fenestra | Titanosauria | Salgado, Coria & Chiappe (2005) and García et al. (2010) | An external mandibular fenestra is absent in neosauropods, with the exception of Nigersaurus (Sereno et al., 1999, 2007). Appearance of this feature in embryonic titanosaurians suggests it is lost over the course of ontogeny, though without adult representatives of the Auca Mahuevo titanosaurian, this remains speculative |
| Decrease in relative size of surangular foramen | Rapetosaurus | Curry Rogers & Forster (2004) | – |
| Displacement anteriorly of the posterior extent of the maxillary tooth row | Diplodocinae; Titanosauria | Salgado, Coria & Chiappe (2005), García & Cerda (2010), García et al. (2010), Whitlock, Wilson & Lamanna (2010) and Tschopp & Mateus (2013) | – |
| Decrease in number of teeth? | Camarasaurus; Bonitasaura; Diplodocidae | This study | See Discussion |
Note:
This list is not exhaustive, and emphasizes ontogenetic hypotheses that are particular to sauropods and can be made for more than one taxon.