A path to gigantism: Three‐dimensional study of the sauropodomorph limb long bone shape variation in the context of the emergence of the sauropod bauplan

Abstract

Sauropodomorph dinosaurs include the largest terrestrial animals that ever lived on Earth. The early representatives of this clade were, however, relatively small and partially to totally bipedal, conversely to the gigantic and quadrupedal sauropods. Although the sauropod bauplan is well defined, notably by the acquisition of columnar limbs, the evolutionary sequence leading to its emergence remains debated. Here, we aim to tackle this evolutionary episode by investigating shape variation in the six limb long bones for the first time using three‐dimensional geometric morphometrics. The morphological features of the forelimb zeugopod bones related to the sauropod bauplan tend to appear abruptly, whereas the pattern is more gradual for the hindlimb zeugopod bones. The stylopod bones tend to show the same pattern as their respective zeugopods. The abrupt emergence of the sauropod forelimb questions the locomotor abilities of non‐sauropodan sauropodomorphs inferred as quadrupeds. Features characterizing sauropods tend to corroborate a view of their locomotion mainly based on stylopod retraction. An allometric investigation of the shape variation in accordance with size highlight differences in hindlimb bone allometries between the sauropods and the non‐sauropodan sauropodomorphs. These differences notably correspond to an unexpected robustness decrease trend in the sauropod hindlimb zeugopod. In addition to forelimb bones that appear to be proportionally more gracile than in non‐sauropodan sauropodomorphs, sauropods may have relied on limb architecture and features related to the size increase, rather than general robustness, to deal with the role of weight‐bearing.

Shape variation in the limb long bones of modern elephants reveals adaptations to body mass and habitat

“…Deinotherium , Mammut , Mammuthus ; see Gheerbrant and Tassy ( 2009 )). At a larger taxonomic and evolutive scale, proboscideans were not the first to display fully graviportal bodies: sauropod dinosaurs were obligatory quadrupeds sharing a general graviportal form (Lefebvre et al, 2022 ; Rauhut et al, 2011 ; Sander et al, 2011 ). Their diversification towards a range of extreme gigantism was made possible by the acquisition of columnar limbs (straighter and positioned almost vertically), allowing to support a multi‐tons' body mass (Hildebrand, 1982 ).…”

“…Harbers et al, 2020 ). Similarly, they have been used to study the effect of high body mass in mammals (Mallet et al, 2019 ) and reptiles (Lefebvre et al, 2022 ; Pintore et al, 2022 ), but not on proboscideans bones.…”

“…Deinotherium, Mammut, Mammuthus; see Gheerbrant and Tassy (2009)). At a larger taxonomic and evolutive scale, proboscideans were not the first to display fully graviportal bodies: sauropod dinosaurs were obligatory quadrupeds sharing a general graviportal form (Lefebvre et al, 2022;Rauhut et al, 2011;Sander et al, 2011).…”

Evolution of hind limb morphology of Titanosauriformes (Dinosauria, Sauropoda) analyzed via 3D Geometric Morphometrics reveals wide-gauge posture as an exaptation for gigantism

“…1) to test whether there is a relationship between wider hind limb posture (as the hind limbs are the main weight support of sauropod body, even after wide-gauge posture acquisition; see Bates et al, 2016) and potential morphological convergence between different titanosauriformes subclades regardless of their body size (as proxied by hind limb size). The analysis presented here can be seen as an expansion of Lefebvre et al (2022) study on sauropod limb evolution as we analyse a sample comprised mostly of Late Cretaceous Lithostrotian sauropods (their study included a broad diversity of sauropodomorph taxa including several titanosaurs).…”

“…Several morphological features can be related to the evolutionary cascade that allowed the acquisition of their colossal sizes, like the vertebral pneumaticity related to avian-like air sacs, high metabolic rates, cranial morphology, feeding mechanisms (among several others, see Sander 2013). There are also several appendicular features related with the development of columnar limbs (Bates et al, 2016; Lefebvre et al, 2022; Salgado et al, 1997; Sander, 2013; Ullmann et al, 2017). The mechanical stability of the columnar limbs allowed them to support their multi-toned body masses.…”