Evolution of the sauropodomorph astragalus: relationships with the emergence of the sauropod bauplan and weight-bearing function, and critical appraisal of evolutionary rate estimation

Abstract Sauropods, the largest terrestrial animals, were a clade of quadrupedal dinosaurs nested within sauropodomorphs, whose early representatives were small bipeds. Their evolution towards gigantism was associated with acquisition of specialized features, notably the columnar limbs. However, their tarsus, mainly represented by the astragalus, has received little attention despite its crucial position and role in limb architecture. We aim to quantify the three-dimensional shape of a large sample of sauropodomorph astragali to assess how its morphology evolved through time and with the emergence of the sauropod bauplan. Acquisition of sauropod-related features was stepwise, congruently with previous results for the hindlimb zeugopod, with marked distinction and diversification of the eusauropod subclade. Size-related variations were congruent with weight-bearing constraints, with flatter distal articular surfaces and more tightly interlocked proximal ones. The subhorizontal pes organization in eusauropods is comparable to that in elephants, suggesting a link with the evolution of an extensive foot pad in both groups. Our evolutionary rates analysis could appear to support this hypothesis. However, our randomization procedure showed that our test was highly sensitive to temporal uncertainties. Our case study therefore encourages acknowledgement of sensitivity in phylogenetically informed palaeobiological analyses to assess more robustly the degree of confidence to put on subsequent interpretations.