Sauropodomorph evolution across the Triassic-Jurassic boundary: body size, locomotion, and their influence on morphological disparity

Abstract

Sauropodomorph dinosaurs were the dominant medium to large-sized herbivores of most Mesozoic continental ecosystems, being characterized by their long necks and reaching a size unparalleled by other terrestrial animals (> 60 tonnes). Our study of morphological disparity across the entire skeleton shows that during the Late Triassic the oldest known sauropodomorphs occupied a small region of morphospace, subsequently diversifying both taxonomically and ecologically, and shifting to a different and broader region of the morphospace. After the Triassic-Jurassic boundary event, there are no substancial changes in sauropodomorph morphospace occupation. Almost all Jurassic sauropodomorph clades stem from ghost lineages that cross the Triassic-Jurassic boundary, indicating that variations after the extinction were more related to changes of pre-existing lineages (massospondylids, non-gravisaurian sauropodiforms) rather than the emergence of distinct clades or body plans. Modifications in the locomotion (bipedal to quadrupedal) and the successive increase in body mass seem to be the main attributes driving sauropodomorph morphospace distribution during the Late Triassic and earliest Jurassic. The extinction of all non-sauropod sauropodomorphs by the Toarcian and the subsequent diversification of gravisaurian sauropods represent a second expansion of the sauropodomorph morphospace, representing the onset of the flourishing of these megaherbivores that subsequently dominated in Middle and Late Jurassic terrestrial assemblages.

Figure 1

Morphospaces and diversity of Sauropodomorpha during the Triassic-Early Jurassic. (a) Randomly selected, time calibrated MPT and morphospace occupation of Sauropodomorpha during (b) Carnian, (c) Norian-Rhaetian, (d) pre-Toarcian, and (e) post-Toarcian periods. Each plot shows the first two principal coordinate axes, with a variance of 9.82% (PCo 1, x axis) and 3.90% (PCo 2, y axis) (see Fig. 2b for more details about morphospace plots).

Figure 2

Morphological disparity of Sauropodomorpha during the Triassic-Early Jurassic. (a) Weighted mean pairwaise dissimilarity (WMPD) and Sum of Ranges (SoR) with their respective 95% confidence intervals. (b) Morphospace distribution in the first two PCOs. Barapasaurus and Isanosaurus (greys circles) were excluded from time bin groups because their temporal uncertainty. (c) Displacement from the centroid of the previous time bin, in which the thickness of the arrows is proportional to the multidimensional distance of the displacement. (d) Phylomorphospace showing locomotion style (mapped in orange circles for bipedal and pink circles for quadrupedal) and body mass optimized through time (colours of branches). Empty circles represent assumed locomotion style based on Fitch optimization in the phylogenetic trees.