The first 50Myr of dinosaur evolution: macroevolutionary pattern and morphological disparity

Abstract

The evolutionary radiation of dinosaurs in the Late Triassic and Early Jurassic was a pivotal event in the Earth's history but is poorly understood, as previous studies have focused on vague driving mechanisms and have not untangled different macroevolutionary components (origination, diversity, abundance and disparity). We calculate the morphological disparity (morphospace occupation) of dinosaurs throughout the Late Triassic and Early Jurassic and present new measures of taxonomic diversity. Crurotarsan archosaurs, the primary dinosaur 'competitors', were significantly more disparate than dinosaurs throughout the Triassic, but underwent a devastating extinction at the Triassic-Jurassic boundary. However, dinosaur disparity showed only a slight non-significant increase after this event, arguing against the hypothesis of ecological release-driven morphospace expansion in the Early Jurassic. Instead, the main jump in dinosaur disparity occurred between the Carnian and Norian stages of the Triassic. Conversely, dinosaur diversity shows a steady increase over this time, and measures of diversification and faunal abundance indicate that the Early Jurassic was a key episode in dinosaur evolution. Thus, different aspects of the dinosaur radiation (diversity, disparity and abundance) were decoupled, and the overall macroevolutionary pattern of the first 50Myr of dinosaur evolution is more complex than often considered.

Figure 1

Morphospace occupation for dinosaurs and crurotarsans. (a) Late Triassic (Carnian–Norian) and (b) Early Jurassic (Hettangian–Toarcian). Triassic and Jurassic taxa are mapped onto a single two-dimensional morphospace, based on the first two principal coordinate axes recovered by the PCO analysis of the entire dataset. Dinosaur morphospace slightly increased in the Early Jurassic, whereas crurotarsan morphospace crashed after the TJ extinction. Note that dinosaur and crurotarsan morphospaces do not overlap: this is an artefact of the discrete character set, which has a phylogenetic structure. Importantly, those crurotarsans heavily convergent with dinosaurs (e.g. poposauroids, ‘rauisuchids’) are intermediate in morphospace between dinosaurs and the bulk of crurotarsans.

Figure 2

Morphological disparity for crurotarsans, dinosaurs and avemetatarsalians across the Triassic and the Early Jurassic. (a) Crurotarsans and dinosaurs, (b) crurotarsans and their sister clade, Avemetatarsalia, (c) disparity and diversity for dinosaurs. Error bars denote 95% bootstrap confidence intervals. Norian measures in (b) are shown offset to prevent the loss of information beneath overlapping error bars. Diversity count in (c) is a phylogenetic estimate based on total observed fossils plus implied ghost ranges.