A Dome-Headed Stem Archosaur Exemplifies Convergence among Dinosaurs and Their Distant Relatives

• Published in: Current biology 2016-10, Vol.26 (19), p.2674-2680
• Main Authors: Stocker, Michelle R., Nesbitt, Sterling J., Criswell, Katharine E., Parker, William G., Witmer, Lawrence M., Rowe, Timothy B., Ridgely, Ryan, Brown, Matthew A.
• Format: Article
• Language: English
• Subjects: Animals; archosauromorph; Biological Evolution; computed tomography; Dinosaurs - anatomy & histology; Fossils - anatomy & histology; mass extinction; Mesozoic; morphology; morphospace; Phylogeny; radiation; Reptiles - anatomy & histology; Texas; Triassic
• ISSN: 0960-9822; 1879-0445
• DOI: 10.1016/j.cub.2016.07.066

Similarities in body plan evolution, such as wings in pterosaurs, birds, and bats or limblessness in snakes and amphisbaenians, can be recognized as classical examples of convergence among animals [1–3]. We introduce a new Triassic stem archosaur that is unexpectedly and remarkably convergent with the “dome-headed” pachycephalosaur dinosaurs that lived over 100 million years later. Surprisingly, numerous additional taxa in the same assemblage (the Otis Chalk assemblage from the Dockum Group of Texas) demonstrate the early acquisition of morphological novelties that were later convergently evolved by post-Triassic dinosaurs. As one of the most successful clades of terrestrial vertebrates, dinosaurs came to occupy an extensive morphospace throughout their diversification in the Mesozoic Era [4, 5], but their distant relatives were first to evolve many of those “dinosaurian” body plans in the Triassic Period [6–8]. Our analysis of convergence between archosauromorphs from the Triassic Period and post-Triassic archosaurs demonstrates the early and extensive exploration of morphospace captured in a single Late Triassic assemblage, and we hypothesize that many of the “novel” morphotypes interpreted to occur among archosaurs later in the Mesozoic already were in place during the initial Triassic archosauromorph, largely non-dinosaurian, radiation and only later convergently evolved in diverse dinosaurian lineages. [Display omitted] •Cretaceous-aged dome-headed dinosaurs are convergent with Triassic-aged Triopticus•Archosauromorph reptiles evolved extreme body plans after the end-Permian extinction•Dinosaurs repeated body plans present in their Triassic-aged relatives•The early evolution of body plans may constrain later body plans in the same group Stocker et al. describe Triopticus primus, a bizarre Late Triassic animal, whose skull structure is converged on by distantly related pachycephalosaur dinosaurs. The unique body plans observed in Triopticus and other archosaurs at the “dawn of the age of dinosaurs” re-evolved in dinosaurs and crocodylians after the end-Triassic mass extinction.