A Dome-Headed Stem Archosaur Exemplifies Convergence among Dinosaurs and Their Distant Relatives.
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.
Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA; Vertebrate Paleontology Laboratory, Jackson School of Geosciences, The University of Texas at Austin, J. J. Pickle Research Campus, R7600, 10100 Burnet Road, Austin, TX 78758, USA. Electronic address: stockerm@vt.edu.
Michelle R
MR
Stocker
Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA; Vertebrate Paleontology Laboratory, Jackson School of Geosciences, The University of Texas at Austin, J. J. Pickle Research Campus, R7600, 10100 Burnet Road, Austin, TX 78758, USA.
Sterling J
SJ
Nesbitt
Department of Organismal Biology and Anatomy, University of Chicago, Chicago, IL 60637, USA.
Katharine E
KE
Criswell
Vertebrate Paleontology Laboratory, Jackson School of Geosciences, The University of Texas at Austin, J. J. Pickle Research Campus, R7600, 10100 Burnet Road, Austin, TX 78758, USA; Petrified Forest National Park, Petrified Forest, AZ 86028, USA.
William G
WG
Parker
Department of Biomedical Sciences, Heritage College of Osteopathic Medicine, Ohio University, Athens, OH 45701, USA.
Lawrence M
LM
Witmer
Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, 1 University Station, C1100, Austin, TX 78712, USA.
Timothy B
TB
Rowe
Department of Biomedical Sciences, Heritage College of Osteopathic Medicine, Ohio University, Athens, OH 45701, USA.
Ryan
R
Ridgely
Vertebrate Paleontology Laboratory, Jackson School of Geosciences, The University of Texas at Austin, J. J. Pickle Research Campus, R7600, 10100 Burnet Road, Austin, TX 78758, USA; Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, 1 University Station, C1100, Austin, TX 78712, USA.
Matthew A
MA
Brown
D000818:Animals; D005075:Biological Evolution; D025061:Dinosaurs / Q000033:anatomy & histology; D005580:Fossils / Q000033:anatomy & histology*; D010802:Phylogeny; D012104:Reptiles / Q000033:anatomy & histology*
Mesozoic; Texas; Triassic; archosauromorph; computed tomography; mass extinction; morphology; morphospace; radiation