Rediscovered Specimen Draws Dinosaur Origins Down Into the Middle Triassic

I've been peripherally involved in the recent renaissance regarding dinosaur origins since my discovery of the skeleton of Revueltosaurus callenderi in 2004 and the subsequent recognition that it was not a dinosaur. With my good friends and colleagues Sterling Nesbitt and Randall Irmis, I proceeded to reexamine much of the Late Triassic dinosaur record using an apomorphy-based approach emphasized by UT Austin professor Chris Bell and his colleagues for Neogene vertebrates. Our findings were that Triassic dinosaurs were actually rarer than previously believed, especially in North America. In addition, extensive fieldwork by Sterling and Randy led to the discoveries of Dromomeron romeri, Tawa hallae, and Asilisaurus kongwe, filling in some key gaps in our understanding of the early diversification of the Ornithodira.  Nonetheless it has become readily apparent that many important specimens crucial to this issue were not weathering out of outcrops in the field, but rather were sitting un- or misidentified in museum collections around the globe. Our new understanding of character states and polarities for early diverging dinosauriforms provided us with specific search criteria leading to the discoveries and/or reinterpretations of taxa such as Eucoelophysis baldwini, Dromomeron gregorii, Technosaurus smalli, Daemonosaurus chauliodus, and of course Effigia okeeffae, all from previously collected material including fossils collected for Edward Cope in the 1800s. Furthermore, Randy's work found that the rise of dinosaurs was diachronous, although the timing is still poorly understood.

The discovery of the silesaurid Asilisaurus kongwe (published in 2010) pulled the split between Silesauridae and Dinosauria into the Middle Triassic creating a significant ghost lineage for Dinosauria as the earliest known bona fide dinosaurs do not appear until the end of the Carnian stage of the Late Triassic. Amazingly it appears that we did not need to wait very long for this ghost lineage to be filled.

Today's issue of Biology Letters has a paper by Sterling Nesbitt, Paul Barrett, Sarah Werning, Christian Sidor, and the late Alan Charig on a probable new dinosaur from the Middle Triassic of Tanzania. Even more amazing is that this is not a new specimen, but was actually collected in the 1930s, and never described, except in Charig's 1950s dissertation, until today. Charig named the new specimen Nyasasaurus parringtoni and until now this name has been a nomen nudum.

Known from a partial humerus and several vertebrate (three cervical, five presacral and three sacral), reanalysis places Nyasasaurus as either a dinosaur or as the sister taxon to Dinosauria. The humerus bears a ventrally elongated deltopectoral crest with a deflected apex, both synapomorphies of Dinosauria. The cervical vertebrae are elongate and possess deep lateral fossae, consistent with character states found in dinosaurs. The presence of three sacral vertebrae, although not restricted to Dinosauria, also supports this placement. This interpretation is supported not only by a phylogenetic analysis but also by bone histology, which shows high, continuous growth rates similar to that of early diverging dinosaurs. 

This material suggests that dinosaurs probably appeared in the fossil record 10 to 15 million years earlier than we expected. Furthermore, Nesbitt et al. argue that this strongly supports the hypothesis that dinosaurs were not a dominant group during their early history. Finally, Nyasasaurus also provides more support for a Gondwanan origin of dinosaurs.

One thing that is now definitely clear is that workers interested in dinosaur origins will need to spend more time in Middle Triassic terrestrial units. Back in 2004 I don't think any of us fathomed what discoveries and interpretations the next decade would bring. As I often state, it is not necessarily what we already know that dirves our work, but what is still out there for us to learn.

Nesbitt, S. J., Barrett, P. M., Werning, S., Sidor, C. A., and A. J. Charig. 2012. The oldest dinosaur? A Middle Triassic dinosauriform from Tanzania. Biology Letters.
 

Abstract - The rise of dinosaurs was a major event in vertebrate history, but the timing of the origin and early diversification of the group remain poorly constrained. Here, we describe Nyasasaurus parringtoni gen. et sp. nov., which is identified as either the earliest known member of, or the sister-taxon to, Dinosauria. Nyasasaurus possesses a unique combination of dinosaur character states and an elevated growth rate similar to that of definitive early dinosaurs. It demonstrates that the initial dinosaur radiation occurred over a longer timescale than previously thought (possibly 15 My earlier), and that dinosaurs and their immediate relatives are better understood as part of a larger Middle Triassic archosauriform radiation. The African provenance of Nyasasaurus supports a southern Pangaean origin for Dinosauria.