Parsing the origins and early history of dinosaurs is a challenging task. A number of prehistoric creatures were a lot like some of the earliest dinosaurs, and sometimes evolutionary cousins of early dinosaurs have been mistaken for dinosaurs on the basis of fragmentary material. As a study published in
Described by scientists John Flynn, Sterling Nesbitt, Michael Parrish, Lovasoa Ranivoharimanana and Andre Wyss, a complete skull of a new species of this enigmatic creature confirms that it is not a dinosaur. Instead, this creature, which lived between approximately 237 and 216 million years ago and is named Azendohsaurus madagaskarensis, was an archosauromorph—a member of a diverse group in which the dinosaur family tree is nested along with other creatures—which had independently evolved some of the features seen in the precursors of the immense sauropod dinosaurs and early ornithischian dinosaurs. This is shown most prominently by its teeth. They are leaf-shaped and are marked by a series of notches well-suited to chopping up leaves, meaning that this type of tooth evolved in several lineages of creatures which had been diverging from each other for millions of years.
This reassessment of Azendohsaurus has important implications for how paleontologists identify early dinosaurs. Previously the peculiar tooth type seen in this creature was thought to be an identifying feature of some early dinosaur types, but if it evolved more than once then isolated teeth and bits of jaw can no longer be taken as those of early dinosaur without further evidence. By being better able to diagnose fragments from the Middle and Late Triassic sites that harbor early dinosaurs, their ancestors and the disparate creatures they lived alongside, paleontologists will better able to understand the evolution of dinosaurs.
FLYNN, J., NESBITT, S., MICHAEL PARRISH, J., RANIVOHARIMANANA, L., & WYSS, A. (2010). A new species of Azendohsaurus (Diapsida: Archosauromorpha) from the Triassic Isalo Group of southwestern Madagascar: cranium and mandible Palaeontology, 53 (3), 669-688 DOI: 10.1111/j.1475-4983.2010.00954.x