Earth Science Student Research Grants Recipients 2006

The early history of the archosaurs: the search for the common relatives of crocodylians and birds

Sterling Nesbitt

Extant birds and crocodiles (archosaurs) represent two body form extremes in a long, complex evolutionary history or archosaurs. Birds and crocodiles split into the bird-line archosaurs (dinosaurs and birds) and the crocodile-line archosaurs (crocodylians and their relatives, e.g., aetosaurs, rauisuchians) by at least 240 million years ago (Early to Middle Triassic)(Gower and Sennikov 2000; Nesbitt 2003). Almost immediately the archosaurs radiated into many morphologically distinct groups that dominated nearly all terrestrial ecosystems worldwide. However, we understand little of the early history, prior to and just after the split between birds and crocodylians. Unfortunately, many of the early archosaur lineages and ghost lineages stretch into the Early Triassic, a time when few early archosaurs have been discovered. The early record of the archosaur radiation is poorly understood for the following reasons 1) few areas preserve Early and Middle Triassic rocks 2) early archosaur remains remain rare in those areas and 3) later Triassic taxa are very derived and preserve few characters that elucidate their relationships. Only the Moenkopi Formation in the southwestern United States, a series of small formations in western Russia, areas in southern Brazil, and the Manda Formation in Tanzania preserve Triassic rocks that contain early archosaurs. Recovery of early archosaur specimens will help better understand character evolution, resolve problematic archosaur relationships, and answer questions about the biogeography history of archosaurs. Here, I propose that new fieldwork is needed to recover early archosaurs and the best place to collect early archosaurs is from the Manda Formation of Tanzania.

The Manda Formation preserves abundant remains archosaurs (Haughton 1932) with partial specimens, good preservation, and a diversity of early archosaur forms. In the 1930's and again in the 1960's expeditions from Germany and the British Museum of Natural History (Attridge et al. 1964) collected many partial specimens of early archosaurs in a rapid period of time. However, most taxa have been named, but none have been fully described in the literature with the exception of Stagonosuchus. These early archosaurs include "Mandasuchus," a small, early rauisuchian, Stagonosuchus, a large early rauisuchian, "Hypselorhachis" a sail-backed archosaur, "Teleocrater," a small, gracile archosaur, and "Parringtonia" an archosaur with unknown affinities. All of the taxa are early members of the Archosauria illustrate the rapid diversification of the archosaurs. The abundance and diversity of archosaurs from the Manda Formation shows great promise for collecting further specimens.