Abstract - The early evolutionary history of Ornithodira (avian-line archosaurs) has hitherto been documented by incomplete (Lagerpeton) or unusually specialized forms (pterosaurs and Silesaurus). Recently, a variety of Silesaurus-like taxa have been reported from the Triassic Period of both Gondwana and Laurasia, but their relationships to each other and to dinosaurs remain a subject of debate. Here we report on a new avian-line archosaur from the early Middle Triassic (Anisian) of Tanzania. Phylogenetic analysis places Asilisaurus kongwe gen. et sp. nov. as an avian-line archosaur and a member of the Silesauridae, which is here considered the sister taxon to Dinosauria. Silesaurids were diverse and had a wide distribution by the Late Triassic, with a novel ornithodiran bauplan including leaf-shaped teeth, a beak-like lower jaw, long, gracile limbs, and a quadrupedal stance. Our analysis suggests that the dentition and diet of silesaurids, ornithischians and sauropodomorphs evolved independently from a plesiomorphic carnivorous form. As the oldest avian-line archosaur, Asilisaurus demonstrates the antiquity of both Ornithodira and the dinosaurian lineage. The initial diversification of Archosauria, previously documented by crocodilian-line archosaurs in the Anisian, can now be shown to include a contemporaneous avian-line radiation. The unparalleled taxonomic diversity of the Manda archosaur assemblage indicates that archosaur diversification was well underway by the Middle Triassic or earlier.
Prior to 2003 the non-dinosaurian dinosauriforms known as silesaurids were unrecognized in the fossil record. Specimens existed in collections, collected as early as the 1930s, while others were given tentative identifications (e.g., the ornithosuchid of Long and Murry, 1995 and the ornithischian Technosaurus). Dzik (2003) described the first, Silesaurus opolensis from the Carnian of Poland, with its very distinctive femoral and morphologies. Suddenly similar forms were recognized from all over the globe (e.g., Eucoelophysis, Sacisaurus; Ezcurra 2006; Nesbitt et al. 2007; Irmis et al. 2007a), whereas new specimens were being discovered from the Chinle Formation of New Mexico and Arizona (Parker et al. 2006; Irmis et al. 2007b).
Still, because the earliest pseudosuchian archosaurs were known from the Anisian (e.g., the Moenkopi Formation of Arizona), whereas the earliest ornithodirans were from the Ladinian of Argentina, there was a proposed ghost lineage for Ornithodira existing back into the Anisian.
Sterling Nesbitt looks over the Ruhuhu Valley in 2007. Photo by L. Tsuji.
One of the strengths of phylogentic analysis is the ability to make predictions as to where in time and space certain groups should and could be found. Sterling Nesbitt clearly recognized the strong possibility that the earliest representatives of ornithodira could be found in the Manda beds of Tanzania as this fauna was known for its pseudosuchian constituents. A few years back this prediction paid off, as a team lead by Christian Sidor in 2007 to explore the Permian and Triassic rocks of Tanzanian, uncovered an amazing deposit of early archosaurs including numerous specimens of a silesaurid. Named Asilisaurus kongwe (Ancient ancestor lizard) these specimens represent the earliest known member of the lineage leading to dinosaurs and strongly supports the diversification of the Archosauria by the early Middle Triassic (~243 million years ago).
The tibia of Asilisaurus, following excavation in 2007. Photo by R. Smith.
This find also strongly suggests that adaptations for an omnivorous or herbivorous diet evolved independently in silesaurids, ornithischians, and sauropodomorphs, from carnivorous ancestors. The paper also phylogenetically defines the Silesauridae and proposes that the South American archosaurs Lewisuchus and Pseudlagosuchus are members of this clade and probably also synonymous. Another early appearance of a non-dinosaurian dinosauriform in
Gondwana also provides further support for a southern origin for this group.
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Irmis, R. B., Nesbitt, S. J., Padian, K., Smith, N. D., Turner, A. H., Woody, D., and A. Downs. 2007a. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science 317:358-361.
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