The two major lineages of extant archosaurs, crocodylians and avians, diverged in the Triassic, but the details and timing of this event are incompletely understood. It is hard to identify early archosaurs because of fragmentary and phylogenetically uninformative specimens, and poor temporal constraints on rock units from the Early and Middle Triassic. One exception, Xilousuchus sapingensis unearthed in the 1970s from the Heshanggou Formation of the Ordos Basin of China (late Early Triassic), stands alone as one of the most complete archosauriforms. It was originally thought to be a distant relative of both birds and crocodiles, turns out to have come from the crocodile family tree after it had already split from the bird family tree, according to research published May 17 online in Earth and Environmental Science Transactions of the Royal Society of Edinburgh 101 (1–14), 2011.

Paleontologists from Columbia University and Institute of Vertebrate Paleontolgy and Paleoanthopology, Chinese Academy of Sciences re-prepared and re-describes the partial skeleton of the only known specimen of Xilousuchus sapingensis. The new examination posits X. sapingensis as a crown-group archosaur within Suchia, thus making this taxon the unequivocally oldest known member of Archosauria. Archosaurs, characterized by skulls with long, narrow snouts and teeth set in sockets, include dinosaurs as well as crocodiles and birds.

The researchers examined bones from the specimen in detail, comparing them to those from the closest relatives of archosaurs, and discovered that X. sapingensis differed from virtually every archosauriform. Among their findings was that bones at the tip of the jaw that bear the teeth likely were not downturned as much as originally thought when the specimen was first described in the 1980s. They also found that neural spines of the neck formed the forward part of a sail similar to that found on another ancient archosaur called Arizonasaurus, a very close relative of Xilousuchus found in Arizona.

“The family trees of birds and crocodiles meet somewhere in the early Triassic and archosauriforms are the closest cousin to those archosaurs”, said Sterling J. Nesbitt, lead author, currently a postdoctoral researcher at the University of Texas at Austin, “But the new research places X. sapingensis firmly within the archosaur family tree, providing evidence that the early members of the crocodile and bird family trees evolved earlier than previously thought.”

“The age and phylogenetic position of X. sapingensis indicate that many archosaurs, including all major clades of non-archosaurian archosauriforms, the avianline, ornithosuchids, aetosaurs and paracrocodylomorph lineages, must have diverged by the end of the Early Triassic. X. sapingensis is part of a possible clade of sail-backed poposauroids that were common components of archosaur assemblages during the Early to Middle Triassic”, said Dr. LIU Jun, co-author, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences.

The research was funded by the National Science Foundation, the Society of Vertebrate Paleontology, the American Museum of Natural History and the Chinese Academy of Sciences.

Fig.1: This drawing depicts the skull and 10 neck vertebrae of Xilousuchus sapingensis. The elongated spines on rearward vertebrae indicate the presence of a "sail" on the animal's back. (Credit: Sterling Nesbitt)

Fig.2: Reconstruction of Xilousuchus sapingensis, based on the fossil. (Credit: Sterling Nesbitt)