Out in eastern Utah, where the rock is laid out in immense swaths of orange and red, there’s a very special impression. The fossil can be a little hard to find, hidden beneath an overhang, but, should you be lucky enough to spot it, there’s no mistaking what it is. Preserved in the ancient sandstone is the triangular outline of a phytosaur skull – a natural mold left by fossil bones from this superficially crocodilesque creature that lived over 200 million years ago.

Paleontologists Michael Morales and Sidney Ash called this fossil The Last Phytosaur. In terms of geological layers, the fossil sits above most other finds of its ilk, pressed into the Wingate Sandstone which – somewhere in its depths – contains the transition between the Triassic and Jurassic world. Whether or not this fossil truly represents the last of these crocodile-like reptiles isn't known. We’d need a time machine to check that. But it stands in as a bookend for a lineage of diverse and widespread aquatic ambush predators that made their living among the Triassic waterways. 

The ultimate phytosaurs, like the Last Phytosaur itself, resembled modern crocodylians in many ways. They were pioneering a life lurking in the shallows while the ancestors of today’s crocodiles were still skittering around on land. But what were the first phytosaurs like? A recent find in China helps fill in the backstory of this oft-neglected group of reptiles.

Back in 2012 paleontologist Chun Li and colleagues named Diandongosuchus fuyuanensis from the Middle Triassic rock of China. The initial analysis of the fossil concluded that this short-snouted, sharp-toothed animal was a poposaur, on a branch within the wider crocodile family. But a new study of the fossil by paleontologist Michelle Stocker, Li, and coauthors has come to a different conclusion. Diandongosuchus is now the earliest known phytosaur.

phytosaur
The skeleton of Diadongosuchus. Credit: Stocker et al. 2017

Up until now, the oldest phytosaurs were about 228 million years old. Diandongosuchus now extends the family’s record back about 10 million years fruther, highlighting just how much these reptiles changed in that interval. 

The fact that Diandongosuchus was initially mistaken for a very different variety of reptile highlights the fact that the earliest phytosaurs didn’t look exactly like their more famous later relatives. The skull of Diandongosuchus is wedge shaped, with a short snout and nasal openings only slightly retracted from the tip of the nose. Still, Stocker and colleagues write, the skull and postcrania of this well-preserved reptile show anatomical hallmarks present among phytosaurs but lacking in early crocodiles. 

Other phytosaur fossils will hopefully fill in the millions of years between Diandongosuchus and the next phytosaurs. For now, though, this revised reptile shows that the postcranial skeletons of phytosaurs evolved some of their key characteristics before their skulls evolved into more elongate, gharial-like forms. Diandongosuchus also shows some specializations of the skull related to increased bite strength, the researchers point out, which may have helped open up the possibility for phytosaurs to become the widespread ambush predators they were. The sharp snout of Diandongosuchus not only points to the successful future of its kind, but how much more there is left to uncover about the dawn of the phytosaurs.

References:

Morales, M., Ash, S. 1993. The last phytosaurs? New Mexico Museum of Natural History and Science Bulletin. 3: 357-358

Stocker, M., Zhao, L., Nesbitt, S., Wu, X., Li, C. 2017. A short-snouted, Middle Triassic phytosaur and its implications for the morphological evolution and biogeography of Phytosauria. Scientific Reports. doi: 10.1038/srep46028