The Jurassic’s Housecat Croc

At long last, Fruitachampsa lives. Sort of. This strange crocodyliform has been extinct for around 150 million years. But, after three decades of waiting, this short-snouted croc has finally been officially named.

The new paper that describes Fruitachampsa callisoni calls the animal “A new shartegosuchid crocodyliform from the Upper Jurassic Morrison Formation of western Colorado.” That’s “new” in a relative sense. Between 1975 and 1979, George Callison and James Clark discovered remains from multiple individuals of this croc near Grand Junction, Colorado. This animal was not like the alligators, caimans, gharials, and crocodiles we know today. (In technical terms, all those living lineages are crocodylians – a remaining portion of the larger and more varied group called Crocodyliformes to which Fruitachampsa also belonged.) Informally referred to as the “Fruita Form” in publications for years, this roughly three-foot-long archosaur had slender legs, a short skull, and rows of flat teeth with wrinkled, horizontal cusps socketed behind a small set of pointed teeth at the front of the jaws. As Jurassic expert John Foster dubbed the animal in his book Jurassic West, Fruitachampsa was “the house cat of the Morrison Formation.”

Yet the long wait for the description of the Fruita croc carried an advantage. Around the time of the animal’s discovery, there was nothing quite like Fruitachampsa. How the animal related to other crocodyliforms was unclear. Since 1975, however, additional discoveries of previously-unknown crocs have put Fruitachampsa in context. These discoveries have not been made elsewhere in the fossil-rich deserts of the American west. The closest relatives of Fruitachampsa – called shartegosuchids – have been found in the Mesozoic sediments of Mongolia, China, and Siberia.

As Clark explains in the new Zoological Journal of the Linnean Society paper, Fruitachampsa appears to be most closely related to Shartegosuchus and Adzhosuchus, both of which lived during the Late Jurassic in what is now southwestern Mongolia. All these animals are united by subtle skull features, such as the absence of a hole in the lower jaw called the mandibular fenestra and the flat, wrinkle-cusped teeth set along the jaws.

On the basis of these relationships, Clark suggests that Fruitachampsa was a North American lineage of shartegosuchid whose ancestors dispersed from prehistoric Asia. How shartegosuchids relate to other crocs, however, is difficult to discern. The group appears to be an archaic branch of the crocodyliform family tree, although relatively closer to the disparate subgroup which contains crocs of modern aspect and their closest relatives – the Mesoeucrocodylia – than to other crocodyliforms – such as Protosuchus – that have generally been thought to be part of an early, primitive grade of croc evolution. There was no single, straight line of early crocodyliform evolution, and paleontologists are still working to untangle the proper placement of all the primeval branches.

But how did Fruitachampsa make a living? This Jurassic croc clearly was not an aquatic ambush predator. Fruitachampsa was a relatively slender animal adapted for a life on the land, but the more precise details of what it ate and how it behaved are unknown. Especially perplexing are those flat-topped teeth which Fruitachampsa shares with other shartegosuchids – what kind of food could such a tooth be suited to? Lizards, mammals, and even baby dinosaurs might have been suitable prey, but no one knows for sure. Hopefully, now that Fruitachampsa has a name, paleontologists can begin studying the natural history of the Jurassic’s housecat croc.

Top Image: A restoration of Fruitachampsa at the Arizona Museum of Natural History. Photo by the author.

References:

CLARK, J. (2011). A new shartegosuchid crocodyliform from the Upper Jurassic Morrison Formation of western Colorado Zoological Journal of the Linnean Society, 163 DOI: 10.1111/j.1096-3642.2011.00719.x