Meet Sonselasuchus: The Toothless Crocodile Cousin That Looked Like An Ostrich

Newly-Named Prehistoric Reptile Roamed Ancient Arizona 215 Million Years Ago

A creature that looked like an ostrich and walked upright on two legs but was actually a close relative of crocodiles just got a name: Sonselasuchus cedrus. Found at Petrified Forest National Park in Arizona, this 215-million-year-old animal is shaking up what scientists know about one of the strangest groups of reptiles that ever lived. Even more surprising, its bones suggest younger animals may have relied more on all four legs before becoming more hind-limb-dominant as they grew.

Paleontologists Elliott Armour Smith and Christian A. Sidor of the University of Washington’s Department of Biology and Burke Museum of Natural History and Culture described the new species in the Journal of Vertebrate Paleontology. Their work draws on more than 950 fossil bones representing at least 36 individuals, the largest collection of its kind from any single site in the world. The animal belonged to a group called shuvosaurids, ancient relatives of crocodiles that independently evolved a body shape eerily similar to the ornithomimids, the famous “ostrich mimic” dinosaurs of the Late Cretaceous. Like those dinosaurs, shuvosaurids were toothless, had large eye sockets, and walked upright. But they weren’t dinosaurs at all.

A Massive Fossil Haul From Ancient Arizona

The fossils come from a single dig site called the Kaye Quarry, located within the rock layers of Petrified Forest National Park. The site preserves a chaotic jumble of nearly all separated bones, meaning the skeletons had been broken apart before burial, from multiple species of ancient animals. Shuvosaurid bones make up roughly 33% of everything recovered from the quarry, an extraordinary concentration.

Dating methods place the site at approximately 215 million years old, during the Late Triassic period, when the supercontinent Pangaea was still largely intact and Arizona looked nothing like it does today. The surrounding rock layers record a period of increasing dryness, with the region shifting from wetter conditions to a more arid environment. The fossils were found in sediments that scientists believe were deposited when a river channel overflowed its banks onto a nearby floodplain, which helps explain why the bones are so thoroughly scattered yet well preserved.

Sharing this ancient ecosystem with Sonselasuchus were large amphibians, early dinosaurs, and a variety of other reptiles, including the armored aquatic reptile Vancleavea campi and early relatives of pterosaurs. The diversity of animals preserved at this single site paints a vivid picture of a busy ecosystem in the deep past.

What Made This Toothless Reptile Unique

Smith and Sidor identified the new species primarily from features of the upper jaw. The main bone of the upper jaw behind the nostril had a reduced and extremely thin front portion, creating a deeply scooped-out rear margin of the nostril opening. An enlarged opening between the nostril and the jaw was bigger than in its relatives. The back end of the upper jaw bone was flattened and paddle-shaped, a feature unlike anything seen in other members of the group.

Like its relatives, Sonselasuchus was completely toothless. Instead of teeth, the edges of its jaws formed sharp cutting ridges, similar to the beak-like structures found in turtles and some dinosaurs. Dense clusters of tiny holes on the jaw surfaces, traces of blood vessels and nerves, suggest the living animal had a horn-like covering over its jaws, much like a bird’s beak. This beak may have been used for eating soft vegetation, consistent with earlier ideas that shuvosaurids may have fed on soft vegetation.

Beyond the skull, much of the rest of the skeleton was broadly similar to what has been described for other shuvosaurids, though subtle differences appeared throughout. The researchers described bones from nearly every region of the body, from the neck down to the toe claws, providing the most detailed look at shuvosaurid anatomy from any single species.

How Sonselasuchus May Have Learned to Walk on Two Legs

One of the most interesting parts of the study is the analysis of how body proportions changed as Sonselasuchus grew. With thigh bones ranging from 7.5 to 17.8 centimeters in length across dozens of individuals, the researchers had an unusually large sample to work with. Most of the individuals appear to have been skeletally immature, meaning they hadn’t finished growing.

By comparing the dimensions of front-leg bones to hind-leg bones across this range of body sizes, Smith and Sidor found that as the animals got bigger, their front legs became proportionally smaller compared to their back legs. This pattern is consistent with the idea that their bones suggest younger animals may have relied more on all four legs before becoming more hind-limb-dominant as they grew.

Walking on two legs is one of the features that made shuvosaurids look so eerily similar to later ornithomimid dinosaurs. Showing that this trait may have developed gradually during growth, rather than being present from hatching, adds a new dimension to how these animals lived and how their body plan came to be.

Filling a Gap in Time and Geography

Before this discovery, only three named shuvosaurid species existed in the scientific literature. Shuvosaurus inexpectatus comes from western Texas and is roughly 219 to 220 million years old. Effigia okeeffeae hails from Ghost Ranch in northern New Mexico and is younger, at about 204 to 208 million years old. Sillosuchus longicervix is known from Argentina but lacks skull material, making detailed comparisons difficult. At approximately 215 million years old, Sonselasuchus cedrus slots neatly between the Texas and New Mexico species in both time and geography.

A family-tree analysis placed Sonselasuchus in a group with Effigia and Shuvosaurus, though the exact branching order among the three couldn’t be fully worked out, likely because different bones are preserved for different species, making direct comparisons incomplete in some areas. The researchers noted that the oldest known shuvosaurid fossils come from the Middle Triassic, including discoveries in Zambia, indicating the group had a long evolutionary history stretching across much of the ancient supercontinent.

With 36 or more individuals of varying sizes from a single site, Sonselasuchus cedrus offers a rare window into the life history of a single species, something almost unheard of for animals this ancient. The sheer volume of material will likely fuel research for years to come, helping scientists understand not just shuvosaurids but the broader story of how crocodile-line reptiles once rivaled dinosaurs in ecological diversity before their dramatic decline. For an animal that looked like an ostrich but was built on a crocodile blueprint, that story remains one of evolution’s most surprising plot twists.

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