Ancient Crocodile Ancestor Evolved Bipedal Walking

More than 200 million years ago, in the forests of what is now Arizona, a small, beaked reptile was doing something remarkable — something that challenges our understanding of how evolution works, and who "invented" bipedal walking. A newly described species, Sonselasuchus cedrus, didn't just walk on two legs. It learned to do so as it grew up, starting life as a four-legged juvenile before rising onto its hind limbs in adulthood. The discovery, published in March 2026 in the Journal of Vertebrate Paleontology, adds an extraordinary chapter to the story of convergent evolution — and forces us to reconsider just how creative nature's engineering can be [1].

A Decade in the Digging

The story begins not in a laboratory, but in the sun-baked badlands of Petrified Forest National Park in northeastern Arizona. Since 2014, paleontologists from the University of Washington and Seattle's Burke Museum of Natural History and Culture have been excavating a remarkably productive fossil site in the Sonsela Member of the Upper Triassic Chinle Formation — sediments laid down roughly 215 million years ago [1][2].

The numbers are staggering. Over the course of more than a decade, the team has unearthed over 3,000 fossil bones from a single multitaxic bonebed — a concentrated deposit containing remains from multiple species. Among those bones, at least 950 belong to Sonselasuchus, representing a minimum of 36 individual animals [3][7]. The vast majority of those individuals were skeletally immature, providing researchers with an exceptionally rare window into how this animal changed as it grew.

"Since starting fieldwork at Petrified Forest in 2014, we have collected over 3,000 fossils from the Sonselasuchus bonebed — and it doesn't seem to show any signs of petering out," said Christian Sidor, a Burke Museum vertebrate paleontologist and professor at the University of Washington, in a statement accompanying the research [1].

Over 30 University of Washington students and volunteers have participated in the excavation over the years, a testament to both the site's richness and the painstaking nature of paleontological fieldwork [1].

From Four Legs to Two: An Ontogenetic Surprise

The most striking finding was not that Sonselasuchus was bipedal — other members of its family, the shuvosaurids, are known to have walked on two legs. What was truly "peculiar," as lead author Elliott Armour Smith put it, was how it became bipedal [2].

By analyzing the proportions of limb bones across specimens of different ages, Smith and Sidor discovered a pattern of differential growth — technically known as allometric scaling — between the forelimbs and hindlimbs. In juveniles, the forelimbs were approximately 75 percent the length of the hindlimbs, a ratio consistent with a quadrupedal stance. But as the animals matured, the hindlimbs lengthened and thickened disproportionately while the forelimbs grew at a slower rate. By adulthood, the forelimbs had shrunk to roughly 50 percent of hindlimb length — proportions incompatible with four-legged walking [3][7].

"Essentially, we think these creatures started out their lives on four legs… they then started walking on two legs as they grew up," Smith explained. "This is particularly peculiar" [1][2].

This kind of ontogenetic locomotor shift — where an animal fundamentally changes the way it moves during its lifetime — is exceptionally rare in the fossil record. While some dinosaur species, including certain sauropodomorphs and early ceratopsians, are thought to have undergone similar transitions, the evidence in Sonselasuchus is among the most compelling ever documented, thanks to the sheer volume of fossil material spanning multiple growth stages [3].

A Poodle-Sized Dinosaur Mimic

Standing just 25 inches tall at maturity, Sonselasuchus cedrus was roughly the size of a standard poodle [1][2]. But its proportions would have made it look nothing like a dog — and everything like a small dinosaur.

The animal possessed a suite of features that, if you encountered them in isolation, would scream "theropod dinosaur": a toothless beak, a large eye socket, hollow bones, and elongated hind limbs built for bipedal locomotion [1][3]. These are hallmark characteristics of ornithomimid dinosaurs — the "ostrich mimics" that would dominate similar ecological niches tens of millions of years later.

Yet Sonselasuchus was not a dinosaur at all. It belonged to the Shuvosauridae, a family within Pseudosuchia — the crocodile line of the archosaur family tree [4][5]. In other words, this animal was more closely related to modern crocodiles and alligators than to any bird or dinosaur.

The genus name, Sonselasuchus, honors the Sonsela Member — the geological unit where the fossils were found. The species name, cedrus, references the cedar trees that would have populated the Late Triassic forests these animals inhabited [7].

Nature's Greatest Plagiarist: Convergent Evolution

The resemblance between shuvosaurids and ornithomimid dinosaurs is so striking that early paleontologists were fooled. When Shuvosaurus inexpectatus — the family's namesake — was first described, it was initially classified as a theropod dinosaur. Only later analysis revealed it was a pseudosuchian, a member of the crocodile lineage that had independently evolved an almost identical body plan [4][5].

This is convergent evolution at its most dramatic. Two entirely separate branches of the archosaur family tree — one leading to modern crocodilians, the other to birds and dinosaurs — independently arrived at the same biomechanical solution: a lightweight, bipedal, beaked herbivore or omnivore optimized for speed and efficiency [4][5].

The implications are profound. Convergent evolution suggests that certain body plans are not accidents of history but represent optimal solutions to specific environmental challenges. The Triassic landscape — with its particular mix of predators, prey, vegetation, and climate — apparently favored this blueprint so strongly that evolution produced it twice, using two completely different sets of biological materials [3][4].

The Triassic: When Croc Cousins Ruled the Earth

To understand why a crocodile relative would evolve to look like a dinosaur, it helps to understand the world in which it lived. The Late Triassic, spanning roughly 237 to 201 million years ago, was a pivotal era in the history of life on land [6].

Early in the Triassic, the archosaurs — the group that includes both the crocodile and dinosaur lineages — split into two major branches. Pseudosuchia, the crocodile line, rose to dominance first. By the Middle and Late Triassic, pseudosuchians occupied virtually every major ecological niche on land [6]. Their ranks included massive predators like Postosuchus, heavily armored herbivores like the aetosaurs, and an array of smaller, more agile forms.

Dinosaurs, by contrast, were late arrivals to the party. The oldest confirmed dinosaur fossils date to around 230 million years ago, and for much of the Triassic, dinosaurs were relatively minor players in ecosystems dominated by their crocodile-line cousins [6].

It was in this crocodile-dominated world that shuvosaurids like Sonselasuchus thrived. As a small herbivore with a beaked, toothless mouth, it likely occupied a low-to-mid tier position in the food chain, browsing on vegetation while remaining vigilant for apex predators like Postosuchus [3].

Not the Only Bipedal Croc

Sonselasuchus is far from the only crocodile relative known to have walked on two legs. The broader group Poposauroidea, to which shuvosaurids belong, includes several bipedal forms. Poposaurus gracilis, a roughly 13-foot-long predator from the Late Triassic, has been described as a "fleet-footed, obligately erect-postured, striding biped" — essentially a large carnivore that moved like a theropod dinosaur [8][9].

Even more remarkably, bipedal crocodile relatives may have persisted far longer than the Triassic. In 2020, a team of researchers reported the discovery of trackways from the Lower Cretaceous Jinju Formation in South Korea — footprints dating to approximately 120 million years ago — that were made by large bipedal crocodylomorphs. Designated Batrachopus grandis, these trackmakers left footprints 18 to 24 centimeters long, indicating animals up to 3 meters in body length, and the consistent absence of forelimb impressions confirmed bipedal locomotion [10].

These Korean trackways were initially mistaken for pterosaur footprints, only adding to the confusion that bipedal crocodile relatives have historically caused among paleontologists [10].

Source: Published paleontological literature

Data as of Mar 9, 2026CSV

What Killed the Walking Crocs?

The end of the Triassic period, around 201 million years ago, was marked by a mass extinction event — the End-Triassic Extinction — that wiped out a vast number of species. Among the casualties were most pseudosuchian groups, including the shuvosaurids, rauisuchians, and aetosaurs [6].

The survivors on the crocodile line were mostly small, aquatic or semi-aquatic forms — the ancestors of modern crocodilians. Meanwhile, the dinosaurs, which had been minor players throughout much of the Triassic, radiated explosively into the vacated ecological niches, launching the Jurassic dynasty that would last another 135 million years.

Why dinosaurs survived and their pseudosuchian rivals did not remains one of paleontology's great questions. Some researchers suggest that dinosaurs may have had physiological advantages — perhaps more efficient respiratory systems or higher metabolic rates — that gave them an edge during the environmental upheaval of the extinction event. Others argue that it may have been largely a matter of chance [6].

The Significance of the Find

The discovery of Sonselasuchus cedrus matters for several reasons beyond the novelty of a bipedal crocodile relative.

First, the ontogenetic locomotor shift — from quadrupedal to bipedal — provides a rare glimpse into the developmental biology of extinct animals. Fossil evidence of how individual organisms changed during their lifetimes is extraordinarily difficult to obtain, requiring large sample sizes of specimens at different growth stages. The 950 Sonselasuchus bones from Petrified Forest represent exactly this kind of dataset [1][3].

Second, the find underscores the power of convergent evolution to produce startlingly similar body plans from vastly different starting materials. The fact that crocodile-line archosaurs independently evolved bipedalism, toothless beaks, hollow bones, and large eyes — the same suite of features found in certain dinosaurs — challenges simplistic narratives about which evolutionary lineages were "innovative" and which were "conservative" [4][5].

Third, the study highlights the continued importance of Petrified Forest National Park as one of the world's premier windows into Triassic ecosystems. With over 500 known vertebrate fossil sites and new discoveries emerging annually, the park's Chinle Formation continues to reshape our understanding of life in the deep past [11].

Looking Forward

The Sonselasuchus bonebed shows no signs of exhaustion. With over 3,000 fossils already recovered and more emerging with each field season, the site promises to yield further insights into Triassic ecology, pseudosuchian biology, and the developmental processes that shaped these remarkable animals.

For Smith and Sidor, the work continues. Each new field season brings the possibility of discovering additional growth stages, associated species, or anatomical details that could further illuminate how a small, beaked reptile learned to stand up and walk — 200 million years before we did.