Peculiar Crocodile Ancestor Transitioned from Four-Legged to Bipedal Walking

When we picture crocodiles, we see armored predators dragging themselves along riverbanks on four stocky legs. The idea of a crocodile relative striding upright on two legs — like a miniature dinosaur — sounds like the plot of a B-movie. But a remarkable new fossil discovery from Arizona's Petrified Forest National Park is forcing scientists to reconsider just how creative evolution can be on the crocodile branch of the family tree.

A team of paleontologists from the University of Washington and the Burke Museum of Natural History and Culture has described Sonselasuchus cedrus, a 25-inch-tall reptile that lived approximately 215 million years ago during the Late Triassic period. What makes this creature truly "peculiar," as the researchers themselves describe it, is not just that it walked on two legs — but that it apparently learned to do so as it grew up, beginning life as a four-legged juvenile before graduating to bipedal locomotion in adulthood [1][2].

The findings, published in the Journal of Vertebrate Paleontology, represent one of the most compelling cases of ontogenetic locomotor transition — a shift in the way an animal moves during its growth — ever documented in the fossil record [3].

The Discovery: 3,000 Bones and Counting

The story of Sonselasuchus cedrus begins in 2014, when excavations led by Professor Christian Sidor of the University of Washington began at a site known as Kaye Quarry, nestled within the Sonsela Member of the Chinle Formation at Petrified Forest National Park in Apache County, Arizona [1][4].

What they found was extraordinary. The bonebed has yielded more than 3,000 fossilized bones belonging to shuvosaurids, with over 950 of those bones belonging specifically to Sonselasuchus cedrus, representing a minimum of 36 individual animals [2][5]. The site has also produced fossils of fish, amphibians, dinosaurs, and other reptiles, painting a rich portrait of a Late Triassic ecosystem deposited by an ancient river system roughly 215 million years ago [1].

"The bonebed doesn't seem to show any signs of petering out," Sidor noted, with more than 30 University of Washington students and volunteers participating in the ongoing excavation over the past decade [6]. The sheer volume of fossils has given the research team an unusual luxury in paleontology: enough specimens across different growth stages to track how an animal's body changed as it matured.

A Body Built for Confusion

At first glance, Sonselasuchus cedrus would have been easy to mistake for a dinosaur. The animal possessed a toothless beak, large eye sockets, and hollow bones — features we typically associate with theropod dinosaurs, particularly the ostrich-like ornithomimids that would appear tens of millions of years later during the Cretaceous period [2][5].

But Sonselasuchus was no dinosaur. It belonged to the Shuvosauridae, a family within the Pseudosuchia — the "crocodile-line" of archosaurs. In the great split of the archosaur family tree, one branch led to birds and dinosaurs (the Avemetatarsalia), while the other led to crocodilians and their relatives (the Pseudosuchia). Sonselasuchus sits firmly on the crocodile side [3][7].

"This animal belongs to a group called the Shuvosauridae, a group of crocodile-line archosaurs that have a number of convergently evolved features with dinosaurs," explained Elliott Armour Smith, a University of Washington graduate student and lead author of the study [4]. The resemblance to ornithomimid dinosaurs is a textbook case of convergent evolution — two unrelated lineages independently arriving at similar body plans because they faced similar ecological pressures.

The species name cedrus references cedar trees, the evergreen conifers that were characteristic of the Late Triassic forests where the animal lived [5]. At roughly 25 inches (63.5 cm) tall, it was about the size of a standard poodle — a far cry from the massive crocodilians we know today [1][2].

From Four Legs to Two: An Unprecedented Developmental Shift

The most striking finding was buried in the mathematics of bone proportions. By analyzing skeletal measurements across dozens of individuals at different growth stages, the research team identified a pattern that, as Armour Smith put it, is "particularly peculiar" [1].

Young Sonselasuchus had relatively proportional forelimbs and hindlimbs — anatomy consistent with walking on all fours. But as the animals matured, their hindlimbs grew significantly longer and more robust while their forelimbs failed to keep pace, a pattern scientists call negative allometry of the forelimb relative to the hindlimb [3][4].

"We think that Sonselasuchus had more proportional forelimbs and hindlimbs as young, and their hindlimb grew longer and more robust through adulthood," Armour Smith explained. "Essentially, we think these creatures started out their lives on four legs... they then started walking on two legs as they grew up" [1][2].

This ontogenetic shift — a change in locomotion tied to growth and development rather than evolutionary time — is exceptionally rare in the fossil record. While some living animals show locomotor changes during development (tadpoles swim, frogs hop), finding clear skeletal evidence of a quadruped-to-biped transition within a single species' lifetime is remarkable.

Not the First Bipedal Croc — But Perhaps the Strangest

Sonselasuchus is not the only example of bipedalism on the crocodile line. The Triassic was, in many ways, the golden age of pseudosuchian diversity. More than 700 pseudosuchian species are known from their 250-million-year fossil record, compared to fewer than 30 living crocodilian species today [8]. During the Late Triassic, pseudosuchians filled ecological roles that would later be dominated by dinosaurs — from armored herbivores (aetosaurs) to massive apex predators (rauisuchians) to the bizarre, ostrich-mimicking shuvosaurids [8][9].

Sonselasuchus joins a family that already included two other known shuvosaurids: Shuvosaurus inexpectatus, discovered in Texas, and Effigia okeeffeae, found in New Mexico. Both were bipedal, beaked, and so strikingly similar to ornithomimid dinosaurs that Shuvosaurus was initially misidentified as a theropod when first described [7][10]. The convergence is so extreme that it represents what paleontologists call a case of "extreme" convergent evolution — two entirely separate evolutionary lineages independently developing bipedalism, toothless beaks, hollow bones, and large orbits [10].

Beyond the shuvosaurids, other crocodile-line archosaurs also experimented with bipedalism. Poposaurus gracilis, another poposauroid, is well-established as a bipedal predator [9]. And in 2020, a team of researchers described Batrachopus grandis, a set of fossilized trackways from the Lower Cretaceous of South Korea — roughly 110 to 120 million years ago — that appeared to show large crocodylomorphs (animals up to 3 meters long) walking exclusively on their hind legs, with no forelimb prints or tail-drag marks visible [11].

But what sets Sonselasuchus apart is the developmental dimension. The other known bipedal pseudosuchians appear to have been bipedal throughout their lives. Sonselasuchus, by contrast, appears to have undergone a transition — starting quadrupedal and becoming bipedal — within a single individual's lifetime. This ontogenetic shift has no clear parallel among known archosaurs.

The Late Triassic: A World of Evolutionary Experiments

To understand why a poodle-sized crocodile relative was running around on two legs in what is now Arizona, it helps to understand the world it inhabited. The Late Triassic, spanning roughly 235 to 201 million years ago, was a period of extraordinary ecological experimentation [8][9].

The supercontinent Pangaea was beginning to break apart. The climate was warm and largely arid, punctuated by seasonal monsoons. Arizona's Petrified Forest preserves a snapshot of a Triassic river system lined with towering conifers and populated by an astonishing menagerie of creatures [12].

The Chinle Formation, where Sonselasuchus was found, is considered one of the richest Late Triassic fossil deposits in the world, containing more than 200 fossil plant taxa and a staggering diversity of vertebrates [12]. Recent discoveries from the same park include North America's oldest known pterosaur (Eotephradactylus mcintireae), an early mammal relative the size of a chipmunk (Kataigidodon venetus), and a tiny lizard with grooved teeth suggestive of venom delivery (Microzemiotes sonselaensis) [13].

In this world, dinosaurs existed but had not yet risen to dominance. Pseudosuchians — the crocodile-line archosaurs — were the ecological heavyweights. They occupied niches from top predator to mid-level herbivore, and their diversity dwarfed that of the early dinosaurs sharing their landscape. The shuvosaurids, with their dinosaur-like body plans, were part of this grand pseudosuchian experiment [8][9].

The experiment ended catastrophically. The Triassic-Jurassic extinction event, approximately 201 million years ago, wiped out most pseudosuchian lineages. Only the crocodylomorphs — a single branch that would eventually give rise to modern crocodilians — survived the bottleneck. The ecological vacancies left behind were filled by dinosaurs, which then dominated terrestrial ecosystems for the next 135 million years [8].

What This Tells Us About Evolution

The discovery of Sonselasuchus cedrus carries implications that extend well beyond a single quirky reptile. It challenges the common perception of crocodilians as evolutionary holdovers — "living fossils" that have barely changed in hundreds of millions of years.

"This challenges the 'living fossil' characterization of modern crocodiles, revealing their ancestors possessed remarkable dietary and ecological diversity," researchers noted, pointing out that ancient crocodile-line animals filled marine, terrestrial, and herbivorous niches alongside their more familiar carnivorous roles [4].

The ontogenetic locomotor shift is also significant for understanding how bipedalism evolves more broadly. In the dinosaur-bird lineage, bipedalism appears to have been an ancestral trait that was maintained and refined. But in the crocodile lineage, bipedalism arose independently — possibly multiple times — and through a different developmental pathway. The fact that Sonselasuchus achieved bipedalism through differential growth rates rather than a wholesale skeletal redesign suggests that the genetic and developmental toolkit for building a biped may be more widely shared among archosaurs than previously recognized [3][9].

There is also the question of why this transition occurred. The researchers have not yet proposed a definitive ecological explanation, but possibilities abound. Bipedalism frees the forelimbs for other functions, potentially improving foraging efficiency. It can also increase an animal's field of vision and allow for faster sprint speeds — advantages for both predators and prey in a competitive Triassic ecosystem.

An Ongoing Excavation, An Evolving Story

The Kaye Quarry bonebed at Petrified Forest National Park continues to produce new material. With more than a decade of excavation behind them and no signs of the fossil supply diminishing, the University of Washington team expects future discoveries to further refine the picture of Sonselasuchus and its world [6].

Additional specimens across a wider range of growth stages could help confirm the quadruped-to-biped transition hypothesis and potentially reveal other developmental changes — shifts in diet, habitat use, or social behavior — that accompanied the locomotor switch. The bonebed's concentration of dozens of individuals also raises questions about the animal's social behavior: did Sonselasuchus live in herds or family groups?

For now, the poodle-sized, beak-faced, two-legged crocodile cousin from the age of dinosaurs stands as a vivid reminder that evolution does not follow a script. The crocodile lineage, far from being a dead-end populated by unchanged relics, was once a laboratory of biological innovation — one that, 215 million years later, is still yielding surprises from the ancient soils of Arizona.

Source: Compiled from Stubbs et al. 2021 & Pseudosuchia literature

Data as of Mar 9, 2026CSV