Rosa Rubicondior: Refuting Creationism - A Relative Of Crocodiles That Only Walked on Two Legs As An Adult

Artist's reconstruction of Sonselasuchus cedrus in its environment in what is now Petrified Forest National Park, 215 million years ago.

Artwork by Gabriel Ugueto.

“Peculiar” ancient ancestor of the crocodile started life on four legs in adolescence before it began walking on two - Taylor & Francis Newsroom

In Greek mythology, Oedipus solved the riddle of the Sphinx and so saved the city of Thebes. The riddle asked, “What goes on four legs in the morning, two in the afternoon and three in the evening?” Oedipus correctly answered “Man” (crawling as a baby, walking upright as an adult and using a walking stick in old age.)

Of course, that myth is no less plausible than the Hebrew origin myths that creationists regard as literal history but, in a way, Oedipus was lucky. Not only did the Sphinx allow the stretch of regarding a walking stick as a leg (for poetic licence?), but the riddle did not stop at two legs, as there are very many bipedal species, and at least one went on four legs in infancy.

Researchers, Elliott Armour Smith from the University of Washington Department of Biology and Professor Christian A. Sidor of the Washington University Burke Museum, have just published the results of a study in the Journal of Paleontology showing that an ancient relative of crocodiles, the poodle-sized Sonselasuchus cedrus, probably began life as a quadruped and later had to learn to walk upright because of different growth patterns in different bones.

This creates a problem for creationists, not so much because other mythologies contain legends such as the riddle of the Sphinx that, treated as a metaphor, describe the human condition better than any of the implausible tales in the Bible, but because the myths it does contain preclude learning any of the fascinating history of life on Earth that requires a realistic timeline running into hundreds of millions, even billions of years. These peculiar crocodiliform shuvosaurids lived between about 225 and 201 million years ago and were contemporaneous with the bipedal ornithomimid dinosaurs that eventually gave rise to birds.

The interesting question then—and something creationists are precluded from considering—is what about the earthly environment made bipedalism such an advantage that it evolved independently in two distantly related groups of reptiles?

This conclusion comes from the examination of 950 Sonselasuchus fossils unearthed in Arizona’s Petrified Forest National Park—a fossil site which has yielded more than 3,000 fossil bones since 2014.

Shuvosaurids – The “Dinosaur-Mimicking” Crocodile Relatives.
The animals known as shuvosaurids were members of a broader group of reptiles called pseudosuchians, part of the archosaur lineage that also includes crocodiles, dinosaurs and birds. During the Late Triassic Period (about 237–201 million years ago), pseudosuchians were among the dominant land animals and evolved into an extraordinary variety of forms.

Shuvosaurids were especially unusual. Although they were closely related to crocodiles, their body plan resembled that of small, lightly built dinosaurs. Many species were bipedal, running on their hind legs, with long hindlimbs and relatively short forelimbs. This gave them a superficial resemblance to ornithomimid dinosaurs, the fast-running “ostrich dinosaurs” that evolved tens of millions of years later.

One well-known member of this group is Shuvosaurus, discovered in Texas, while Effigia from New Mexico is another striking example. These animals had long legs, slender bodies and lightly built skulls, suggesting they were agile terrestrial runners rather than semi-aquatic predators like modern crocodiles.

The newly studied species, Sonselasuchus cedrus, comes from the Chinle Formation exposed in Arizona’s Petrified Forest National Park, one of the richest Late Triassic fossil deposits in North America. The site preserves a diverse ecosystem that included early dinosaurs, crocodile relatives, amphibians, reptiles and plants from a landscape of river systems and seasonal floodplains.

What makes Sonselasuchus particularly interesting is evidence that its locomotion may have changed as it grew. Juveniles appear to have been quadrupedal, walking on all four limbs, while adults likely became bipedal as their hindlimbs grew proportionally longer than their forelimbs. This kind of developmental shift, known as ontogenetic change, highlights how complex the evolution of body plans can be. It also illustrates how similar adaptations—such as bipedal running—can evolve independently in very different evolutionary lineages when animals adapt to similar ecological opportunities.

The work of the research group is explained in a news item from Taylor & Francis.

“Peculiar” ancient ancestor of the crocodile started life on four legs in adolescence before it began walking on two
Newly discovered Late Triassic reptile was among creatures that had physical features mimicking the late-evolving dinosaurs it lived beside
A “peculiar” ancient relative of the crocodile which experts believe began life on four legs before, in adulthood, it learnt how to walk on just two has been revealed in a new study.

Named Sonselasuchus cedrus, this archaic reptile was part of the shuvosaurid group, most of which had an appearance mimicking that of the ornithomimid dinosaurs that it shared the landscape with during Late Triassic time (approximately 225-201 million years ago).

In peer-reviewed findings, published today in the Journal of Vertebrate Paleontology, experts from University of Washington Department of Biology and Burke Museum reveal that unusual proportions of some of the fossils led them to believe that this poodle-sized creature had to learn how to walk on two feet.

By analyzing the proportions of the limb skeletons of different animals, they determined its bipedal stance (standing on two feet) may have been the result of a differential growth pattern. We think that Sonselasuchus had more proportional forelimbs and hindlimbs as young, and their hindlimb grew longer and more robust through adulthood. Essentially, we think these creatures started out their lives on four legs… they then started walking on two legs as they grew up.

This is particularly peculiar.

Elliott Armour Smith, lead author.
University of Washington
Seattle, WA, USA.

Armour Smith, a graduate student, carried out the study alongside Burke Museum colleague Professor Christian Sidor.

Professor Sidor was among the dig team that unearthed the 950 Sonselasuchus fossils, in 2014, from Arizona’s Petrified Forest National Park – an extraordinary fossil site which in 10 years of excavation and preparation has revealed more than 3,000 fossil bones.

Sonselasuchus’ fossils also reveal many clues about its appearance and 25-inch tall size. It had a toothless beak, a large eye socket, hollow bones, the experts believe.

Although similar to the ornithomimid dinosaurs these features would have evolved separately, and this similarity was probably due to the fact that croc-line and bird-line archosaurs evolved in the same ecosystems and converged upon similar ecological roles. Also, despite the fact that features like bipedalism, a toothless beak, hollow bones and a large orbit are characteristic of ornithomimid theropod dinosaurs, shuvosaurids like Sonselasuchus show that these features evolved on the croc-line as well.

Elliott Armour Smith.

Sonselasuchus would have lived in the forest, and its name cedrus represents the cedar tree, an evergreen conifer similar to those of Late Triassic forests.

The Sonselasuchus part of the name (pronounced “sawn-SAY-la-SOOK-us”) is in recognition of the geologic unit (the Sonsela Member of the Upper Triassic Chinle Formation) from which the animal originates.

This bedrock has presented many finds to-date.

For Professor Sidor, this project is a culmination of over a decade of fieldwork in collaboration with the National Park Service.

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. In addition to Sonselasuchus, the bonebed has yielded fossils of fish, amphibians, as well as dinosaurs and other reptiles. Over 30 University of Washington students and volunteers have been involved over the years. It’s exciting to see that the site continues to produce new and interesting fossils.

Professor Christian A. Sidoe, co-author
Burke Museum
University of Washington
Seattle, WA, USA.

Publication:
Armour Smith, E. & Sidor, C. A. (2026)
Osteology and relationships of a new shuvosaurid (Pseudosuchia, Poposauroidea) from the Upper Triassic Chinle Formation of Petrified Forest National Park, Arizona, U.S.A.
Journal of Vertebrate Paleontology e2604859. https://doi.org/10.1080/02724634.2025.2604859

ABSTRACT
Shuvosauridae is a clade of pseudosuchian archosaurs currently represented by three named species characterized by a body plan strikingly convergent with that of ornithomimid theropod dinosaurs. This paper documents a new genus and species of shuvosaurid from the Upper Triassic Chinle Formation of Petrified Forest National Park, Arizona. Sonselasuchus cedrus, gen. et sp. nov., is largely diagnosed by features of the maxilla, including a reduced body and anterior process, an enlarged subnarial foramen, and an expanded facet on the posterior process, although other unique cranial features are also noted herein. By contrast, its postcranial anatomy is largely similar to what has been described for other shuvosaurids, although subtle differences are noted. A phylogenetic analysis finds S. cedrus in an unresolved clade with Effigia okeeffeae and Shuvosaurus inexpectatus, likely as a result of non-overlapping missing data, particularly for the skull of Shuvosaurus. A review of the shuvosaurid fossil record indicates that shuvosaurids were persistent components of Late Triassic terrestrial vertebrate faunas in North America. S. cedrus is represented by a minimum number of 36 individuals, mostly skeletally immature, that occur within a multitaxic bonebed of almost exclusively disarticulated elements. An analysis of the relative size change of limb dimensions indicates the forelimb was growing on a negative allometric trajectory relative to the hindlimb, which is consistent with a transition in locomotory mode from quadrupedal to bipedal in S. cedrus during ontogeny.

http://zoobank.org/urn:lsid:zoobank.org:pub:2EA6306F-CF54-40A1-98D4-CFE1DFC4FDBC

Armour Smith, E. & Sidor, C. A. (2026)
Osteology and relationships of a new shuvosaurid (Pseudosuchia, Poposauroidea) from the Upper Triassic Chinle Formation of Petrified Forest National Park, Arizona, U.S.A.
Journal of Vertebrate Paleontology e2604859. https://doi.org/10.1080/02724634.2025.2604859

© 2026 by the Society of Vertebrate Paleontology. Published by Informa Plc.
Reprinted under the terms of s60 of the Copyright, Designs and Patents Act 1988.

Finds such as this are a reminder that the history of life on Earth is far richer and more complex than the simplistic accounts preserved in ancient religious texts. The fossil record repeatedly reveals lineages experimenting with different ways of moving, feeding and surviving in changing environments. In the case of *Sonselasuchus*, we see evidence not just of evolutionary change across millions of years, but even of dramatic changes during the lifetime of an individual animal as its body proportions developed.

It also highlights an important evolutionary principle: similar solutions can arise independently in very different groups when they face similar ecological pressures. Bipedalism, often thought of as a hallmark of dinosaurs and later birds, clearly evolved more than once among Triassic reptiles. The resemblance between these crocodile-line archosaurs and the later bipedal dinosaurs is therefore not evidence of special creation but of **convergent evolution**—nature repeatedly discovering similar mechanical solutions to similar problems.

And this is precisely the sort of discovery that creationist mythology cannot accommodate. In their worldview, species were created more or less in their present forms only a few thousand years ago, leaving no room for the deep timescales, transitional forms and evolutionary experimentation that the fossil record reveals. Yet every year palaeontologists uncover more fossils like those from Petrified Forest National Park, each one adding another piece to the vast and intricate puzzle of life’s history.

Far from presenting a tidy catalogue of separately created “kinds”, the fossil record instead documents a dynamic evolutionary story stretching back hundreds of millions of years. It is a story of adaptation, experimentation and occasional dead ends—one that continues to grow clearer as new discoveries like *Sonselasuchus cedrus* emerge from the rocks.

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