240-million-year-old reptile fossil reveals a lost chapter before dinosaurs and crocodiles

The discovery of a 240-million-year-old reptile fossil in Brazil is shedding new light on the evolution of archosauriformes, a group of reptiles that gave rise to the archosaurs. The archosaurs, in turn, are the lineage that encompasses two of the best-known groups of terrestrial vertebrates: crocodiles and dinosaurs, including birds.

The fossil, named *Silescelida acristata*, was found in the interior of Brazil's Rio Grande do Sul state. It lived during the Middle Triassic Period, a time when terrestrial ecosystems were still recovering from the Permian-Triassic extinction event.

## A small animal in a world under reconstruction

When *Silescelida acristata* lived, the planet was still recovering from the devastating Permian-Triassic extinction event. This event wiped out much of the life on Earth and paved the way for the diversification of new groups of animals. It was against this backdrop of ecological recovery that different lineages of reptiles began to take on new roles in terrestrial environments.

Among them were the archosauriforms, which underwent extensive diversification during the Triassic. *Silescelida acristata* was a relatively small, slender animal that moved on all four limbs. Its size can be compared to that of a small alligator. Although it was neither a dinosaur nor a crocodile, it belonged to a lineage closely related to the forms that preceded the origin of these groups.

## Why is this fossil important?

The fossil primarily preserves parts of the limbs, and at first glance, this type of material may seem limited. However, it carries important information about the animal's locomotion and evolutionary relationships. One of the most significant characteristics of *Silescelida acristata* lies in its femur, the thigh bone. Like some of its close relatives, it had legs in a more semi-erect position, lower on the body than to the side.

This change allowed for more efficient locomotion, reducing drag as the body moved across the ground. In evolutionary terms, this type of transformation is related to the set of anatomical changes that would later prove fundamental to the success of archosaurs.

## Rare phylogenetic relationship

Phylogenetic analyses indicate that *Silescelida acristata* may be related to the *Euparkeriidae*, a rare group of archosauriforms that is still poorly understood by science. Until now, fossils associated with this group were known primarily from the continents of Africa, Asia, and Europe. The presence of a form related to the *Euparkeriidae* in South America significantly expands the known geographic distribution of these animals.

This suggests that these reptiles were more widely distributed across the world during the Triassic than the fossil record indicated. It also reinforces the importance of South America for the study of the origin and diversification of major groups of terrestrial vertebrates.

## The ‘rediscovery’ of a forgotten fossil

The discovery of *Silescelida acristata* also has an unusual history. Part of the fossil - precisely the portion that preserved essential information about its provenance - was accidentally lost for more than two decades. It was not until 2022, during a technical visit to the scientific collection at the Pontifical Catholic University of Rio Grande do Sul (PUCRS), that researchers located the missing fragment.

This rediscovery made it possible to confirm the specimen's origin and formally describe it as a new species. The animal's very name refers to this story. *Silescelida* combines words associated with "silence" and "leg." "Silence" refers to the long period during which part of the fossil lay forgotten, while "leg" refers to the type of preserved material, consisting mainly of limb bones.

## What does the discovery change?

The presence of *Silescelida acristata* in the Middle Triassic of Brazil shows that the evolutionary history of archosauriforms was broader and more complex than previously thought. The fossil indicates that lineages closely related to the *Euparkeriidae* were also present in South America, highlighting the continent's role in the early diversification of the relatives of dinosaurs and crocodiles.

Furthermore, the discovery reinforces Rio Grande do Sul's status as one of the world's most important regions for the study of Triassic fauna. The region's rocks preserve fossils from different stages in the evolution of terrestrial vertebrates, including some of the oldest known dinosaurs and large predators that lived before the Age of Dinosaurs.

Every new fossil found in this context helps reconstruct how terrestrial ecosystems reorganized after the Permian-Triassic extinction and how the groups that would later dominate the continents emerged.

## Small bones, big stories

Incomplete fossils are often viewed as less informative than nearly complete skeletons. However, it is essential to remember that the vast majority of fossils found during excavations are fragmentary. Discoveries such as *Silescelida acristata* show that even isolated parts of the skeleton can reveal fundamental information about evolution.

In this case, limb bones made it possible to identify a new species, infer aspects of its locomotion, investigate its phylogenetic relationships, and expand the known distribution of a rare group of Triassic reptiles.

More than just describing a new animal, the study shows how Brazilian paleontology continues to contribute to our understanding of central chapters in the history of life on Earth. In rocks from the interior of Rio Grande do Sul, 240-million-year-old fossils still hold clues about the origin of the animal groups that would transform terrestrial ecosystems forever.