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

Long before dinosaurs dominated the continents and modern crocodiles appeared, their ancestors were already going through a decisive stage. In this scenario, shortly after the largest mass extinction in Earth's history, a new species of fossil reptile emerged in the interior of Rio Grande do Sul, Brazil.

This is Silescelida acristata, an animal that lived about 240 million years ago, during the Middle Triassic period. The specimen was found in Dona Francisca, in rocks of the UNESCO Geopark Quarta Colônia. The discovery helps fill an important gap in the evolution of archosauriforms, the group that gave rise to archosaurs, a lineage that includes crocodiles, dinosaurs, and birds.

The work was carried out by researchers from the Quarta Colônia Paleontological Research Center at the Federal University of Santa Maria, along with teams from UFRGS and PUCRS, and was published in the journal Scientific Reports.

When this reptile inhabited the Earth, terrestrial ecosystems were still recovering from the Permian-Triassic extinction, which occurred about 252 million years ago. This event wiped out much of life and paved the way for the diversification of new groups. In this context of ecological reconstruction, different lines of reptiles began to occupy different roles.

A small predator with special limbs

Silescelida acristata was a relatively small and slender animal that moved on four limbs. Its size can be compared to that of a small caiman. Although it was neither a dinosaur nor a crocodile, it belonged to a lineage closely related to the forms that preceded the origin of those groups. Its diet likely included smaller animals, fulfilling the role of a small predator in the Triassic ecosystems of southern Brazil.

The fossil primarily preserves parts of the limbs. At first glance, this material may seem limited, but it provides key data about the locomotion and evolutionary relationships of the animal. One of the most relevant features is found in the femur. Like some of its close relatives, it had its limbs in a more semi-erect position, more underneath the body than to the sides. This change allowed for more efficient movement, with less drag on the ground.

In evolutionary terms, this type of transformation is related to the set of anatomical changes that later proved fundamental for the success of archosaurs. The new fossil helps to understand a phase prior to the rise of dinosaurs and crocodiles, when their close relatives were still experiencing different body shapes, postures, and modes of movement.

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

A geographic redistribution and a rediscovered fossil

This suggests that these reptiles were more widespread across the Triassic world 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 story of the discovery has a particular twist. Part of the fossil, specifically the portion that held essential information about its provenance, was accidentally lost for over two decades. Only in 2022, during a technical visit to the scientific collection of PUCRS, did researchers locate the missing fragment. This reunion allowed for the confirmation of the specimen's origin and its formal description as a new species.

The very name of the animal alludes to this story. Silescelida combines words associated with “silence” and “limb.” The silence refers to the long period during which part of the fossil remained forgotten, while “limb” refers to the type of preserved material, composed mainly of limb bones. The epithet acristata means “without crest,” because the femur lacks a bony protrusion where part of the tail musculature would attach. This absence distinguishes it from almost all its close relatives.