A 240-million-year-old reptile fossil discovered in southern Brazil is offering new clues about the evolution of the animals that eventually gave rise to dinosaurs and modern crocodiles.
Named Silescelida acristata, the small predator lived during the Middle Triassic, about 12 million years after the Permian–Triassic mass extinction, the most severe extinction event in Earth’s history. The fossil was found in Dona Francisca, in Brazil’s Rio Grande do Sul state, within rocks belonging to the UNESCO Quarta Colônia Geopark.
The study, published in Scientific Reports, was conducted by researchers from the Quarta Colônia Center for Paleontological Research at the Federal University of Santa Maria, in collaboration with scientists from the Federal University of Rio Grande do Sul and the Pontifical Catholic University of Rio Grande do Sul.
Silescelida acristata was neither a dinosaur nor a crocodile. Instead, it belonged to the archosauriforms, a group of reptiles closely related to the evolutionary lineage that eventually produced archosaurs — the group that includes dinosaurs, birds and crocodiles.
Roughly the size of a small alligator, the animal was a slender, four-legged predator that likely fed on smaller animals. Its fossil preserves important parts of its limbs, providing clues about how early archosauriforms moved.
One of its most significant features is the structure of its femur, or thigh bone. Its legs appear to have been positioned in a more semi-erect posture rather than extending fully sideways. Such positioning could have made movement more efficient by reducing drag as the animal moved across the ground.
The discovery also has implications for the geographic history of these reptiles. Phylogenetic analysis suggests that Silescelida acristata may have been related to the Euparkeriidae, a poorly understood group previously known mainly from fossils found in Africa, Asia and Europe. Its presence in South America suggests that these reptiles had a much wider distribution across the Triassic world than previously recognised.
The fossil itself has an unusual history. Part of the specimen, including material crucial for establishing its origin, had been missing for more than two decades. Researchers rediscovered the fragment during a 2022 visit to the scientific collection of the Pontifical Catholic University of Rio Grande do Sul, allowing the specimen to be formally identified as a new species.
The findings highlight the importance of Brazil’s fossil-rich Triassic rocks in reconstructing how terrestrial ecosystems recovered after the mass extinction and how the ancestors of some of today’s most significant vertebrate groups evolved.
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