Fossil Discovery of Silescelida acristata Provides Insights into Archosauriform Evolution

A new species named Silescelida acristata has been identified from fossils dating back approximately 240 million years, during the Middle Triassic Period. Discovered in Dona Francisca, central Rio Grande do Sul, the fossil resides within rocks that belong to the UNESCO Quarta Colônia Geopark. This find contributes significantly to our understanding of archosauriformes, a group of reptiles that later gave rise to archosaurs, which include well-known terrestrial vertebrates such as crocodiles and dinosaurs, including birds.

The research was carried out by the Quarta Colônia Center for Paleontological Research at the Federal University of Santa Maria (CAPPA/UFSM), in collaboration with the Federal University of Rio Grande do Sul (UFRGS) and the Pontifical Catholic University of Rio Grande do Sul (PUCRS). The findings were published in the journal Scientific Reports.

Ecological Recovery in the Middle Triassic

When Silescelida acristata existed, ecosystems were still recovering from the Permian–Triassic extinction event, which occurred around 252 million years ago. This dramatic extinction eliminated much of the planet’s life and set the stage for the proliferation of new animal groups. The Triassic Period saw archosauriforms diversify extensively, with species like Silescelida acristata adapting to new roles in these recovering environments.

This small, slender reptile measured about the size of a small alligator and moved on all fours. While neither a dinosaur nor a crocodile, it belonged to a lineage closely related to those that preceded these groups. Its diet likely consisted of smaller creatures, suggesting it was a small predator in the Triassic ecosystems of southern Brazil.

Significance of the Fossil

Although the fossil primarily includes limb bones, it provides valuable information regarding Silescelida acristata‘s locomotion and evolutionary connections. Notably, its femur exhibits a semi-erect posture, lower on the body, which enhances mobility and reduces drag while moving. This anatomical adaptation is linked to evolutionary changes that would later benefit archosaurs.

The fossil enhances our comprehension of an era preceding the rise of dinosaurs and crocodiles, showcasing a time when their relatives experimented with diverse body structures and movement patterns.

New Phylogenetic Insights

Phylogenetic studies suggest that Silescelida acristata might be allied with the Euparkeriidae, a rare and not well-studied group of archosauriforms. Previously, fossils from this group were mostly found on continents such as Africa, Asia, and Europe. This South American find substantially broadens the geographical distribution of these reptiles, indicating a more widespread lineage during the Triassic than what fossil records have shown.

The Rediscovery Process

The backstory of Silescelida acristata is also noteworthy; a part of the fossil containing key provenance information was lost for over twenty years. Its rediscovery in 2022 during a technical visit to the Pontifical Catholic University of Rio Grande do Sul allowed researchers to confirm the fossil’s origin and recognize it as a new species. The name Silescelida combines “silence” and “leg,” reflecting the fossil’s long period of neglect, while acristata means “without a crest,” referencing the unique absence of a trochanter on its femur, distinguishing it from similar species.

Broader Implications of the Discovery

The presence of Silescelida acristata in the Middle Triassic of Brazil suggests a more intricate evolutionary history for archosauriforms than previously recognized. This fossil underscores South America’s critical role in understanding the early diversification of ancestors to dinosaurs and crocodiles. Furthermore, the find strengthens Rio Grande do Sul’s status as a significant region for Triassic fauna research, revealing fossils from various evolutionary stages, including some of the oldest known dinosaurs.

Each new fossil discovery adds to the reconstruction of ecosystems that reorganized following the Permian–Triassic extinction, shedding light on how groups that would dominate terrestrial landscapes later emerged. Silescelida acristata illustrates how even fragmentary bones can yield crucial insights into the history of life on Earth.