New fossil assemblage highlights the complexity of classifying silesaurid phylogeny

A set of fossils discovered in Rio Grande do Sul, Brazil’s southernmost state, has brought an additional layer of complexity to the study of the evolutionary history of silesaurids, a family of dinosaurs (dinosaurs and their close relatives ) who lived during the Triassic period between 247 million and 208 million years ago.

In an article published in the Journal of Vertebrate PaleontologyResearchers affiliated with institutions in Brazil and the United States show that even with the new fossil assemblage, it is difficult to confirm that these animals were part of the evolutionary history of dinosaurs.

The fossil assemblage was discovered in 2014 at a site called Waldsanga in the Santa Maria Formation, one of the most fossil-rich rock bodies in Brazil. The bones belonged to more than one individual and it is impossible to be sure that they all belonged to the same species, although the evidence suggests that this is the case. Above all, it is an essential testimony to the animals that lived in the region during the Triassic.

The assemblage is the fourth discovery relating to silesaurids in Brazil and the second from the Carnian age (237-227 mya). It is labeled UFSM 11579 and is deposited at the Laboratory of Stratigraphy and Paleobiology of the Federal University of Santa Maria (UFSM).

“When we inserted the assemblage features into various phylogenies (evolutionary histories) of the group, we did not specify whether the silesaurids were dinosaurs or close relatives of dinosaurs. Regardless, the anatomical evidence “And phylogenetic studies validated the discovery as belonging to the silesaurid lineage, although it is not designated as a new species,” said Gabriel Mestriner, first author of the paper.

“As the bones are disarticulated and given the uncertainty about the evolution of the group, we concluded that adding another species in this case would not be a solution but would make the problem worse,” Mestriner said .

Silesaurids were mostly quadrupeds measuring 1 to 3 m long. They had long hind legs and their front legs were thin. Remains have been discovered today in South America, North America, Africa and Europe.

The first species, described in 2003, was Silesaurus opolensis. His remains were found near Opole in Silesia, Poland, hence his name. “It is the species with the most complete skeleton to date. In addition, the discovery included several hundred well-preserved bones belonging to several individuals. About ten other species have since been described, but their remains were more fragmented,” said Júlio Marsola. , second author of the article.

The study was part of the thematic project “Diversity of dinosaurs and associated faunas in the Cretaceous of South America”, led by Max Langer, professor at FFCLRP-USP and last author of the article.

“Although many species have been described on the basis of few bones, the main problem with this group is not lack of material, but ambiguous anatomy: some parts of the skeleton are similar to those of dinosaurs, “Others less so. It’s difficult to resolve their phylogenetic relationships,” Langer said.

Different teeth

In another study, published in 2021, researchers focused on dental anatomy to look for new evidence for silesaurids’ place in the dinosaur family tree. Their analysis of tooth attachment and implantation in four species, including UFSM 11579, concluded that the teeth of these species were primarily fused to the jaw, lacking the periodontal ligament (the soft connective tissue between the teeth and bones) present in extant dinosaurs and crocodiles.

“At the same time, however, some of the teeth we analyzed were anatomically closer to those of dinosaurs and crocodiles, as if silesaurids evolved in that direction. If so, they could represent an intermediate step between ancestral state (fused teeth) and the derived condition (teeth anchored to alveolar bone sockets by ligaments),” Mestriner said.

The new tooth configuration, also seen in humans and other mammals, is considered an important evolutionary advantage for ancestral non-mammals since periodontal ligaments act as shock absorbers that help reduce the mechanical impact of biting and chewing.

The findings on tooth implantation are not enough to differentiate silesaurids from other dinosaurs, but it is more likely that they were very closely related to dinosaurs. For Langer, who was the principal investigator on both studies, understanding the evolutionary history of groups is more important than continuing to name new species and can be accomplished using existing data, such as fossils deposited in museums.

“We need more detailed phylogenetic studies by researchers who examine collections to analyze all the fossils of a group for features that indicate relatedness within the group or to other groups. The basics of “The data we have now is the result of this type of research, and there hasn’t been enough of it. It’s hard work, but we won’t move forward without it,” he said.