Fossil shows complexity of classifying silessaurids – 11/02/2023 – Science

A set of fossils recovered in Rio Grande do Sul brings a new layer of complexity to the study of the evolutionary history of silessaurids, a lineage of dinosaurs (dinosaurs and close relatives) that lived between approximately 247 million and 208 million years ago, in the Triassic period. .

In a study published in the Journal of Vertebrate Paleontology, researchers from Brazil and the United States show that, even with the new fossil, it is still difficult to say where these animals fit into the evolutionary line of dinosaurs.

The fossil was found in 2014 at a site called Waldsanga, in Rio Grande do Sul, part of the Santa Maria Formation, one of the rock bodies richest in fossils in Brazil. The bones correspond to more than one individual, which creates some uncertainty about whether they belonged to a single species, although the evidence indicates that they did. The fossil, first of all, is an important record about the animals that lived in the location in the Triassic period.

The animal is the fourth silessaurid found in Brazil, the second from the Carnian age, between 237 million and 227 million years ago. It is known by the acronym UFSM 11579, as it is deposited at the Stratigraphy and Paleobiology Laboratory of the Federal University of Santa Maria (UFSM).

“When we insert the characteristics of the specimen into different phylogenies [histórias evolutivas] of the group, it remained open whether the silessaurids were dinosaurs or their close relatives. In any case, the anatomical and phylogenetic evidence validates the new findings as belonging to the silessaurid lineage, even if not named as a new species”, says Gabriel Mestriner, first author of the study, carried out as part of his doctorate at the Faculty of Philosophy, Sciences and Letras de Ribeirão Preto from the University of São Paulo (FFCLRP-USP) with a scholarship from Fapesp.

“Because this is disjointed material, and considering the unstable evolutionary context of the group, we concluded that, in this case, the addition of another species would only worsen the existing problem, instead of providing solutions”, he adds.

Silessaurids were mostly quadrupedal animals, between one and three meters long. They had long hind legs, while the front ones were slender. They are recorded in what are now South America, North America, Africa and Europe.

The first species, Silesaurus opolensis, was described in 2003, from fossils unearthed in Silesia, Poland. Hence the name of the species that gave rise to the group.

“This first species described is the one with the most complete skeleton, in addition to several individuals serving as reference. Since then, the other ten or so species have been described from more fragmented material”, reports Júlio Marsola, co-author of the study, part of his postdoctoral studies at FFCLRP-USP carried out with a scholarship from Fapesp. Currently, Marsola is a professor at the Federal Technological University of Paraná (UTFPR), in Dois Vizinhos.

The work is part of the project “Exploring the diversity of South American Cretaceous dinosaurs and their associated faunas”, supported by Fapesp and coordinated by Max Langer, professor at FFCLRP-USP who supervised the study now published.

“Although many species have been described based on a few bones, the biggest problem with this group is not a lack of material. They were animals with ambiguous anatomy, with parts of the skeleton similar to those of dinosaurs and others not so much. So it is difficult to define their relationship” , explains Langer.

Different tooth

In another study, published in 2021, researchers delved deeper into understanding a feature that provided new evidence about the silessaurids’ parentage.

By analyzing the teeth of four species, including specimens of the fossil now described, the group observed that, in general, the dental implantation of silessaurids was fused into the bone. There was, therefore, no layer of ligament (a soft tissue) between the alveolar bone and the tooth, as occurs in current dinosaurs and crocodiles.

“However, at the same time, some of the teeth analyzed presented a configuration closer to that of dinosaurs and crocodiles, as if silessaurids were evolutionarily heading in that direction, representing an intermediate stage between the ancestral condition [dentes fusionados] and the derived condition [dentes sustentados por ligamentos]”, explains Mestriner, who carried out the work during his master’s degree at FFCLRP-USP, with internships at Virginia Tech (United States) and the University of Alberta (Canada).

The new configuration is seen in the teeth of mammals, such as humans themselves, and is considered an important evolutionary advantage over our non-mammalian ancestors, as the ligament complex functions as a damping system that helps reduce the mechanical impact caused when biting into a hard food, for example.

The information on dental implantation, however, was not sufficient to differentiate silessaurids from other dinosaurs. However, it takes them a little further away from dinosaurs, making it more likely that they are a lineage of animals very close to, but not integral to, dinos.

For Langer, who coordinated the studies, more important than continuing to name new species is understanding the evolutionary history of the groups. This can be done based on existing data, such as fossils deposited in museums.

“We need phylogenetic studies with a greater level of detail. Researchers who look into the collections, study all the fossils from that group and find characteristics that indicate kinship within the group or between groups. The databases we have today are created based on of these surveys, which have been lacking. It’s hard work, but without it we can’t move forward”, concludes the researcher.

The article “Anatomy and Phylogenetic Affinities of a New Silesaurid Assemblage From the Carnian Beds of South Brazil” can be read here.