The rocks of the interior of Rio Grande do Sul, crucial for understanding the origin of dinosaurs, have just revealed another small treasure: the femur (thigh bone) of the oldest precursor of these animals in South America, aged around 240 million years. .
At this stage of their evolutionary history, the ancestors of the gigantic reptiles that would one day be the lords of all terrestrial environments on our planet were small and rare — so discreet, in fact, that this makes it difficult for paleontologists trying to find them. The femur with 11 centimeters in length, found in the municipality of Dona Francisca (RS), probably corresponded to an animal of 1 m in length or less.
The finding is described in an article recently published in the specialized journal Gondwana Research. Rodrigo Temp Müller and MaurÃcio Garcia, from the Federal University of Santa Maria, sign the study. The duo classifies the animal as a dinosaurmorph, that is, a member of a group that encompasses all species closer to dinosaurs than to crocodiles (current or extinct) or pterosaurs (flying reptiles).
To find out where the owner of the femur fits into the Triassic reptile family album (the geological period in which he lived), the researchers analyzed the details of the bone’s anatomy. Among the most important are the so-called trochanters, protuberances of the femur that function as support sites for the muscles that command movements in areas such as the thigh and tail. It’s also interesting to get an idea of ​​how the femur fit into the animal’s pelvis.
It so happens that, throughout the evolution of the group that would give rise to the dinosaurs, these details underwent transformations dedicated to facilitating bipedal posture and more cursory habits (that is, typical of agile animals, which did well in the race). It’s details like these that appear in the bone excavated in the interior of Rio Grande do Sul. “But we still haven’t been able to say that this specific animal was already bipedal”, explains Müller.
Dinosaurmorphs with characteristics similar to those present in the new fossil had already been found in rocks from Argentina and also in Africa, in countries such as Tanzania and Zambia. It makes sense when you consider that, at that remote time, South America and the African continent were glued together, without the presence of the Atlantic that separates them today. It is not by chance that the Brazilian coast and the West African coast seem to fit together, like the pieces of a jigsaw puzzle.
It is already clear that the femur from Rio Grande do Sul is the oldest record of these animals in South America, surpassing the Argentine specimens in age. In theory, African fossils would be a little older, but their age is being re-discussed in recent times, which means that the Brazilian record of dinosauromorphs is perhaps also the oldest in the entire world.
What is known about the animals of the group at this time indicates that they were small carnivores or omnivores (eating both insects and small vertebrates as well as plants). There is little data on the vegetation existing in the region at this stage of the Triassic, but the characteristics of the rocks indicate that the climate was relatively humid, says Müller.
Other groups of reptiles were far more numerous and diverse than dinosaurmorphs at that time. There were also precursors of mammals and, among predators, the Prestosuchus chiniquensis, a distant relative of the current alligators and crocodiles that reached 7 meters in length. The increase in size and ecological importance of the dinosaur lineage would not happen until millions of years later.
