Dinosaur ‘dwarf’ neck is found in the interior of São Paulo

Although it belonged to the group of the largest terrestrial vertebrates of all time, the dinosaur Ibirania parva he was just a runt—relatively, at least. Measuring between 5 m and 6 m from the tip of the snout to the tip of the tail, the species, found in the interior of São Paulo, is the first “dwarf” form of sauropods (necked and herbivorous dinosaurs) to be identified in the American continent.

“Since the fossils were found, it was already known that it was a small animal, but the first hypothesis is always that it could be a juvenile individual”, explains paleontologist Aline Ghilardi, a professor at UFRN (Federal University do Rio Grande do Norte) and co-author of the study that describes the new species.

The detailed analysis of the bone structure of the small sauropod, however, made it clear that it was an adult individual, justifying the baptism with a new scientific name. The name ibirania comes from the municipality of Ibirá, which is 420 km from the capital of São Paulo, in the region of São José do Rio Preto, while silly derives from a Latin term for “small”, the authors explain in an article in the specialized magazine Ameghiniana.

The scientific birth of the animal took a long time — the first bone elements were identified in the midst of the pasture in rural Ibirá in the 2000s by Marcelo Fernandes, a paleontologist at UFSCar (Federal University of São Carlos), and were initially studied by Ghilardi when she was his undergraduate biology student.

Other collaborators helped obtain more fossils of the species. The description analyzed vertebrae from different regions of the spine, bones of the front legs (the radius and ulna, which, in humans, are those of the part of the arm connected to the hand) and hind legs, among others. The set of anatomical details makes it clear that the animal belonged to a subgroup of titanosaurs, animals that dominated the South American herbivore fauna in the final phase of the Dinosaur era. It is estimated that the I. Silly lived just over 80 million years ago.

According to Bruno Navarro, a researcher at USP and the study’s first author, a series of signs help to demonstrate that the animal had already reached maturity when it died and, therefore, belonged to a naturally small species. A first clue comes from the fusion of sutures (joints between bony areas) in the animal’s vertebrae. As the animals become adults, the presence of cartilage in these regions decreases and they unite.

“The problem is that this is very variable and follows a certain order in development, depending on the group to which the animal belongs. In the case of sauropods, for example, it usually happens first in the neck and then goes towards the tail”, he explains. That is, the “mature” vertebrae in the neck would not necessarily indicate that the animal was in fact an adult when it died.

The solution was to analyze the cellular structures of the bones, by means of computed tomography. “This showed that his bone structure was already that of an adult who had stopped growing, and even similar to that of ‘dwarf’ forms in other parts of the world”, summarizes Ghilardi.

Everything indicates, however, that some special mechanisms came into play in the evolutionary trajectory of the I. silly, at least when the animal is compared to its dwarf cousins. It was already known that some Brazilian titanosaurs were relatively modest in size (about 10 m) when compared to their megalomaniac cousins ​​from Argentina, which could exceed 30 m in length in some cases.

Truly dwarf forms, however, had only been found in Europe, in regions that were, at the time, an archipelago. It was thought, therefore, that these species would have been forged by insular dwarfism, a phenomenon in which large animals “trapped” on islands become smaller over the generations, since natural selection would favor those that required fewer resources to survive in the restricted environment in which they live. This happened, for example, on the islands of the Ice Age Mediterranean, which took on various forms of dwarf elephants.

But Ibirá was not an island in the Cretaceous period. On the other hand, it is known that the region was semi-desert and very poor in resources during the drought phases. “In addition, it seems to have been surrounded by mountains that would make it difficult for the animals that lived there to move to other places”, highlights Marcelo Fernandes.

All this, according to the researchers, may have triggered the “shrinkage” of the species. “Both things, the tendency towards smaller size in his lineage and the very hostile environment, may have contributed to his becoming a dwarf”, concludes Navarro.