Long-necked dinosaurs reached largest known size – 5/9/2023 – Science

Sauropod dinosaurs lived between the Jurassic and the end of the Cretaceous, from 247 to 66 million years ago, and were the largest animals ever to exist on Earth.

Some of them, like the argentinosaurusreached almost 40 meters in length and up to 110 tons, giants compared to the largest land animal that exists today, the African elephant (3 meters tall and weighing about 6 tons for an adult male).

Understanding how representatives of this group reached such dimensions, however, is not an easy task. At the same time that many of the organisms were giants, so-called dwarfism—reduced body size—also evolved several times in the group known as Sauropoda.

A study published on Monday (8) in the scientific journal Current Biology revealed that gigantism in long-necked dinosaurs evolved independently (that is, in unrelated lineages) at least 36 times in the group’s evolutionary history.

The research thus brings a new understanding of the evolution of gigantism in the group, which was previously considered as a phenomenon restricted to a few clades (or lineages).

The study was led by Michael D’Emic, a paleontologist and associate professor at Adelphi University in New York. From analyzing the dimensions of the long bones of more than 200 fossils, he found that the average size of the group was about 11.7 metric tons (about 12,000 kilograms), or less than half that reached by the largest mammals that ever lived. existed (now defunct). This weight is also equivalent to one-sixth of that observed in the largest sauropod ever described, the argentinosaurus.

However, over the course of about a hundred million of their evolutionary history, the size of long-necked dinosaurs reached proportions far beyond that observed for lineages of land mammals (such as hippos and elephants).

Origin of gigantism

Gigantism arose twice in so-called non-neosauropod dinosaurs (the most recent lineage of sauropods); twice in the group known as Turiasauria; seven times in Diplodoicodea dinosaurs (like the diplodocus, from the Jurassic of the United States); 25 times in the group called Macronaria (including brachiosaurs); and 15 times in the so-called titanosaurs (which includes representatives also found in Brazil).

According to D’Emic, long-necked dinosaurs may have reached the maximum observed size in a much larger number than previously thought, thus opening up a possibility of investigation into the mechanisms involved in these giant sizes.

“An animal’s weight increases by a cubic power, but the diameter calculated by cutting the bones increases by a square power. This means that the bones would have to get much thicker to support the continually growing weight in the new strains, but there is a limit to how thick a bone can be,” he says.

Some research, including with fossils found in Brazil, has already revealed that in exceptionally sized long-necked dinosaurs, the bones were pneumatic, that is, they contained air sacs inside, similar to that observed in birds and pterosaurs —which also reached large sizes and needed stay in flight.

“Each event [evolutivos] What caused sauropods to reach the largest body sizes ever experienced were unique, because species differ greatly in their habitats, diets, growth rate, and body proportions. There is no single set of factors that favored such a body size, including environmental ones, that could explain the evolutionary course,” he said.

The paleontologist, however, points out that it is possible that some sizes were limited in environments with less energy resources than others. “Metabolically speaking, some sauropods would have to spend 24 hours a day feeding themselves in order to obtain the necessary caloric intake for their growth. So there are still some limitations that we need to understand better”, he says.

what is missing to understand

Research that advances in the knowledge of how these animals developed must follow, as for example with the analysis in microscope of histological slides (tissue) of the bones of the dinosaurs. “Since the blades are a very small fragment of the bone that is much larger, it is difficult to make this understanding of the whole, because to study the complete organism it would take years of research. But the new technologies can help in this study”, he concluded.

For Bruno Navarro, a paleontologist and doctoral student at the Laboratory of Paleontology at the Museum of Zoology at USP, the study is remarkable in that it concentrates an unprecedented amount of fossils in a set of data to assess the evolutionary patterns of sauropods. “The main result of the research is to identify how the 36 gigantism events occurred, distributed in several lineages, but it also shows that the data can be used to infer, for example, the effects of temperature, oxygen availability and resources on body size of these dinosaurs”, he explains.

According to him, the so-called “bauplan” (body plan, which would be the set of morphological characteristics that define the body of a sauropod) was very effective. “The considered average size of approximately 15 tons would be the ancestral state found in many of these lineages that managed to outgrow and reach even larger sizes, and this was also somewhat limited by the so-called ‘continental rifting’, which separated the continents in the Mesozoic period” , he says.

Giant neck dinosaurs are known from all continents except Antarctica and India, probably related to a taphonomic bias (processes occurring after death that prevent preservation), explains Navarro. “In Antarctica, the only known record is a vertebra in a poor state of conservation, thus making it impossible to identify the fossil, and in India the described species are of medium size.”