The T. Rex wasn’t the only dinosaur to have … tiny arms!


What the terrible Tyrannosaurus Rex he consoles himself, he’s not the only large predator to have had ridiculously tiny arms. The discovery of Merax gigaspresenting the same specificities, however, it poses the problem of this morphological convergence, despite the distance between the two species on the evolutionary tree.

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From dinosaurs carnivorous giants, tyrannosaurs are characterized by arms ridiculously disproportionate to the rest of the body. However, a team of scientists has just discovered a new dinosaur species with the same morphological peculiarity. It concerns Merax gigas. The most surprising thing is that this new dinosaur has nothing to do with tyrannosaurs. The two species would have evolved completely independently towards a morphology similar, raising the question of the origin of this feature and his interest.

A new skeleton discovered in Patagonia

the Merax gigas belongs to the group of Carcharodontosaurids, which lived at the beginning of the Cretaceous period, that is, from 129 to 89 million years ago. Carcharodontosaurids are among the largest terrestrial predators that have existed, like the Giganotausoro where is the Tyrannotitan, For instance. But not another known predator, the Tyrannosaurus Rex, which is part of a different group. Despite their characteristics physicist are relatively close, Mr. Giga And T.rex in fact he did not live at all at the same time, Mr. Giga having disappeared about 20 million years before the appearance of T.rex. The two species are also quite distant on the evolutionary tree and have no direct link. Having very small weapons is not tied to a transmission genetichow to explain that such a morphology is found on these two species?

The skeleton of Merax gigas recently discovered in northern Patagonia (Argentina) may provide some clues. the fossil found belongs to an adult individual, certainly 45 years old, 11 meters long and probably weighing more than five tons. The skeleton is particularly complete at the level of the arms and legs and shows the presence of large muscular insertions as well as a very developed pectoral belt. The arms of Merax gigas they were therefore small, but strong. Important fact as it means that this morphology is not the result of a atrophy gradual reduction of the upper limbs due to their uselessness. Despite their size, these little arms had to be used for something.

A genetic peculiarity linked to the size of the head?

For Juan Ignacio Canale, lead author of a study published in the journal Current biology, Mr. Gigajust like the T.rexhe could have used his small arms during breeding, especially to hold the female during thecoupling. Other theories suggest that they could have been useful for getting up after a fall. However, it is impossible for the moment to be sure of their exact usefulness.

None of these theories, however, explain why the arms of these giant monsters were so short. For Juan Ignacio Canale it could be the result of a genetic selection linked to the size of the head, the two elements seem closely linked. The researchers indeed noted that in general, among large predators such as Mr. Giga And T.rexthe bigger the head, the smaller the arms.

It would therefore appear that in these dinosaurs a large head implies a negative interaction with the length of the arms. In large predators, a big head is an asset and certainly a breeding criterion, it would have in fact selected individuals with small arms. A new track to dig.

The fossil also provides other valuable information on the social life of Merax gigas and on the importance of the head in this species. the skull it is in fact decorated with various “ornaments”: crests, protuberances, furrows and small horns, characteristics found only in adult individuals. Hence the hypothesis that these cranial ornaments certainly played a role in the male’s sexual attractiveness.

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