Discovery of fossils of a new species between a snake and a lizard

The fossil, named Nagini mazonense, is both a new genus and a species belonging to a group known as Molgophyda. Experts believe that this species was without arms, but with legs. transitional form between four-legged animals and snakes that we know today.

Previous research has shown that among the earliest four-legged land animals, many lineages evolved into a serpentine build in which excessive axial elongation was accompanied by reduction or even complete loss of limbs. However, when and how this body structure arose remains poorly understood. Aryan Mann, Jason Pardo and Hillary Maddin unveil new fossils today in the journal Ecology of nature and evolutionwhich may provide some answers.

These fossils are the oldest examples of limb loss.

The researchers unearthed two fossils they found at Mazon Creek, in the Francis Creek Shale, a fossil site located in the American Midwest that is well known to experts. These samples are between 309 and 307 million years old.

© A. Mann et al., Natural Ecology and Evolution (2022)

Photos fossils Nagini masonense recently discovered.

Each is about 10 centimeters long; they lack forelimbs or even the pectoral girdle, the bony structure that, in tetrapods, connects the forelimbs to the spine. Both specimens, however, have small hind legs, feet, and four toes. Thus, this is the oldest example of complete loss of snake-like limbs seen in the amniote fossil record. on his Twitter account paleobiologist Arjan Mann.

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Amniotes refer to a clade including mammals, reptiles, and birds in which the embryo (or fetus) is protected by an amniotic sac; this is one of the evolutionary characteristics that allowed this group of animals to free themselves from the aquatic environment (unlike amphibians). This key characteristic allowed amniotes to become one of the vertebrate groups. largest and most variedanimals of all shapes and sizes that we meet today.

But so far, scientists have gathered little evidence for the ability of very early amniotes to evolve into different body structures. The reduction or complete loss of limbs is one way to diversify them, says Dr. Aryan Mann in his work. article accompanying the publication.

In particular, one group of tetrapods known as Recumbirostra, is considered one of the first varieties of reptiles. This group is distinguished, in particular, by the variety of teeth and strong skulls, probably adapted for digging the earth. Among the fossils of the primitive group of amniotes are the earliest examples of extreme body elongation; most of them have already shown some form of limb reduction. But Nagini masonense is the first of its kind.

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Ability that most vertebrates can have

According to the team, the loss of forelimbs suggests that N. mazonense mostly walked sideways as a means of locomotion and probably rarely used its hind legs. An analysis of depressions carved into the rock around the fossil, presumably created by the soft parts of the animal’s body, shows that N. mazonense had a round muzzle; however, there was no evidence of soft tissue where the forelimbs might have been. ” This contraction of the forelimbs is consistent with the pattern of limb contraction seen in modern snakes. ‘, the researchers write. Ecology of nature and evolution. In snakes, this limb loss is associated with altered expression of the Tbx5 gene. It should be noted that in many other groups of tetrapods, reduction of the hind limbs was observed first of all.

Although this ancient animal bears some resemblance to snakes, it is not ancestor of modern snakes, say the researchers. However, this discovery may help explain how they lost their limbs; it also explains some of the curious phenomena observed so far: In snake embryos, as in pythons, the rudiments of the hind limbs are still being formed, which disappear in the process of development. » announced New scientist Rolf Zellerresearcher at the University of Basel.

>>Also read: Tardigrades: the discovery of an entirely new fossil genus

Today, only snakes, some lizards and a few amphibians are completely limbless. But this discovery suggests that the ability to lose limbs is present in most vertebrates. If the genes that control this ability are still written into their genomes, then theoretically other groups of animals could evolve into a body structure with fewer (or no) legs at all, Zeller said. ” Our results suggest that the serpentine limb reduction mechanism may operate more widely in the amniote tree. confirm Mann and two of his employees.

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