The skin skeleton of the fins helped create the limbs of terrestrial vertebrates

The skin skeleton of the fins helped create the limbs of terrestrial vertebrates
The skin skeleton of the fins helped create the limbs of terrestrial vertebrates
Anonim

Even before the formation of fingers in the limbs of tetrapods, the network of skin rays of paired fins in some lobe-finned was simplified, the number of these rays decreased, and the upper side of the fin acquired differences from the lower one, according to the Proceedings of the National Academy of Sciences. These transformations made it easier for ancient vertebrates to move on a silty substrate and became an important stage on the way to the formation of multi-toed tetrapods.

The structure of vertebrates when they came to land has changed greatly, and perhaps the most modifications have undergone their limbs. If in fish these are fins with many bony and skin rays, then in tetrapods (this is a common name for all terrestrial vertebrates, including birds) there are no rays in the limbs, but there are fingers.

When studying the evolution of limbs, most often attention is paid to the bony rays in the fins of lobe-finned fish, the alleged ancestors of tetrapods. Skin rays are studied in far less detail. They are worse preserved, and sometimes they are even removed from the fossil remains so that the bony elements of the fins are better visible.

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Tiktaalik's fin. Dorsal rays (on the dorsal side), blue - ventral rays (on the ventral side) are shown in orange.

In contrast, paleontologists from the University of Chicago, the University of Wisconsin at Parkside and Drexel University, led by Neil Shubin, decided to compare the details of the structure of the skin rays of the fins of animals close to the first tetrapods. To do this, they used computer microtomography.

Scientists made a series of virtual cuts of the pectoral fins of the remains of adults and juveniles of Sauripterus taylori, Eusthenopteron foordi, and Tiktaalik roseae. These species are referred to as tetrapodomorphs - a group that includes supposed transitional forms from lobe-finned fish to tetrapods (and the tetrapods themselves). Sauripterus is considered the most ancient and primitive of the three, Tiktaalik the most advanced and tetrapod-like.

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The explored views as seen by the artist. From top to bottom: Sauripterus taylori, Eusthenopteron foordi, Tiktaalik roseae. The dorsal rays of the pectoral fins are shown in orange, the ventral rays are in blue.

The researchers focused on the relative position of the cutaneous and bony rays of the pectoral fins. They determined how these rays are located in relation to the bone ones, assessed their number and length.

It turned out that the closer a species is to tetrapods, the less skin rays in its pectoral fins. What is even more interesting is that the cutaneous rays of the dorsal (dorsal) and ventral (ventral) sides of these fins have different lengths and are "superimposed" on the bony rays to varying degrees. This asymmetry is most pronounced in Tiktaalik, and according to it, the authors suggested that this animal had a functional semblance of palms, that is, the lower side of the pectoral fin carried a lot of muscles.

Probably, the muscles of the "palm" of Tiktaalik helped him to push off from the muddy bottom of shallow water bodies, where he supposedly lived, and even to make short movements on land. Previously, the asymmetry of the dorsal and ventral sides of paired fins in those who had not yet stepped ashore and had no fingers was not studied in detail.

Modern benthic teleost fish also have differences in the arrangement of rays from different sides of the pectoral fin. This is important because the limbs of those who keep close to the bottom experience similar stresses to those of those who try to walk on land or in very shallow water. All such fish need to push off from a hard substrate, and this can be helped by the muscles on the ventral side of the fin. The authors remind that this must be taken into account when studying the emergence of vertebrates on land; therefore, it is wrong to limit ourselves to considering only the bony rays of the fins of tetrapodomorphs.

In 2016, Neil Shubin and colleagues showed that the same genes are responsible for the formation of rays in the fins of teleosts as those for the formation of fingers in tetrapods. They hypothesized that the fingers of terrestrial vertebrates are formed from the same structures from which fin rays appear in fish. Then the experiments were carried out on the living zebrafish fish.

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