2018-12-14 13:29:56
A little over a century ago, in 1893, the Franco-Belgian paleontologist Louis Dollo presented his ideas on evolution to the Belgian Society for Geology, Paleontology and Hydrology. There he enunciated for the first time what has since been known as Dollo’s law, an empirical principle according to which an organism cannot return to a previous state of its evolutionary line. In short, Dollo viewed evolution as an irreversible process. This is not to say that an animal that has evolved, for example, from a terrestrial to an aerial way of life, such as birds, cannot re-adapt to a completely terrestrial life; there are countless cases of birds that have lost the ability to fly, such as the dodo, which we have already discussed here.
What Dollo’s law means is that in a process as complex as evolution, traces of the intermediate stages will always remain, like the atrophied wings of the dodo in our example. In the words of British biologist Richard Dawkins, the law “is a statement about the statistical improbability of following exactly the same evolutionary path twice in any direction.” If evolution is the successive accumulation of infinitely many small changes, the probability of reversing those changes in reverse order is infinitesimal.
When there is an apparent evolutionary reversal, such as that of the dodo, the evolutionary path that leads to the result is completely different from the original path; that’s why the dodo is a dodo, and not a dinosaur, which is what a flying bird would become if it followed its evolutionary path in reverse.
There are many cases similar to the dodo. One of the best known is that of tetrapods that have returned to the sea, such as dolphins, whales, seals, turtles and, in times past, plesiosaurs and ichthyosaurs. None of these have turned into a fish; they retain characters that clearly indicate their mammalian or reptilian affiliation. What had never been seen, until last year, is that one of these aquatic reptiles turned back into a terrestrial animal.
According to the most recent research, plesiosaurs and ichthyosaurs are part of a large group of aquatic reptiles with a common ancestor. In this group are also the thalatosaurs, with a long tail and flattened like an oar, the armored saurosphárgidos and the placodonts, like Henodus, which we talked about here a few years ago. Well, of all these aquatic reptiles, only one, that we know of, returned to the mainland: Eusaurosphargis.
In 2003, paleontologists Stefania Nosotti and Olivier Rieppel described a new species of reptile from the Middle Triassic, discovered in the oil shale of Besano in northern Italy, near the Swiss border. The fossil remains, a partial disarticulated skeleton, correspond to a single individual that lived about 243 million years ago. This new species received the name of Eusaurosphargis dalsassoi. The specific name, dalsassoi, pays homage to paleontologist Cristiano dal Sasso, from the Natural History Museum in Milan. The generic name comes from the Greek eu, “good” or “true”, and from Saurosphargis, another species with which it bears many similarities. The name Saurosphargis is made up of sauros, “lizard”, and sphargis, an old name for the leatherback turtle, since the species presents certain features that seem intermediate between turtles and other reptiles, although we now know that it is not a turtle ancestor.
More disarticulated remains of Eusaurosphargis turned up in the Netherlands in 2014, but it wasn’t until 2017 that a complete and almost fully articulated specimen was found in the Swiss Alps near Davos. It was a young specimen, less than eight inches long, with an elongated barrel-shaped body and a very short tail. Adults are estimated to reach half a meter in length. Thanks to this discovery we have a fairly accurate description of Eusaurosphargis: The skull is short and tall, with very robust jaws; the upper jaw is equipped with a double row of leaf-shaped teeth, while the lower jaw has only one row of teeth. From the shoulders to the base of the tail, the skin is covered with osteoderms, bone plates of various kinds: along the sides of the body are rows of partially overlapping oval plates; on the back, along the vertebral column, conical plates with a broad base, more or less triangular; on the sides of these, from the shoulders to the last dorsal vertebra, there are two rows of large conical plates with a broad ovoid base; the pelvis is covered with rectangular plates; and shoulders and forelegs of partially overlapping triangular osteoderms, forming a serrated edge, with some rounded plates further back.
The highly developed and robust front legs of Eusaurosphargis, as well as the shape of its hip, indicate that it is not an aquatic animal, although its closest relative, Helveticosaurus, is. Helveticosaurus, discovered in the Swiss site of Mount San Giorgio, very close to where the first Eusaurosphargis fossils were found, is an aquatic reptile with an elongated body about two meters in length, which lived in the shallow sea that covered much of the Europe around the same time, in the Middle Triassic. It has a long and flexible tail that serves as a propeller through lateral undulations, but also the front legs are transformed into strong fins; This combination of characters, with a double propulsion system, by undulation and flapping, is very unusual among aquatic reptiles. The sharp teeth of Helveticosaurus suggest that it is a predator, although its square, robust head lacks the elongated snout found in many fish-eating aquatic mammals and reptiles: dolphins, crocodiles, ichthyosaurs… We don’t know what Helveticosaurus preyed on. .
Evolution never repeats itself, but neither does it rest. Since an ancestral fish left the water nearly four hundred million years ago, several of its descendants, terrestrial tetrapods, have returned to the water. And of their descendants, we now know that at least one, Eusaurosphargis, returned to the mainland again.
CONSTRUCTION OF GERMAN FERNÁNDEZ:
reticular infiltrate
Reticular Infiltrator is the first novel in the trilogy The Saga of the Borelians. Do you want to see how it starts? Here you can read the first two chapters.
The Karnak file. Ed.Rubeo
The hanged man and other fantastic tales. Ed.Rubeo
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