Four limbs, a beak, a carapace.
These are the tools needed to colonize almost the entire planet, on land or in the sea. With a little less than 350 existing species, turtles are found on all continents except Antarctica.
But the conquest of the planet is not the only distinction of these animals: they also have an extremely long life span, are extremely tolerant to anoxia, are active until very late in the reproduction and some species are able to freeze almost completely, to thaw with barely a scratch or two.
Origins of the turtle
The fossil record offers turtles with very strange physical characteristics. One of the oldest fossils ever found is that of Odontochelys semitestacea. The particularity of this animal lies in the presence of teeth instead of the beak characteristic of the turtles, and in the long tail instead of the reduced one of the current turtles. But the strangest thing is probably its lack of upper shell. Dated 220 million years ago, it lived alongside the Ichthyosaurs, marine reptiles that resembled the current dolphins, and the Thalattosaurs, sea lizards that could reach 4 meters in length.
Image: Odontochelys semitestacea, a turtle with a carapace already formed on its belly, but not on its back.
In addition to having remarkable characteristics on the physical level, the turtle is also a mysterious being on the evolutionary level, it poses more questions than it brings answers. They seem to have been at first aquatic beings, dominating the fresh waters, to adapt thereafter to the terrestrial and marine environment. However, scientists do not know how many times the turtle has adapted to marine life, it is very possible that this characteristic appeared several times during their evolution.
This marine adaptation is characterized by a change in the structure of the animal: a lighter, flatter shell for aerodynamics, tear glands that evacuate sea salt, and bones adapted to rapid changes in depth.
A long and complex evolutionary history...
Turtles are divided into several groups, respectively the terrestrial turtles (Testudinidae), the aquatic turtles, and the marine turtles (Chelonioidea). This last group was composed of five families in the past, but only two of them have survived: the hard-shelled sea turtles, called chelonians, and the dermochelyids, which do not have scales on their shells, but a skin instead. Today the only representative of this group is the leatherback turtle.
The classification of turtles remains a thorny problem even today. This enigma comes from the fact that it was for a long time very complicated to determine who were its close cousins.
Part of the classification of living organisms is based on the skull. The latter sometimes has openings behind the orbit, "fenestra" that lighten the skull and allow the insertion of muscles. There are 4 types of skulls: anapsids, which have no temporal pits, euryapsids, which have one towards the top of the skull, synapsids which have one towards the bottom of the skull, and diapsids which have two temporal pits. All of these morphologies originate from an ancestral anapsid skull, which then drifted to give all the others.
Turtles have an anapsid skull, i.e. without temporal pits. Logically, one would think that they are classified as anapsids, yet they are classified as diapsids... Crazy, really! Or not quite, once we know that they were probably diapsids before, and that they lost their temporal fossae afterwards. They are therefore close cousins of the archosaurs, a group that includes birds and crocodiles today; they would have separated from this group 255 million years ago.
Image: The skull differences all come from an anapsid skull. Turtles share a common ancestor with lizards and snakes (a diapsid skull).
Turtles have a great evolutionary history and diversity. But not surprisingly, like most of the current species, it is threatened by the hand of man, especially by fishing. Industrial fishing kills some 250,000 loggerhead and leatherback turtles each year. They are considered as bycatch, because they are not wanted but there is no way to precisely target the fished species. Today, the bycatch phenomenon represents the first global threat to endangered marine mammals.