Found in a water hole, a turtle fossil delivers interesting insights into the evolutionary history of Chelonoidis, a kind of South American turtle.
1000 years ago, the islands of the Caribbean were virgins. The endemic species of birds, mammals and reptiles that lived there disappeared within a few centuries after the arrival of humans. Among them, Chelonoidis alburyorum, a giant tortoise species whose carapace could reach a size of nearly 50 centimeters.
Usually, in the tropics, due to climatic conditions, fossil DNA is not conserved. But a German-American team led by Christian Kehlmaier of the Dresden Zoological Museum was able to sequence almost completely the genome of a C. alburyorum fossil found in the Sawmill Sink Abyss in the Bahamas. An advance that provides useful insight into the dispersal of terrestrial turtles on the surface of the Earth.
The Sawmill Sink Abyss, more than 30 meters deep, is what is called a blue hole: a natural excavation filled with water (mostly sea water). Rarely, Sawmill Sink is a terrestrial blue hole, but it has already been invaded by the sea. It contains deep, sweet, salty layers of water, which, as in other abysses of the genus, are anoxic; without oxygen.
This explains why collagen containing DNA has been preserved in the bones of the fossil turtle. Even if this genetic material was contaminated, the researchers were able to restore almost all of it with a good confidence index. What does it show?
First, that C. alburyorum forms a clade – a group of species sharing the same ancestral traits – with the Argentine tortoise (Chelonoidis chilensis) and the giant tortoise of the Galapagos (Chelonoidis vicina). It thus appears as a manifestation of the genus Chelonoidis, comprising 13 species, all South American.
The calculations of the researchers lead them to conclude that the first common ancestor of the three species lived about 15.5 million years ago, making it about 7 million years after the divergence between the genus now south American Chelonoidis and the African genus Geochelone at the end of the Oligocene (33.9 to 23.03 million years) and 3.5 million years before the divergence between the turtles of Argentina and the turtles of the Galapagos towards The middle of the Miocene (23.03 to 5.332 million years).
Thus, C. alburyorum appears to be one of the forms produced by Chelonoidis radiation by the oceanic route (towards the Galapagos and the West Indies), long after another oceanic radiation led the Geochelons Africa in America some 25 million years ago. How was this possible across an Atlantic Ocean that was already at least 1500 kilometers wide? How long did it take? Mystery.