The oldest known fossil sea turtle has been discovered near the town of Villa de Leyva in Colombia. Dated to around 120 million-years-old, the specimen outdates the previously oldest known example by 25 million years. The researchers were able to study an “almost complete” skeleton of the animal, plus an additional four skulls and two partially preserved shells, which were actually found in 2007 but had been sitting in collections in both Colombia and California since.   

The creature measured around two meters long and showed all of the characteristics and traits of modern marine turtles, adding an important link in the evolution of turtles. “The turtle described by us as Desmatochelys padillai sp. originates from Cretaceous sediments and is at least 120 million years old,” said Dr Edwin Cadena, co-author of the study to be published in PaleoBios. “This lends a special importance to every fossil discovery that can contribute to clarifying the phylogeny of the sea turtles.”

The evolution of turtles is a convoluted and hotly debated story. With few intermediary fossils showing how they got their shells, it’s difficult to trace their origin. One theory, based on fossils of the oldest known extinct species of turtle, called Odontochelys, suggests that the group started to evolve their distinctive anatomy in water, before moving back onto land. This is because Odontochelys had a well-developed plastron, or bottom plate, but lacked a carapace, or top of the shell.

It’s suggested that this could be because a plastron might be better to protect against underwater predators that the semi-aquatic Odontochelys faced, and only when they returned to land did a carapace then become necessary, as the predatory threats then came from above. It was then that marine turtles evolved from fully shelled land turtles that had returned to the sea.

Based on its physical characteristics, Cadena and his colleague have placed Desmatochelys in the turtle group Chelonioidea. This group is still around today, with six out of the seven living species of sea turtle found within it. This would therefore suggest that the only other living sea turtle, the huge leatherback, split from the other species over 120 million-years-ago, when ichthyosaurs and plesiosaurs were still cruising the oceans.