The discovery of a rare Cretaceous fossil that may have been a missing link in the evolution of modern snakes made headlines in 2015 Tetrapodophis (“Four-footed serpent”) and proved controversial from the start, with some paleontologists questioning the interpretation that it was a protosnake. Now there is strong evidence that this latter view might be correct and that according to a. is more of an early species of lizard a new paper published in the Journal of Systematic Paleontology.
Paleontologists have long suspected that at some point in the distant past, snakes evolved from lizards and gradually shed their limbs. So there should be an evolutionary predecessor with four limbs. This prediction was confirmed in 2006 with the discovery of a snake-like transition fossil (Najash rionegrina) with two hind legs that are around 95 million years old. There is also an ongoing debate about whether snakes came from a marine or terrestrial environment, and the 2006 fossil supported the latter hypothesis.
Then, in 2015, David Martill of the University of Portsmouth and co-author Nicholas Longrich of the University of Bath published a description of a four-legged fossil, they claimed, was the first known example of a four-legged protosnake with front and rear legs in the fossil record. Martill had stumbled upon the fossil at the Solnhofen Museum in Germany, which was part of a larger fossil exhibition from the Cretaceous Period.
According to Martill, the fossil had many familiar features of a snake, with the exception of its tiny arms and legs, each with oddly long fingers and toes that would have been useful for digging – additional evidence to support the arguments for terrestrial origin. It had 160 spinal vertebrae and a further 112 vertebrae in the cylindrical (as opposed to the flattened) tail. There were also scales extending across the abdomen, an elongated body, sharp hook teeth, and a skull (about the size of a human fingernail) with a short snout and a long brain case. Another animal’s bones in the gut suggested the creature was likely a carnivore.
While some paleontologists described the find as exceptional, others expressed skepticism, most notably Michael Caldwell of the University of Alberta in Edmonton, Canada, co-author of this latest article. At the time, he pointed out that most known snakes and lizards have vertebrae with concave front surfaces and convex rear surfaces, but this did not appear to be the case with Tetrapodophis. The specimen ‘s vertebrae also appeared to be missing a small bone called the intercentrum. Caldwell suggested that Tetrapodophis likely belonged to another large group of amphibians that became extinct about 251 million years ago.
Caldwell presented a formal rebuttal at the Society of Vertebrate Paleontology (SVP) meeting in 2016, based on his further observations of the slab of rock containing the fossil, which was a natural mold. This gave him a clearer view of the skull in particular, as this shape retained several features that were not visible in the original study. As Science reports At that time, “in snake skulls, a bone called quadrature is elongated, allowing snakes to open their jaws very wide. The square bone of this fossil is more C-shaped and surrounds the animal’s hearing aid – a characteristic of a group of lizards called squamates. “
There was one additional wrinkle in the story that fueled the controversy. Questions about the origin of the fossil had already arisen. Its composition coincided with the fact that it was excavated in limestone quarries in Brazil, much of it in the second half of the 20th century. However, Brazilian laws passed in the 1940s made all fossils the property of the state and it was unclear how the specimen was had reached the Museum Solnhofen.
And when Caldwell contacted the museum to get access to the fossil for further study, his request was denied. It turned out that the copy belonged to a private collector and had only been on loan to the museum. The owner removed the fossil after it was damaged during CT scanning at the European Synchrotron Radiation Facility in Germany. Some researchers felt that this should be further investigation Tetrapodophis scientifically controversial, as any findings would not be verifiable if the fossil remained inaccessible.