A 'transitional fossil' debunked
Snakes are a very diverse group of present-day reptiles, with nearly 3,600 known species. They are readily recognized by their long bodies and lack of limbs. The origin of snakes from lizard-like precursors with paired limbs has long been a controversial subject. This reflects the lack of fossils and conflicting results from phylogenetic assessments using molecules and anatomy, respectively. Thus a 2015 report announcing discovery of a 110-million-year-old skeleton of a snake-like reptile from the Cretaceous of Brazil generated worldwide interest.
|Tetrapodophis specimen [Credit: Dave Martill/University of Portsmouth]|
The subject of the 2015 report is an articulated 20 cm long skeleton of a snake-like reptile from the Cretaceous of Brazil. Named Tetrapodophis ("four-footed snake"), this fossil has a long body with some 160 vertebrae in front of the tail but also four very small limbs. Such a combination of features had never been observed in any lizard or snake. Tetrapodophis purportedly showed that body elongation preceded loss of limbs in the evolution of snakes and thus gained international attention as a "transitional fossil."
|Tetrapodophis hindlimb [Credit: Dave Martill/University of Portsmouth]|
Caldwell concludes: "This specimen challenges a number of long held ideas about the evolution of elongation and limb reduction in tetrapods—it displays anatomical features that seem to break all the rules. To make it more complicated, the skull is poorly preserved and the animal is extremely small, thus making it hard to pinpoint morphological features linking it to any one particular group of squamates [the group comprising lizards and snakes]. It is a perfect paleontological storm in every way."
Source: Society of Vertebrate Paleontology [October 29, 2016]