In collaboration with a colleague from Zürich, Ingmar Werneburg from the Museum für Naturkunde Berlin discovered how snakes lost their limbs. Those surprising results were published in the international journal Evolution. Inside vertebrate embryos, the specific bones of the body develop in a particular sequence. Those sequences differ among species and the comparison of shared and derived characters permits conclusions about evolutionary changes of bone formation. A phylogenetic analysis uncovered that, through reptilian evolution, several bones appear later and later until, in snakes, they do not form at all. This scenario explains the reduction of legs and of several skull bones in snakes.
Snakes belong to the most fascinating reptiles. Based on their scurrile body form and their behavior they are an integrative part of mythology and human psyche. Extant snakes do not have limbs. However, they can move by winding or by swimming without a problem. Also several skull elements are reduced, which enabled the great mobility within the head. Snakes are able to expand and stretch their jaws in order to feed on much larger objects in regard to body size. Many species have poison fangs, which are anchored inside the jaws and help to paralyze or kill their prey.
The origin of snakes is highly debated. Either they originate from lizard-like land vertebrates, from fossorial forms, or from marine reptiles. Anatomical and genetic studies resulted in different results in regard to snake origin. Interestingly, many fossil snakes and even some living species such as pythons and boas exhibit rudiments of hip bones and even hind limb bones. An unsolved issue concerns the question on how the ancestors of snakes lost their limbs. In the case that the fossil record and comparative anatomy do not provide a solution of such problems, it is worth to study embryonic development of animals. For that, the scientists studied how the patterns in the sequence of bone ossification changed through reptilian evolution. They compared micro-computer-tomography scans and skeletal staining preparations of different reptilian embryos and mapped the data on a phylogenetic tree. Independent of which hypothesis on snake origin was used (terrestrial, fossorial, or marine), the pattern was always the same for all reptiles.
When compared to their ancestors, extant reptiles show a temporally late appearance of many bones. This is culminated in snakes, in which several bones appear extremely late in the embryo. Afterwards, those bones only have little time to develop and consequently they are only small in the adult animal. That explains the reduced shape of the snake skull. The first embryonic appearance of some bones, however, is so late in reptilian evolution that, in snakes, they do not form at all. Among others these are bones of the shoulder girdle and the forelimbs and finally those of the pelvic and the hind limbs. The evolutionary tendency of late appearing bones in embryos is correlated to the phenomenon that lizards evolved a gracile body form through evolution. In this regard, snakes resemble the most extreme example.
Published in: Werneburg I, Sánchez-Villagra. 2015. Skeletal heterochrony is associated with the anatomical specializations of snakes among squamate reptiles. Evolution 69(1): 254–263. doi: 10.1111/evo.12559