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SVP 9 – Origin of snakes: Da Silva
Da Silva 2015 takes on the origin of snakes using skull shape. Notes follow (*):
From the abstract:
“The origin of snakes is a contentious topic with three competing hypotheses: aquatic, terrestrial or fossorial*. The snake fossil record is poor with a few preserved complete skulls dated back to the Cretaceous**. Phylogenies using discrete morphological data and including fossils are contradictory regarding the ophidian ancestor***. Thus, alternative approaches that aid tracking down the lizard-snake ancestral transition are necessary****. Comparisons of quantitative data such as skull shape of extant and fossil taxa but also ontogenetic trajectories of skull development are relevant alternative approaches. In this study, we analyzed for the first time more than 600 extant and extinct taxa representative of all major Squamata families using two- and three-dimensional landmarked-based geometric morphometrics. We also mapped a consensus phylogeny onto the morphospace and estimated ancestral shapes with Parsimony. Lastly, we traced 61 skull
ontogenetic trajectories with principal component analysis. We first found that snakes and lizards occupy different parts of the morphospace, except for many convergent fossorial forms. Shape transitions are gradual and strongly linked with ecology. The first axis of variation largely accounts for changes in the braincase and quadrate. Interestingly, ancestral estimations recovered the most common ancestor of snakes as a small fossorial similar to Anomochilus*****, while Cretaceous snakes show intermediate skull shapes similar to boas and pythons. Ontogenetic trajectories of snakes and lizards are linear and overall parallel phylogeny in snakes. Young embryos of Alethinophidia have similar shape to terrestrial adult lizards and trajectories are clearly peramorphic. Adults and embryos of Scolecophidia are located at the base of lizard ontogenetic trajectories, likely indicating neoteny. Altogether, our data indicate that skull shape and ecology are strongly connected, supporting the hypothesis that modern snakes lineages originated from a fossorial snake ancestor through an early transition from terrestrial lizards. Lastly, natural selection fine-tuned skull ecological function upon variation generated by heterochrony.”
*The large reptile tree solved this problem. The answer is secondarily terrestrial from aquatic ancestors. For more information, start here.
** Not so. the record is wonderfully complete. Da Silva just did not recognize it yet.
*** True. But then, none but the large reptile tree employ Jucaraseps and Tetraopodophis.
**** Not so. Just taxon inclusion and phylogenetic analysis will solve the problem.
***** No. Anomochilus is a highly derived burrowing form, not a basal snake in the large reptile tree.
References
Da Silva FO 2015. Skull shape supports a terrestrial – fossorial transition in the early evolution of snakes through heterochrony. Journal of Vertebrate Paleontology abstracts.
Source: https://pterosaurheresies.wordpress.com/2015/11/04/svp-9-origin-of-snakes-da-silva/
