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.

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2 thoughts on “SVP 9 – Origin of snakes: Da Silva

  1. Hi, here is Da Silva writing.

    Thank you for mentioning my abstract, the work was still in progress when I wrote the abstract, it would have been nice if you attended to my talk in the conference. Also, I appreciate your short comments on the parts you think are incorrect but I would like to make a few things clear here regarding your comments.

    *The large reptile tree solved this problem. The answer is secondarily terrestrial from aquatic ancestors. For more information, start here.
    Well, I don’t think it is the case. I think you are just assuming it. The literature is full of disagreements regarding the origin of snakes: phylogeny, ecology, skull evolution, and development. Trust me, we would not been working on a problem if it would be well-understood.

    ** Not so. the record is wonderfully complete. Da Silva just did not recognize it yet.
    Neither wonderful not complete, I really do not known what wonderful would mean here for you or complete, this implies you have a clear expectations of how the snake transition took place. That is not the case though considering the literature. Moreover, the truth is a dangerous concept to to rely on. But above all, I mentioned there that a few skulls have well-preserved shape, remember, that is what I was investigating. Most early fossils, whether they are snakes or not, have fragmented remains, just like early-branched snakes such as Coniophis and Najash (this one better preserved and new skulls will be published, but I did not have access to them). Skulls that are complete enough and have skull-shape preserved or that has been reconstructed are all from the Cretaceous or Cenozoic.

    *** True. But then, none but the large reptile tree employ Jucaraseps and Tetraopodophis.
    I am not adding Tetrapodophis in my analysis because its skull shape is incomplete (back of the skull) and has the shape only partially preserved.

    **** Not so. Just taxon inclusion and phylogenetic analysis will solve the problem.
    That is your background, I respect your opinion. But it is certainly limited in the approach. First, relationships on the tree do not imply that the ecology of the ancestor is similar to the sister group, we need to estimate the ancestor. Phylogenetic analysis provided the framework where I could analyze skull shape evolution and track the shape of the ancestor skull. As shape and ecology are partially associated, I could then predict the ecology of the ancestor based on the skull shape. Development has not been used to address the problem of how the skull of snakes evolved in the first place. I used large-scale sampling for that, which is a new approach and really revealed new insights about snake evolution previously unmentioned as you can read in the abstract. There is more to come, we hope to get the paper published soon, so you can evaluate it in its full potential.

    ***** No. Anomochilus is a highly derived burrowing form, not a basal snake in the large reptile tree.

    I did not say that, if you read it again I said the shape is similar to, not that Anomochilus is basal. But in fact, the story changed a bit when I added lizards to the analyses, let’s wait for the paper to come out.

    Hope I have addressed some of the misunderstandings regarding my abstract.
    Cheers
    Filipe

    • Thank you for your informed comments, Filipe. Whether a snake is above ground, below ground or in the in-between ground (compressed and decaying leaf litter) is going to be difficult to assess. I look forward to your paper. The phylogeny here seems to produce a gradual accumulation of derived traits and niches, which is all to the good. Dave

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