Dinilysia and the burrowing(?) origin of snakes

A recent paper
by Yi and Norell 2015 attempted to answer the controversy whether ancestral snakes were terrestrial burrowers or marine swimmers while looking at their ear vestibules. They looked at the stem snake Dinilysia (1.8m in length) along with other reptiles and noted that the spherical vestibule portion of the inner ear was large (Fig. 1), like those of other burrowing reptiles.

Then they put together a cladogram (Fig.1) that was missing pertinent taxa in the ancestry of snakes, as determined by the large reptile tree (Fig. 2).

Figure 1. Colored chart originally published in Yi and Norell 2015 with their cladogram of snake origins tied to burrowing, aquatic and generalst niches, I have added black bars for legless taxa and gray bars for vestigial leg taxa. The inner ear of Dinilysia is shown at the bottom.

Figure 1. Colored chart originally published in Yi and Norell 2015 with their cladogram of snake origins tied to burrowing, aquatic and generalst niches, I have added dark green bars for burrowers and light green for other taxa, black bars for legless taxa and gray bars for vestigial leg taxa. The inner ear of Dinilysia is shown at the bottom. It does resemble the inner ear of burrowers. Note the large number of legless taxa preceding leg taxa, revealing this as an untenable family tree. See figure 2 for a cladogram that makes more sense.

 

Legless taxa
are primitive in the Yi and Norell tree (Fig. 1). That marks their cladogram as untenable or at best, unlikely because legs would have to redevelop in Varanus and Heloderma in the Yi and Norell tree. By contrast, legless taxa are largely derived in the scleroglossan subset of the large reptile tree (Fig. 2, Bipes is a notable exception).

Figure 2. Scleroglossan subset of the large reptile tree. Generalists taxa duplicated in the Yi and Norell tree are shown in bright green. Burrowers shared in the Yi and Norell tree are in dark green. Legless taxa are black. Vestiges are in gray. Unknown are striped.

Figure 2. Scleroglossan subset of the large reptile tree. Generalists taxa duplicated in the Yi and Norell tree are shown in bright green. Burrowers shared in the Yi and Norell tree are in dark green. Legless taxa are black. Vestiges are in gray. Unknown are striped. Here legless taxa are largely derived/

The cladogram of Yi and Norell 
cannot tell us whether ancestral (= pre-Dinilysia) snakes were burrowers or not because Yi and Norell did not include any pre-Dinilysia ancestral snakes in their cladogram. That’s a major oversight.

By contrast,
the large reptile (Fig. 2) tree indicates that many pre-snakes, like Adriosaurus, were not burrowers — BUT BUT BUT — teeny tiny Tetrapodophis was a likely burrower, derived from a series of much larger swimmers, like AdriosaurusTetrapodophis was the size and shape of living burrowing snakes AND Dinilysia was the proximal descendant taxon in the large reptile tree. No one has yet revealed what sort of inner ear vestibule Tetrapodophis had. But IF it followed the pattern shown by Yi and Norell and had a burrowing-type large vestibule, it would follow that its proximal descendant, Dinilysia, might have shared that trait, no matter what its niche was. So the large inner ear vestibule of Dinilysia indicates that it could have been a burrower (at least until other data, like its large size, trumps the Yi and Norell results) or it could have been the descendant of a burrower.

Unfortunately, the large vestibule of Dinilysia cannot tell us whether or not pre-Dinilysia taxa were burrowers or not. We should look at the vestibule in those taxa from the large reptile tree. IMHO tiny Tetrapodophis was a likely burrower and Dinilysia was not by virtue of its much greater size.

Remember when we wondered if pterosaurs were quadrupeds or bipeds? 
And then we discovered they were both!

Were pre-snakes aquatic or burrowing?
Again, they were likely both!

References
Yi H and Norell MA 2015. The burrowing origin of modern snakes. Research Article Science Advances 2015;1:e1500743  27 November 2015

 

 

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