The loss of hands in proto sea snakes documented in Adriosaurus
In contrast to widely-held traditional theories
that report snakes had a controversial single origin (Lee and Caldwell 2000, Lee and Scanlon 2002), the large reptile tree recovers a heretical two origins for modern snakes.
One set of snakes, led by the Gila monster, Heloderma, are largely burrowing forms, such as Cylindrophis, with increasingly odd jaws that ultimately rotate medially, like Leptotyphlops.
The other set of snakes, led by leggy Ardeosaurus and the elongated marine Adriosaurus (Fig. 1) ultimately led to non-burrowing snakes, like Boa and Pachyrhachis.
Today we’ll look at the reduction of the forelimbs in various species of Adriosaurus (Fig.1) as documented in Palci and Caldwell (2007).
Important to remember,
as noted by Palci and Caldwell, this forelimb reduction in Adriosaurus may be the sequence in which protosnakes lost their forelimbs, OR it may be a parallel and convergent sequence, since limb loss happens so often in elongated lepidosaurs (like skinks that evolve to become amphisbaenids).
Figure 1. Click to enlarge. From Palci and Caldwell 2007, various specimens of Adriosaurus documenting the reduction of large clawed hands to small clawless paddles, then ultimately disappearing completely. Below, all the humeri are scaled to the same length.
Palci and Caldwell (2007) wisely reported, “Any attempts to understand the evolution of anatomy, such as that displayed by Adriosaurus microbrachis, will be brought up short by the limitations of a static system, that is, the limits of an individual, when the real goal is to understand the transformation of those systems within populations, and between species and other higher taxa.”
This blog and ReptileEvolution.com are devoted to showing and explaining the gradual accumulation of derived traits (which sometimes includes loss of bones). Sometimes no explanation is necessary as the series in Adriosaurus (Fig. 1) so grandly shows.
Interesting
that despite the major differences in fore limb morphology in these Adriosaurus specimens, all four are considered congeneric by Palci and Caldwell 2007.
Short note:
Earlier I posted three phylogenetic matrices on MorphoBank.org. Several interested parties downloaded work from there. Recently, however, the regents there decided they wanted to restrict posted matrices to those that had been peer-reviewed and published. So, my matrices are no longer available there. Nevertheless, anyone interested can always write to me directly through this website and request a matrix.
References
Lee MSY and Caldwell MW 2000. Adriosaurus and the affinities of mosasaurs, dolichosaurs, and snakes. Journal of Paleontology 74:915-937.
Lee MSY and Scanlon JD 2002. Snake phylogeny base on osteology, soft anatomy and ecology. Biological Reviews 77:333-401.
Palci A and Caldwell MW 2007. Vestigial forelimbs and axial elongation in a 95 million-year-old non-snake squamate. Journal of Vertebrate Paleontology 27(1):1-7.
Source: http://pterosaurheresies.wordpress.com/2013/11/30/the-loss-of-hands-in-proto-sea-snakes-documented-in-adriosaurus/
