Unexpected centralia in the ichthyosaur, Chaohusaurus

A recent paper
by Montani et al. 2015 purported to indicate the presence of two centralia in the wrist of a juvenile basal ichthyosaur, Chaohusaurus (AGM CH-6628-22). It was not present in the adult. That, on its face of it is odd. The specimen, despite appearing to be undisturbed, lacked an ulna. That is also odd. Finally, no sister taxa have centralia. So the appearance here (Fig. 1) is triply odd.

Figure 1. Reinterpretation of Motani et al. 2015 showing how the purported radius could be a radius and ulna overlapping, the lateral centrale (lc) could be distal tarsals 3 and 4, and the medial centrale (mc) could be m4.2. Metacarpal "0" could be a part of the ulna. No sister taxa have centralia.

Figure 1. Reinterpretation of Motani et al. 2015 showing how the purported radius could be a radius and ulna overlapping, the lateral centrale (lc) could be distal tarsals 3 and 4, and the medial centrale (mc) could be m4.2. Metacarpal “0” could be a part of the ulna. No sister taxa have centralia. Click to enlarge.

Montani et al. report. 
“no amphibious sister taxa to ichthyopterygians have been discovered so far.”

Not so.
For the last four years the large reptile tree lists many sister taxa of increasing distance to the Ichthyopterygia, beginning with Wumengosaurus, basal mesosaurs and the several pachypleurosaurs that led to these taxa. The centralia is absent over several nodes prior to their appearance.

Centralia last appear
in Claudiosaurus, Adelosaurus, Sinosaurosphargis and Largocephalosaurus, but not thereafter. As the Enaliosauria becomes more aquatic, carpals are lost, beginning with the two centralia. Pachypleurosaurs do not have centralia. Neither do mesosaurs.

The (AGM CH-6628-22) specimen
of a juvenile Chaohusaurus that Motani et al. believe to have centralia (Fig. 1) has widely spaced and largely cartilaginous (poorly ossified) elements, some of which, like the ulnare and radiale are clearly disturbed from their in vivo placements. There is a long bone they label the lateral centrale and a short bone they label a medial centrale where such bones belong and this is the basis for their claim. There is even a medial metacarpal “0”, which anchors a sixth medial digit in the related Hupehsuchus, but is not known in Chaohusaurus, which is not a basal ichthyosaur.

We saw a similar reappearance
of digit “0” in Limusaurus, a theropod with embryonic hands retained into adulthood. Claudiosaurus, Adelosaurus and Sinosaurosphargis have a pisiform lateral to the ulnare, but it is similar in size and shape to the ulnare. More derived enaliosaurs lack a pisiform along with the centralia.

The problem is,
the lateral centrale in the above named enaliosaurs is not elongated (double wide), as it is purported to be in the juvenile Chaohusaurus, but rounded and similar in size and shape to the medial centrale.

A solution:
In my experience with missing bones alongside extra bones the answer might be to reinterpret the extra bones as the missing bones, only displaced. Perhaps the ulna is not missing from the juvenile Chaohusaurus, but instead is resting partly atop the radius (Fig. 1) as it appears to do so with that white line dividing the pair. If so the ulna can be restored, but the medial part is damaged. Here the purported metacarpal “0” might be part of the ulna. That’s a guess. The lateral centrale might be distal tarsals 3 and 4, as in the adult “D” specimen (Fig. 1), double wide. The purported distal tarsal 4 then is reinterpreted as distal tarsal 2 here. The medial centrale is reinterpreted as m4.2, which is also missing. The new restoration more closely matches adult specimens and sister taxa. It would also be nice if somehow we could determine that more radius was hidden beneath that portion of the displaced ulna.

This is parsimony and phylogenetic bracketing at its best.
If correct, this scenario just requires you to accept that a certain amount of displacement occurred during taphonomy, as in our old friend, Sordes, the pterosaur. I have not seen the specimen. So, if correct, this is another example of DGS, digital graphic segregation, and an example of pulling more data out of a photograph than was pulled out with the specimen in hand.

References:
Motani R et al. 2015. New evidence of centralia in Ichthyopterygia reiterating bias from paedomorphic characters on marine reptile phylogenetic reconstruction. Journal of Vertebrate Paleontology. 6 pp.

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