Ran across this image of a Coelophysis manus
(Figs. 1, 2) in the SuppData to Xu et al. 2009 (the paper that introduces us to Limusaurus. Fig. 4), which we looked at earlier here. The fossil appears to be one of the best preserved examples of a Coelophysis manus. Every bone is laid out without disturbance.
6 possible phalanges here. Three are vestiges.
Xu et al. (2009)
introduced the concept of a “phase shift” in theropod digits to account for the medial bud in Limusaurus, which they counted as digit 1 and the similar bud in chicken embryos. In their hypothesis the other digits changed their appearance to resemble the digit that was medial to each one. One through Three became Two through Four.
Xu et al. 2009 did not clarify, but only muddied avian digital homologies. As we learned earlier, that medial digit is actually a ‘throwback” to basal tetrapods that once had an extra digit medially and is lost on almost all other taxa after hatching. Embryos retained it for a short time.
In this revised hypothesis
the appearance of the bud does not represent a “phase shift” of phalanges, but rather the appearance of a very old medial digit, digit “0”. The other digits retain their original pentadactyl numbers 1-5.
That brings us to
a specimen of Coelophysis (Figs. 1,2) that appears to preserve the manus without disturbance and with that rare medial bud. Perhaps this is another digit “0” — and this time with vestigial phalanges — if interpreted correctly.
Digit 0 on theropods appears to be retained as a fused medial element in some modern birds (Fig. 3). It’s that little bump medial to metacarpal 1.
I presume, are lost during excavation every so often. Are they worth scoring in phylogenetic analysis? I score them.
There goes that hypothesis
Earlier I thought digit 0 appeared on Limusaurus because it was an embryological artifact retained on a vestigial manus retained after hatching and maturation (Fig. 4). As everyone knows, the manus is not vestigial in Coelophysis, so… there goes that hypothesis – if that medial bud is indeed the ephemeral digit “0” and not something else.
Cope ED 1889. On a new genus of Triassic Dinosauria. American Naturalist 23: 626
Late Triassic Norian
Colbert E. 1989. The Triassic Dinosaur Coelophysis. Museum of Northern Arizona Bulletin 57: 160.
Xu X, et al 2009. A Jurassic ceratosaur from China helps clarify avian digital homologies. Nature 459, 940-944. doi:10.1038/nature08124