Chalcides ocellatus is a basal extant skink. It has a basic (plesiomorphic) hand and foot, each with five digits and very few derived traits (Fig. 1, like the fusion of m4.3 + m4.4 and the fusion of mc1 + c1).
By contrast, another species, C. chalcides, the Italian three-toed skink, has but three toes on both the manus and the pes, which are, at best, tiny vestiges on tiny vestigial limbs.
From the Young et al. (2009) abstract:
“Digit identity in the avian wing is a classical example of conflicting anatomical and embryological evidence regarding digit homology. In recent years, gene expression as well as experimental evidence was published that supports the hypothesis that this discrepancy arose from a digit identity shift in the evolution of the bird wing. A similar but less well-known controversy has been ongoing since the late 19th century regarding the identity of the digits of the three-toed Italian skink, Chalcides chalcides.The data confirm that the adult and the embryological evidence for digit identity are in conflict, and the expression of Hoxd11 suggests that digits 1, 2, and 3 develop in positions 2, 3, and 4. We conclude that in C. chalcides, and likely in its close relatives, a digit identity frame shift has occurred, similar to the one in avian evolution.”
The three-toed skink (Chalcides chalcides, Fig. 2) is more derived than the five-toed skink. Digits 4 and 5 are outwardly absent from both the manus and pes. But note the buried vestige of digit 4 in both the manus and pes. The phase shift described by Young et al. happens during embryology (not shown here). In the adult basic homologies are maintained. The medial digit is #1.
Here’s a possible explanation for apparent “Phase Shift” during embryogenesis.
Looking at the entire family tree of amniotes and tetrapods, it appears that this so-called “phase shift” may have its roots in basal tetrapods which had more than 5 fingers and toes (Fig. 3) and our genes remember that part of our ancestry. Remember there are gills in tetrapod embryos, too~! And gills goes back in further in our genetic ancestry.
As in Acanthostega,
that little bud developing medially on the embryo bird manus (Fig. 3) is probably not digit 1. Rather, it appears to be homologous to digit “pre-1” on Acanthostega (Fig. 3), making a brief appearance during embryo development before disappearing as embryo growth continues. I don’t think the proper term for this is “phase shift”. Rather it is an ephemeral and short-lived appearance of digit pre-1 that probably occurs in most tetrapods.
Has this been considered before in academic publications? If so, it’s a convergent hypothesis.
Young RL, Caputo V, Giovannotti M, Kohlsdorf T, Vargas AO, May GE ,a and Wagner GP 2009. Evolution of digit identity in the three-toed Italian skink Chalcides chalcides: a new case of digit identity frame shift. Evolution and Development 11:6, 647–658.