Development of Skink Toes
A collaboration between myself and Jonathan Kane. I drew the images, and he created the text, all of the formatting, and the caption below. This image relates to an explanation for the reason why theropod dinosaurs appear to have lost their two outermost fingers, but bird embryos form and then lose the inner- and outermost fingers. If birds evolved from dinosaurs, and ontogeny recapitulates phylogeny, then one would expect that the two long-lost fingers that briefly appear in the embryo of a bird to be the same two lost in primitive theropod dinosaurs. However, this is not the case, and this seeming inconsistency has been a common argument of both creationists and BANDits for decades.
However, it was eventually established that a digit frameshift is responsible for this exact sort of inconsistency. Of course the genetics of long-extinct theropods cannot be examined, but the same mutation with the same effect has been observed in a number of living organisms, and these skinks are perhaps the best example.
This diagram shows the embryonic development in the two-toed earless skink compared to in the four-toed earless skink. Although the two toes of the two-toed skink have the same size, shape, and number of joints as the inner two toes of the four-toed skink, they develop from the middle two toes of their embryo. This difference is caused by a type of frameshift. Diagram adapted from Shapiro 2002.*
*Shapiro, Michael. “Developmental Morphology of Limb Reduction in Hemiergis (Squamata: Scinidae) Chondrogenesis, Osteology, and Heterochrony.” Journal of Morphology 254 (2002): 211-231.