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Submitted: July 11, 2012
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Development of Skink Toesby *EWilloughby

Digital Art / Drawings / Technical Drawings

This is another diagram for the book, and is a collaboration between myself and ~Agahnim. 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.

Caption reads: 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.

~Sinande Jul 12, 2012

Nice! I hadn't known that the developmental frameshift thing also happened elsewhere!

BTW, I kinda hate it that it's called a frameshift. A frameshift mutation already means something, and it's not this. *grumps*

~Agahnim Jul 12, 2012

Are you sure it's not the same actual type of mutation? I'd always figured that it was, and that frameshift mutations can just have all sorts of different effects.

~Sinande Jul 13, 2012

A frameshift mutation is a technical term. It only refers to changes at the DNA level that change the reading frame of a protein-coding sequence (i.e. indels that aren't in multiples of three). AFAIK their most common consequence is a malfunctioning protein courtesy of a premature stop codon.

There is absolutely no reason a developmental frameshift should be caused by a frameshift *mutation* - I'm pretty sure it is instead underlain by shifts in gene expression patterns. It *could* be that some of those result from frameshifts in some transcription factor gene involved in the regulation of digit identity genes, but I see no reason to think this is more likely than a plain old enhancer mutation, or a change in a morphogen gradient.

~Agahnim Jul 14, 2012

I'd known what frameshift mutations are, but from the fact that the change in digit identity between theropods and birds is also referred to as a "frameshift", I'd always assumed it had been determined somehow that a frameshift mutation was what caused it. Thanks for explaining that this is a separate, unrelated meaning of the word. I guess I should probably explain that in the chapter, since earlier in the same chapter I also discuss an actual frameshift mutation, when describing the nylon-eating bacteria.

!FatCaiman Jul 12, 2012 Hobbyist Traditional Artist

That is a really interesting and relevant example! I figured there had to be some explanation for why birds have a different toe arrangement than theropod dinosaurs. Yeah, my ornithology professor was a BANDit and he posited this as a reason birds are not descended from dinosaurs.

I can't wait to see this book.

~electreel Jul 12, 2012

Quite interesting!

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