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>D. Squires asked about the relationship between mammalian hair and
>reptilian scales. There are a copule of theories banging about. One suggests
>that hair "is homologous with "certain sensory horny appendages of aphibia and
>reptiles". Histolologically, the warts of toads are different from hairs, but
>the PROTOTRICHs in lizards have been considered a possibility (argued mainly
>by Elias & Bortner 1957 (not a type, I know my typing is bad!). The alternative
>stated in numerous places by RIC Spearman (died last year, sadly) is as
>follows: "in the absence of fossil material...the weight of evidence from
>embryology, histology, and histochemistry supports the...theory that hairs are
>entirely new horny appendages which arise not from scales, but from the
> epidermal hinge regions posterior to each scale."
> This is not unlike what has been suggested for feathers. On the other
>hand, feather and hair follicles work very differently. On the positive side,
>the proteins derived from hair, wool (and there is a lot of work here), etc
>is much closer to the alpha-keratin of skin than is the phi-keratin of
Actually, it should not be expected that hair is derived from modern
sauropsid scales. It is indeed possible (as has been argued by J. Gauthier
and N. Hotton) that no ancestor of mammals above fish had scales. All
modern tetrapods (four-footed vertebrates) with reptilian scales share a
common ancestor more recent than the split with the ancestors of mammals.
Both amphibians and mammals (but not turtles, lepidosaurs, crocodilians, or
birds) have vascular skin, which is probably a primitive feature for
tetrapods. There is no evidence that the non-mammalian synapsids (things
like the sail-backed Dimetrodon and Edaphosaurus, dicynodonts, Cynognathus,
etc.) had scales. In fact, paintings to the contrary, they probably had
naked skin, proto-fur, or fur itself.
The assumption that Dimetrodon, dicynodonts, Cynognathus, etc. had scales
stems from the old concept of "Reptilia" (i.e., any hard-shelled egg laying
vertebrate that isn't a bird or mammal). Under a cladistic definition, the
so-called "mammal-like reptiles" aren't reptiles at all, and attribution of
lizard- or crocodile-like characteristics to them is probably unwarranted.
Thomas R. Holtz, Jr.
Vertebrate Paleontologist in Exile Phone: 703-648-5280
U.S. Geological Survey FAX: 703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA 22092