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Dermal structures



Dinogeorge wrote:
<Actually, I see feathers/featherlike dermal structures in the
dinosaur/archosaur clade as far back as _Longisquama_. And I see keeled
ankylosaur scutes and spines, stegosaur plates and spines, sauropod dorsal
spikes, and other such dinosaurian dermal structures as probably deriving
from the protofeathers of the ur-dinosaur. True feathers, of a kind we would
recognize as feathers were we to see them fossilized, probably first appeared
sometime during the Triassic in the theropod-bird clade. (Rank speculation of
the most egregious kind, of course.)>

Cool speculation though.  As a molecular biologist/cell biologist who attended
Allan Brush's excellent Dinofest lecture on feather molecular biology and the
ease with which gene mutations can rearrange feather morphology, I tend to
agree.  The mineralization that occurs in dermal bones would definitely follow
the cellular patterns that pre-exist where scutes, etc. are laid down, and if
these cells were in feather-follicle arrangements, then voila! - ridged and
keeled dermal bones are LIKELY, let alone possible.  Good thinking.
    May I add a speculation perhaps more rank than yours?  Since feathers keep
getting ancient-er, and you want to put related, follicle-derived structures
on ornithiscians etc., let's go further.  I can think of another follicle-
bearing lineage, namely, mammalia, what with all the hair and whiskers and
stuff.  Let's do some thinking along an inevitable line of logic: follicles
(indentations in the skin that arise embryologically by the invagination of a
dermal placode or small "plug" of epidermal cells) may have arisen only once
in evolution, to give rise to three notable things; 1) dermal scutes, 2)
feathers, and 3) hair.  I can tell you one thing I know from my days as a cell
biologist, the follicle structures of hair and feathers are incredibly
similar, hence, I think they are likely to have a common evolutionary origin.
    My only qualm is about where to stop.  The common, follicled ancestor of
mammals and birds would be found among Permian reptiles, I think.  But images
of fuzzy  Devonian salamanders come to mind.