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Re: dermal structures



In a message dated 98-07-07 10:02:06 EDT, you write:

<< Just read Tom Hopp`s mention of "one time" evolution of dermal scutes,
follicles etc., I myself have a strong feeling about a once only development
of warm-bloodedness, and that being mainly for the incubation of the young in
a colder enviorn. In fact, I`m looking at endothermy before the synapsid-
diapsid split. (I don`t believe we should place diapsid beginnings way after
synapsid development based upon the scant fossil record of early diapsid
forms).>>

But based on the relatively good fossil record of early synapsid forms and
their distinct separation from all sauropsids I think it is pretty safe to
distantly separate the two groups .

<< Endothermy is complex, involving many organ systems, and requires large
expenditure of energy. It would be hard for a new form to develop it
independently, when already established in existing species due to
competition. So...I think it evolved only once, and that diapsids probably are
derived from the synapsid condition. And feathers, as well as hair, or at
least "fuzzy scales" would soon follow. (so we`re talking Pennsylvanian).>>

I really don't think this is a possibility (both common origin for hair and
feathers and a single Carboniferouas endothermic evolution), based on dermal
state of the fossil animals and comparison with modern forms. Synapsids, based
on what skin impressions there are, had a soft lissamphibian-style skin that
was secondarily derived from an amphibian scale structure, apparently.
Diapsids and their ancestors were certainly scaled, if not all with modern
reptile-style squamation. Hair came from the soft therocephalian or probably
cynodont therapsid skin, in an origin vastly separate from feather origins.
Also, on the topic of Carboniferous endothermy, why would all the living
representatives of this group be ectothermic? If an animal that for all
intents and purposes lived and looked like a lizard was endothermic back then,
why wouldn't animals living today that occupied the exact same lifestyle and
evolved from such a thing retain this adaptation? Diapsids are definitely not
derived from a synapsid condition, and I find it exceedingly doubtful that
endothermy could have come about in the sprawling insectivores akin to a
salamander that had no use for such a great energy expenditure and whose
fossils show bones geared for pure ectothermic life.

Sincerely,
Christian Kammerer