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Re: Triassic mammal-like reptiles?
<<I consistently see them called "mammal like reptiles".>>
You may do, Dora, but you'd struggle to find the term in current scientific
studies except in exclamation marks. I can't help it if other people get
the terminology incorrect. I suppose if Reptilia were expanded to include
all amniotes (including mammals), then the phrase could be justifiable.
This isn't a proposal I've heard being made, nor is it one I'd be likely to
take much notice of.
If you'd care to check the leg of a chicken, tortoise or crocodile, you'll
find the thing has heavy scales. These contain keratin alpha below and a
much harder layer of keratin beta on top. That's a derived condition among
amniotes, and its restricted to sauropsids. Those are sauropsid scales, and
similar structures are known from old skin impressions of sauropsids.
Fossilised synapsid skin is available from the Lower Permian, and there's no
sign of heavy scales. Rather, there's "a dense pattern of concave lens-like
structures", (/Estemmosuchus/ -with thanks to: Oftedal OT (2002), The
mammary gland and its origin during synapsid evolution, Journal of Mammary
Gland Biology and Neoplasia, 7(3), p.237). Living synapsid skin is
available from somewhere between 4,500 to 5,000 species. While some of us
favour having scales (eg. on the tails of mice), these tend to be restricted
to contact surfaces and don't come with the armour plating provided by
keratin beta. They're soft and elegant in comparison.
Our synapsid skin is relatively primitive stuff. It's glandular,
amphibian-like hide. We aren't descended from reptiles, whatever the
commentary on Discovery Channel might have said.