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Re: Eodromaeus, new basal theropod from Triassic in Argentina



That there are differences between structures does not make them
non-homologous. For example, some bat hair is more complicated than
common mammal hair, yet it is homologous with the latter. If
pterosaur, ornithischian, and coelurosaur "hair" resemble each other
in at least one aspect (being thin and elongate), they may be
considered homologous as far as we do not found a sufficient number of
very well preserved fossils of the clade in question (Ornithodira)
where the complete integumental cover is found, and which thus
explicitly indicate the at least slightly similar integumental
features are not homologous.
If it is accepted that the Pterosauria lies outside of the Archosauria
(crown-group definition), it does not have to follow that all the
non-dinosaurian archosaurifoms more related to the Dinosauria than to
the Pterosauria are more parsimoniously considered as presenting
"hair". It would be equally parsimonious to accept the group defined
by the most immediate ancestor of the Pterosauria and Archosauria
acquired "hair" and it was later lost by crocodiles (two steps,
appearance and later reversal to lacking hair), and to alternatively
accept "hair" independently appeared in the Pterosauria and Dinosauria
(two steps, appearance of "hair" in Pterosauria, and appearance of
"hair" in Dinosauria).