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Re: Molecular clocks and avian diversification
evelyn sobielski wrote-
But *is* an ossified sternum a monophyletic trait? I'd
rather be tempted to put it on hold in the "avian
characters theropods were prone to evolve one way or
another" list. Just as with feathering, flight, beaks
or fusion of distal caudal vertebrae.
Ossified sterna certainly seem to have arisen only once - in
maniraptoriformes. Alvarezsaurids (Mononykus, Shuvuuia), Pelecanimimus,
oviraptorosaurs (Caudipteryx, Protarchaeopteryx, the CMNH caenagnathid,
Rinchenia, Citipati, Conchoraptor, Khaan, "Ingenia", Heyuannia),
dromaeosaurids (Microraptor/Cryptovolans, Sinornithosaurus, Bambiraptor,
Velociraptor, Adasaurus) and birds all have them. The only exceptions are
archaeopteryids, Sapeornis and Jinfengopteryx (and perhaps other troodontids
like Mei and Sinornithoides). My own feeling is that archaeopterygids and
troodontids may be sister taxa, while Sapeornis is a separate lineage that
may be more basal than its pygostyle leads us to believe.
I'd have to see specimens to decide whether ossified
sterna are symplesiomorphies or parallelisms. There
are far too many of the latter in the whole theropod
clade to make me cautious. Archie and Rahonavis both
had flight-enhancing pelves, but looking at specimens
it is quite obvious they are very much homoplasies as
far as adaptations for flight are concerned (the
hypopubic cup of Archie and the hypertrophied pubic
boot of Rahonavis, to be precise). Possibly very much
equivalent - but not at all equal.
Neither Archaeopteryx nor ANY other bird has a hypopubic cup. In
Archaeopteryx, the structure is a mass of calcite with an artificial
concavity (Norell and Makovicky, 1999).