Re: Molecular clocks and avian diversification

[Michael Mortimer <mickey_mortimer111@msn.com>]

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).

Mickey Mortimer