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Re: Mononykus



In a message dated 97-03-28 16:28:28 EST, znc14@ttacs1.ttu.edu (Jonathan R.
Wagner) writes:

<< Ceteris
 paribus it is more parsimonious to consider that within Coelurosauria the
 keeled sternum evolved only once (contra the implications of Hou, Zhou,
 Martin, and Feduccia 1996). >>

Sure, except that the keeled sternum is >plesiomorphic< to Coeluria, since it
is known in at least one pre-coelurian theropod (the no-longer-headless
wonder from Dinosaur National Monument, which also displays a giant furcula).
The keeled sternum was lost repeatedly, along with the furcula and volant
wings, in many if not most Cenozoic flightless birds, and it was likewise
lost in most cursorial, flightless Mesozoic non-avialan theropods--which
never had such strongly keeled sterna as the ancestors of the various
Cenozoic flightless avian groups--as well. In the non-avialan theropod
_Mononykus_, the keeled sternum was incompletely lost and probably retained
some function related to the highly derived forelimbs. Vestigialization of
the fibula also likely occurred repeatedly among the more volant birds.

Vestigialization of a body part through functional loss is quite common and
rapid in vertebrate evolution, so great care must be taken in assessing the
presence or absence of characters such as a keeled sternum and a reduced
fibula in cladistic analyses of theropods and birds. Nature need not be
parsimonious at all and can frequently be convergent when it comes to
vestigialization.