[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: New Jeholornis specimen

Gregory S. Paul wrote:

Yet another example that cladistics is at best hard pressed to handle the degree of reversal, parallalism, and convergence going on in the mosaic development of basal birds that had not yet evolved far from the dinosaurian grade, and were often losing and perhaps even reevolving flight.

I see what you're driving at - and having read both PDW and DA, I'm sympathetic. It is conceptually difficult to distinguish the incipient flight apparatus of "pre-flight" theropods, with the "post-flight" apparatus of secondarily flightless birds close to the origin of flight. Some of the latter may be unfairly pigeonholed into the former under the invocation of parsimony. However...

Cladistics is one important tool for restoring paleophylogentics, but not the only one, and in this case is inferior to assessing the relative derived degree of flight adaptations that are present.

I sense a subjectiveness in this method, which relies too heavily on assessing the "flightworthiness" of individual osteological characters. Take the more lateral orientation of the glenoid (for more anterolateral excursion of the humerus) or the broader second manual digit (for attachment of primaries). Both *might* be directly related to the inception of powered flight, specifically the flight stroke.

However, these and other maniraptoran / avepectoran characters may initially have evolved for other purposes. For example, the lateral glenoid allowed the arms to sweep forward to an improved degree - an advantage when grabbing large prey with both hands, or snatching flying prey out of the air with one or both hands. The improved attachment of primaries to the hand may not necessarily be an adaptation for resisting torsional forces during flapping flight, but for resisting the rigors of other behaviors that involved the hands. Grappling prey, for example. Or more rudimentary forms of aerial locomotion than flapping flight - ground-to-air leaps, or tree-to-tree (or tree-to-ground) glides.

Long story short, I think your model understates the role of exaptation in the assembly of the avian flight apparatus. As I said before, I don't dismiss the idea that many non-flighted maniraptorans / avepectorans were secondarily flightless. But proposing a volant pedigree for all these taxa is throwing the baby out with the bathwater, IMHO. Personally, I think maniraptorans underwent a long period of experimentation with aerial and arboreal behavior prior to the advent of powered flight; and that oviraptorosaurs, therizinosaurs, and deinonychosaurs are relicts of this process. But only birds took this process to the next (and most demanding) level of aerial locomotion: powered flight.

Fire away!


Tired of spam? Get advanced junk mail protection with MSN 8. http://join.msn.com/?page=features/junkmail