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Re: Archaeopteryx or Velociraptor



At 12:17 PM 7/8/97 -0400, Mark Shelly wrote:
>Thomas R. Holtz, Jr. wrote:
>>Archaeopteryx is exceedingly primitive, and the common ancestor of
>>dromaeosaurids and birds probably looked like something very similar
>>to Archaeopteryx.
>
>   As such, early bird ancestors and related groups probably still retained
>the theropod socketed teeth, tail, and hand claws.  In other words, they
>were flying theropods.

Well, even hummingbirds are flying theropods.  Very derived flying
theropods, but theropods nonetheless.

>It seems highly probable that many of these
>flying theropods flew to isolated islands and reverted to the predatory
>bipedal predator niche that their theropod features allowed, becoming
>flightless, but still retaining some of the flight adaptations.

Very likely, but we do not as yet have evidence that this did indeed occur.

>   With the primary locomotion means being flight in this ancestral group,
>the need for tail based Caudi-femoralis running muscles would have been
>reduced, possibly resulting in the rear facing pubic bones (for balance)
>and thinner, stiffer tails.

There are other biomechanical explanations for this reorganization of the
pelvic/hindlimb/tail muscle complex, which may have been exapted into the
flight tail of birds.

>   Since not all flying theropod groups would have adapted the same weight
>reducing changes in the same sequence, there may be many theropod groups
>such as the velociraptors or ostrich mimics that do not look similar to each
>other but evolved from flying theropod dinosaurs.

Although this is possible, the data that would be necessary for me to accept
this hypothesis is a phylogenetic analysis placing one or more of the
typical "non-avian" theropods within a clade with known flying members.
This has not yet been the result of theropod phylogenetic analyses.

>   If modern birds had not given up their teeth and hand claws during their
>evolution, the predominant predator might still be very theropod-like.

At various points in the Cenozoic, large predatory flightless bitds did
become the local dominant predators.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661