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Re: Birds and mosasaurs [Rather long and theoretical]



[Deletion of discussion of a particular genetic scheme]

>If this scenario makes any sense and if it can fairly apply to the case
>under discussion -- two very big ifs -- it pushes the range of possible bird
>ancestors back down the theropod line.  Its pretty hypothetical, but I'm
>attracted to the idea because it explains the strange, mosaic evolution of
>avian characteristics in the coelosaurian radiation

Which, to be fair, is not uncommon in other lines (i.e., therapsids close to
mammals, for example, or primates near the base of Anthropoidea).

>AND explains why we see
>these characteristics popping up anew, in different combinations, and in
>different lineages,

Although the pattern is fairly consistent, as dromaeosaurids clearly share
the vast majority of these with basal birds, and other taxa varying degrees
fewer.

>right into the latest Cretaceous,

As stated plenty of times, Late K members of the various theropod groups are
NOT more "birdy" than their Early Cretaceous members.  There isn't a trend
towards more birdlike features *within* Oviraptorosauria or
Ornithomimosauria or Troodontidae or Dromaeosauridae over time (with the
exception of a very few features previously mentioned, such as the loss of
teeth within ornithomimids).  What there is is a difference in the amount of
birdlike features between these groups.

By sedimentological fate, we are blessed with very good Late K deposits in
western North America and Mongolia, so we know a lot about these dinosaurs.
Deposits from earlier times are less extensive and/or less well exposed, and
we know correspondingly less about the dinosaurs from these time.  Not
coincidentally, we also know less about the mammals, lizards, amphibians,
unionid molluscs, etc. from these units.  This isn't surprising, and says
little about the organisms.

>all without requiring
>very much in the way of missing links or introducing biostratigraphic
anomalies.

Although it does require an entirely hypothetical genetic scheme.

To put things in perspective: dinosaur fossils have been recovered from the
Morrison since the 1870s, and pretty much from EVERY summer since the 1910s.
And yet this best sampled of North American dinosaur producing units (okay,
the Judith River might rival it...) still produces entirely new taxa:
Comodactylus and Mymoorapelta and Koparion, all named in just the last five
years or so.

These supposed biostratigraphic "anomalies" should not be worrying people.
However, they do suggest that basing an evolutionary scheme primarily on the
lack of recovery of particular terrestrial vertebrate taxa in a particular
epoch is probably not a very secure methodology.

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