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>> [Note: at the Seattle GSA meeting molecular phylogenetic evidence was
>> presented which suggests that all modern orders of birds (btw birds are
>> dinosaurs) had diversified prior to the K/T boundary. This is the sort
>> of independent evidence that Jere means.]
>
>GSA? I wondered where the molecular clock folks had gone. These studies
>depend on pulling a starting date (for some branch point in the lineage)
>from the fossil record, so to some extent they aren't independent. And
>there's a lot of debate in the molecular biology community about whether
>these studies are sound, if they assume that the clock runs at the same
>speed in all lineages, etc., which other evidence (variation in abundance,
>diversity, and morphological change) suggests is not the case. What does
>the fossil record of birds look like? How many orders are recognized in
>the Cretaceous? (Gee, I should know this, being at Yale...)
Actually, earlier that same meeting (actually, the paleo Short Course this
year), Alan Feduccia stated that no Mesozoic bird could confidently be
refered to any modern order (i.e., the clade Neornithes of most authors =
Aves of Gauthier 1986 = the most recent common ancestor of all modern birds
and all descendants of that ancestor). Many of the Cretaceous birds now
appear to be enantiornithines (a very diverse group), while others
(Icthyornis, Hesperornis, etc.) seem to be closer to neornithines. There
is a volume on Mesozoic Birds in the works as we speak, so hopefully in a
few years we will have a better picture of post-Archaeopteryx avifauna.
>In sci.geo.geology (Usenet) right now, there's a raging debate about
>the K/T boundary, the demise of presumably open-ground egg-laying
>dinosaurs and presumably egg-eating small mammals. Some lay readers
>are in favor of putting the dinosaur extinction at the feet of hungry
>mammals, while various geologists defend the idea that a catastrophic
>event could have seriously interfered with dinosaur reproduction via
>lower temperatures or some other effect. How this might have worked
>is not clear to me, though I think the idea is worth serious thought.
Ugh... there are people still seriously talking about the "Class warfare"
(underdog Mammalia vs nasty old Reptilia) idea?!? Glad they haven't made
their way to the dinosaur listserv group (yet). ;-)
The idea about temperatures causing dinosaur reproduction failure goes
along these lines: it has been observed that in some (but by no means all)
modern reptiles, sex determination of the embryo is not primarily genetic,
but somatic, based on the average temperature at which the egg was kept
during incubation. The theory is that a severe shift in global climate up
or down would skew the sex ratios of the post-impact dinosaur populations
so out of whack (all males, all females, whatever), that there would be no
generation after that.
However, members of lineages known to possess this temperature-linked sex
determination (some turtle and crocodilian families) survived the crisis.
Furthermore, this character has a very disparate distribution in modern
reptilian clades. Is it really reasonable to assume that all two dozen or
so latest Maastrichtian dinosaur species worldwide, representing lineages
which diverged some 140 million years previously, would all possess this
trait? Lastly, at least one dinosaurian lineage (Neornithes) has no modern
representatives possessing this character.
Thomas R. Holtz, Jr. tholtz@geochange.er.usgs.gov
Vertebrate Paleontologist in Exile Phone: 703-648-5280
U.S. Geological Survey FAX: 703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA 22092
U.S.A.
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