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Re: Subject: Bird hibernation/torpor



At 12:58 PM 3/23/97 -0500, John Bois wrote:
>
>> Their ability to excrete uric acid allows them to conserve water quite
>> efficiently - but I would hardly call the desert an avian "refugium" but an
>> area they have successfully invaded.
>
>Who can adjudicate the distinction you have made?  One would have to be
>present at the moment of desert colonization.

Well, you could use the Extant Phylogenetic Bracket method, and discover
that ALL living reptiles (turtles, lepidosaurs, crocs, and birds) have the
capacity for uricotely and, by parsimony, all extinct reptiles (including
nonavian dinosaurs) would be expected to have the same condition.

Although arid conditions may have been responsible for the origin of this
adaptation, it was much more likely the arid conditions around the time of
the Synapsid-Sauropsid split (ACCTRANS) or at least by the Anapsid-Diaspsid
split (DELTRANS) than anywhere within the bird radiation itself!!

>       But how about this:  ostriches, with their enhanced osmoregulatory
>abilities were able to range and reproduce in habitats unavailable to
>many of their predators.  This being true, desert ostriches enjoyed
>greater reproductive success than tropical rain forest ostriches.

Given that all reptiles have this capacity, the deserts are no safe refuge
from reptilian predators.


>     This still leaves ostriches.  And I think an explanation
>might again be found in the grasses.  I am going to argue that
>deserts were not as available to dinosaurs as they are to
>ostriches; and that this meant they could not exploit or hide in
>areas as far away from those who would eat them, their eggs, or
>their offspring.

Ummm, the famous Djadokhta Formation nests from Mongolia are from desert
environments, and include specimens from several different dinosaur lineages
(nonavian theropods, ankylosaurids, ceratopsians).  Deserts were hardly
prohibited to dinosaurs!!

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