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Re: Theropod "migrations"



I've got a lot of catching up to do here, so I'll try to summarize it all in
one post:

Larry Febo:
>These late Cretaceous theropods (Oviraptor and Velociraptor)
>have a pretty wide distribution across many unconnected continental masses.

Okay, first thing to do is to be *specific* with regards to your taxonomy.
_Oviraptor_ and _Velociraptor_ proper are at present known only from Asia.
However, oviraptorosaurs and dromoaeosaurs do have a wider distribution.

If you don't get the distinction here, we aren't going to be able to go any
further.

No one (to my knowledge) has argued that supraspecific taxa (genera on up)
of herbivores have a less restricted species areal range than do carnivores.
The argument applies only at the species level.  Example: Bovidae (antelopes
& cattle) and canids (dogs) both have a huge, comparable range.  However, I
can't think of any wild bovid *species* which has a range anywhere near as
big as _Canis lupus_ (the wolf).

>And Tyrannosaursa are also found both in Asia and North America. This seems
>possible only if you believe in many "land bridges " connecting these
>continents, in which case the seeming lack of exchange of Herbivores  seems
>a bit unusual.

Did you not read what I wrote?  If you aren't reading it, there's no point
in me posting any further on the subject.  Please go back and see what I
wrote on the subject in the posts on Friday.

Yes, there was interchange of herbivores.  That isn't the issue.

>> Tyrannosaurs seem to have been restricted to Asia prior to the earliest
>Late
>> Cretaceous (unless _Stokesosaurus_ is a tyrannosaurid).  Tyrannosaur teeth
>> (as well as teeth of other groups known in Early Cretaceous Asia but not
>in
>> Early K North America) first show up in North America in the earliest
>> Cenomanian deposits of the Cedar Mountain Formation.
>
>So, how did it cross over to N America? Land bridge??

Almost certainly yes, although there is the change it was actually an
island-hopping event (unlikely given the large numbers and diversity of
fauna that crossed).

Febo again:
>
>I still have my old copy of "Evolution of the Earth" by Dott and Batten
>(1971). I suppose I should look for something more up to date concerning
>plate techtonics and paleoenviornments. Can anyone suggest some good titles.
>I`ll try the local library, but I know they won`t have everything that`s
>available, and I may have to go interlibrary loan.

Umm, yes.  You should look at something more up to date.  (The analogy is
looking in a copy of a 1971 book on planetary astromony to debate current
concepts in the Oort Cloud & Kuiper Belt...).

The successor to Dott & Batten is Dott & Prothero's Evolution of the Earth
(McGraw Hill).  I prefer Steven M. Stanley's Earth System History (W.H.
Freeman), for various reasons.  There are some other good introductory books
on historical geology out there, too.

HOWEVER, these only give very general synoptic maps for these time periods:
they don't show the various alternative arrangements for smaller scale
regional structures.

Furthermore, for this particular situation (the Asian-American connection),
both the western part of North America and the eastern part of Asia are
patchworks of small microcontinents accreted onto the landmasses during the
Mesozoic, so the original outlines are not well understood.  Additionally,
the current coastlines do not show the geologic boundaries of plates (parts
of northeastern Asia are tectonically part of the North American plate).
So, unfortunately, the region that interests us the most for our purposes
here is one of the most complicated.


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