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Paleogeography (was Re: Early Ceratopian)
At 05:00 PM 10/16/98 -0500, Rob Meyerson wrote:
>Thanks to all for the multiple corrections stating that _Psittacosaurus_ is
still the oldest ceratopian (perhaps we can call Zuni the "oldest known
quadrepedal ceratopian"; how many variations can we come up with? :^)
*sigh*. Nope, we don't know for certain that _Zuni._ is older than other
non-horned quadrupedal ceratopsians, due to the poor resolution of the ages
of most Late Cretaceous central Asian formations.
It REMAINS however, the oldest known brow-horned ceratopsian, which is what
Wolfe & Kirkland called it.
>With the earth having high sea levels, it seems problematic to have a
structural land bridge (or was Alaska and Russia actually joined at that time?).
As Mike Brett-Surman was saying to a bunch of teachers this weekend,
remember that geography as changed DRASTICALLY over geologic time. It isn't
just a matter of the same geography with different level seas and different
plants on top.
Mountain ranges are being uplifted and eroded; the Appalachians, for
example, are not the core of an old mountain range: they are some
secondarily (or tertiarily...) uplifted part of deformed belts around the
old mountain chain (which was further to the East, basically running along
I95). Drainage patterns and sediments might be coming in from the west
during some period, and then the east soon afterward; etc. Just about
everything that can change does change. Sections of continents get
fractured, extended, rumpled, crushed, and so forth.
Furthermore, plate boundaries are not 1:1 maps with modern geographic
boundaries. Who here thinks that the border between the North American and
Asian plates is along the Bering Straights (raise your hand)? Okay, put
them down: you are incorrect. The boundary is within Asia: much of eastern
Siberia, Korea, even parts of Japan are techincally part of the North
American plate. So, techincally speaking, Alaska and Siberia have been
joined for a very, very long time (well before the Cretaceous).
Additionally, the recognition of microplates (aka "terranes") has modified
our understanding of continental reconstruction. A LOT of the western part
of North America (and especially a lot of Alaska) seems to be composed of
microplates which collided with the North American plate sometime during the
later Mesozoic (and possibly early Cenozoic): any given collision would
potentially thrust up a mountain range, which later eroded away.
There are some spots that can be relatively well reconstructed in terms of
paleogeography: the Western Interior of North America, for one. The active
edges of continents, however, have much poorer resolution, because their
very activity destroys (or at least transforms) much of the evidence for
which we're looking.
So, to answer your question: we do not know for certain exactly when there
was dry land connection between Asia and North America. It can only be
inferred by other evidence, primarily paleontology.
Thomas R. Holtz, Jr.
Vertebrate Paleontologist Webpage: http://www.geol.umd.edu
Dept. of Geology Email:email@example.com
University of Maryland Phone:301-405-4084
College Park, MD 20742 Fax: 301-314-9661