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Re: Albertosaurine Survival and the End of the Cretaceous



OK, I had a little trouble with determining who sent what to whom
(just switched to Gmail, so I'm still working out some of the
functions), but I think this message is appropriate to answer.


On Wed, Jun 25, 2008 at 1:28 AM, Andrew Simpson <deathspresso@yahoo.com> wrote:

"I have very little understanding of what you guys are talking about
but I am so facinated by the last couple million years of the Mesozoic
in North America I would be interested in knowing anything and
everything on this era. What animals have we found from this time?
Where were they located?


Andrew Simpson"

Well, that's a tall order, but I'll relate a few major points relevant
to this discussion. These are stated as best as I can remember them,
so no guarantees about every detail being exactly right.

Basically, right at the very end of the Cretaceous, only a handful of
major dinosaur-bearing formations are known from North America, and
the most famous is Hell Creek - which, IIRC, preserves one of the most
complete sequences from earlier Maastrichtian right up to the close of
the Cretaceous. Sites in the northwest U.S. (in Montana and South
Dakota, for example) and in southwest Canada (i.e. Alberta) provide
much of what we know not just about the end-Cretaceous in North
America, but also for the entire globe.

So what my question has been is, more or less, whether a sort of
locality bias is involved in our understanding of latest-Cretaceous N.
American faunas (or at least, those from the western half of the
continent, which was separated from the eastern half since the Albian
but which was starting to re-connect toward the very end of the
Maastrichtian). Sites like the ones I mentioned turn up a set of
loosely associated dinosaurs in this timeframe - tyrannosaurine
tyrannosaurs, chasmosaurine ceratopsids, hadrosaurine hadrosaurs, and
ankylosaurid ankylosaurians (and dromaeosaurine dromaeosaurs, as
well). This same association of dinosaurs is also present as early as
the Campanian; in the southwestern U.S., they are the only assemblage
to be found, but in the north during the Campanian, they overlapped
with another assemblage - one composed of albertosaurine tyrannosaurs,
centrosaurine ceratopsids, lambeosaurine hadrosaurs, and nodosaurid
ankylosaurians (as well as velociraptorine dromaeosaurs).

I had been wondering if the reason that only the tyrannosaurine
assemblage is known from latest-Cretaceous regions that, formerly,
*also* supported the albertosaurine assemblage might have something to
do with climate change, and/or the retreat of the Western Interior
Seaway - which was drying up and in the early stages of re-connecting
western N. America (Laramidia) with eastern N. America (Appalachia).

Further, my conjecture (in its current modified form) is that such
climate change simply drove the albertosaurine assemblage farther
north, into northern Canada and Alaska, and that those regions were
therefore refuges for this previously more widespread faunal
association. BUT since those northern regions are by and large
inadequately sampled, my suspicion is that poor sampling, not
Cretaceous ecology, is why we do not yet know of any
late-Maastrichtian albertosaurines.

If you're interested in more details of the latest Maastrichtian in
North America, I'd start by looking up some of the things I just
mentioned on Wikipedia, and then branching out into Google/Google
Scholar searches. The chapters on tyrannosaurs and hadrosaurs in The
Dinosauria, second edition, also talk about some of what I've been
talking about here.

As for myself, I fear my n00bism about how and who to respond to may
have already killed my thread . . .

I guess my key questions are:

- Are we certain that truly albertosaurine material has been found in Alaska?

- Has tyrannosaurine material ever been identified from Alaska/northern Canada?

- Have any deposits from late Maastrichtian Alaska/northern Canada
ever demonstrated flora or fauna associated with Campanian (or early
Maastrichtian) Alberta, etc?

- How certain are we that Maastrichtian Asian lambeosaurs (e.g.
*Olorotitan*, *Charonosaurus*) are from the *late* Maastrichtian
specifically?

- Is it right to say that late Cretaceous deposits in Alaska/northern
Canada are truly under-sampled?

Any educated response to these questions, or heck, even a response
saying "this stuff is on Google if you look harder," would be
appreciated.

Thanks to any who respond.