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RE: Albertosaurine Survival and the End of the Cretaceous

Second verse, same as the first. In plain text this time.

Hi, new poster, glad to be here, etc, etc.

I have a series of interrelated questions with some solid speculation
tossed in. But first, some context.

I understand that albertosaurine tyrannosaurs have only been
officially confirmed in the Campanian/early Maastrichtian of what was,
at that time, the more northerly reaches of Laramidia. (With some
reported but not yet described from New Mexico and Alaska.) I also
understand that they seem to have been part of a sort of loose
association with lambeosaurines and centrosaurines (and nodosaurs and
velociraptorines, perhaps, even more loosely?) Later Maastrichtian
rocks don't record assemblages like that, instead showing an
exclusively tyrannosaurine-hadrosaurine-chasmosaurine (and
?ankylosaurid and ?dromaeosaurine) association which had also
overlapped with the albertosaurine territory during the Campanian.

OK, so, I have a very conjectural idea here that I'd like to get some
clarification on so I can confirm or deny my suspicions. So far, it
seems that albertosaurines may have had more limited habitat
tolerances than tyrannosaurines, and the same might also be said of
the other taxa in their respective assemblages. Those tolerances could
be linked to one or both of the following (or other, unknown factors,
I suppose): one, they lived in cooler, more northern habitats and
(hypothetically) didn't deal well with southern heat; and/or two, they
lived in comparatively moist, coastal lowlands, where large tracts of
deltas, lagoons, swamps, etc were present, but did not live in drier,
more inland environments. Tyrannosaurines apparently did fine either
way, as evinced by the co-occurence of *Daspletosaurus* and
*Gorgosaurus*, for example.

Is that basically correct so far?

Anyway, here's the conjectural part. We can, perhaps, surmise that the
absence of the albertosaurine-lambeosaurine-centrosaurine association
from the later-Maastrichtian American northwest was due to local
ecological changes, rather than truly global ones, since a fair
diversity of lambeosaurines survived right up to the end in Asia (e.g.
*Olorotitan*, *Charonosaurus*). We also know that the Western Interior
Seaway was retreating during the Maastrichtian, causing many coastal
habitats to disappear and leaving the continental interior a drier
place with more severe temperature fluctuations. (Does anyone have any
good resources on the timing of the WIS retreat? I haven't been able
to pin down the exact sequence of retreat during this timeframe from
my Google searches.)

The conjecture, therefore, is that albertosaurines (and their
associates) did not simply die out wholesale during this span of time,
but instead became restricted to one or both of two areas: one, they
followed the coastline of the WIS as it retreated, therefore moving
generally east and perhaps south, and/or they became limited to even
more northerly regions, such as Alaska, where a more favorable
temperature regime was still present. I think *Edmontosaurus*-type
faunas have been found in coastal habitats too, right? That would
speak against the former option . . .

Now, of course, this is once again totally conjectural, but do I have
the basic facts right? Is it at least plausible that the lack of
albertosaurines in the northwest interior is simply because they moved
elsewhere as climates/habitats shifted?

Going farther afield, is it possible that they survived in Asia? Given
the presence of both corythosaurin and parasaurolophin lambeosaurines
in Asia at the end of the Maastrichtian, and the presence of
apparently albertosaurine material in Alaska, I guess I personally
wouldn't be the least bit surprised to hear that albertosaurines had
been found in northeast Asia. Of course, no evidence of that yet . . .
(Might as well ask if anyone could direct me to some resources for the
timing of Cretaceous land-bridge connections in Beringia, too.)

The main reason I ask all this is because I've taken a thoroughly
informal interest in what may (or may not) have been an incipient
faunal exchange between Laramidia and Appalachia at the
end-Cretaceous, for which of course there is exceedingly little to
even speculate on . . .

So, thoughts?