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Re: dino listing
>What might the 12 genera right at the Hell Creek/Cenozoic
>boundary be? I'm pretty sure _Tyrannosaurus_ and _Triceratops_ qualify,
For the record book:
A T. rex femur has been found about 3 meters below the boundary clay
Ditto for Triceratops sp. (a large part of the skull, found near
Brownie Butte). I don't know if that is good enough for you,
but it is a bit ambiguous to me.
> _Pachycephalosaurus_, _Stygimoloch_, _Ornithomimus_, and maybe
> _Ankylosaurus_. How about_Thescelosaurus_? _Ricardoestesia_?
> _Torosaurus_? It's pretty easy to compile a list of North American
> Maastrichtian dinosaurs (see, for example, the stratigraphy section of
> _The Dinosauria_), but figuring out which of those might be the very,
> very last ones known is a bit trickier. (Unless of course you find the
> right paper.)
Andy Farke wrote:
>I don't know if it is easy (or possible) to determine what these 12
>genera might be. Many of those mentioned (e.g., Torosaurus, Stygimoloch,
>Ankylosaurus, et al.) are rare below the boundary, let alone AT it.
Since I haven't read Sloan's Dinofest paper (yet), I will withhold
judgment on his new conclusions. But if his new conclusions are
anything like the previous conclusions reached by Sloan and Rigby
regarding dinosaurian biostratigraphy in the H.C., I'm afraid I won't
be easily convinced.
My main beef with previous Rigby and Sloan results can be summarized
thusly: How does one deal with any effects of dinosaur material being
reworked from the lower part of the Hell Creek Formation into the upper
part of the Formation? From the degree of transport abrasion? Nope.
Farlow et al. showed that evidence of transport-induced abrasion isn't
clear-cut (Farlow et al. rock-tumbled tyrannosaur and fossil croc teeth,
and looked for evidence of abrasive transport. They found
some minor evidence that pointed directly to transport, but many teeth
in the tumbler didn't show any abrasion at all).
And if Sloan is still relying on screen-washed sites (and therefore by
inference he is also relying on disarticulated material),
I think his conclusions are ambiguous (at best).
Since Ankylosaurus is uncommon even in the base and middle part of the
H.C. Formation, I think that potential reworking (along with the
Signor-Lipps Effect) effectively "swamps-out" (maybe "blurs" is a
better word) the real temporal range of Ankylosaurus. Ditto Stygimoloch
and the other two pachys.
It is not at all atypical to find ankylosaur teeth 3 meters below the
boundary clay. So, does this mean that Ankylosaurus or Edmontonia made
it to within 3 meters of the boundary clay? Beats me. Did Ankys
make it to the boundary itself? We may never know for sure.
> Also, I have a hunch that some animals in the same family (such as
> Triceratops and Torosaurus) may be confused or misidentified when
> collected (such as that awful problem with any Maastrichtian ceratopsid
> fragment being referred to Triceratops).
Yes, another fly in the biostratigraphic ointment.
> Then, there's that nasty
> problem of inflating/deflating species counts depending on the opinions
> of the investigator (e.g., Triceratops horridus vs. Triceratops
I myself would like to know how Sloan dealt with the two slightly
different Troodon tooth morphotypes present in the H.C.
Did he count them as two taxa (presently undescribed, by the way),
or did he take the alternative route and count them as teeth from
different positions on the jaws of one taxon? (after all,
T. formosus from the Judith River Fm. has heterodont dentition;
to a very minor degree in the laterals, but clearly so in the premax/
> Could local environmental changes (such as mountain building,
> change in climate) affect bone preservation? Could this skew counts?
I'll tell you what could have really messed-up Sloan's counts....
that "barren zone" below the impact clay layer. That layer varies from
one to 3 meters thick. It is not only devoid of dinos, it's devoid of
other fossils as well. Retallack has evidence that this zone represents
an acid-leached zone. Others aren't so sure *what* that zone
Some even think that the "zone" isn't real!
Sort of makes ambiguities of the Signor-Lipps Effect pale in comparison,
doesn't it? ;-)
> It's nice that they're tracking dinosaur diversity in the Hell Creek of
> Western North America, but what about the rest of the world? How was
> diversity affected in Asia or South America? Or even the American
> Southwest? Did all dinosaurs world-wide go extinct simultaneously? Were
> there variations between northern and southern hemispheres? This was a
> global extinction, and I doubt much more progress will be made (at least
> in the land animal area) until global faunal counts are done.
The best evidence for a sudden K/T extinction is in the marine
sections (because reworking is much less, as well as fewer
unconformities). In contrast, reworking is rampant in the fluvially-
dominated H.C. Fm. (IMHO at least), and Retallack, Fastofsky, Sheehan
and a host of other workers have found many small intraformational
unconformities in the H.C. Fm. This makes the otherwise straight-
forward process of laterally-tracing a particular rock layer
("time-horizon") into a nightmarish-process.
Frankly, after looking at this problem for a while now, I don't think
that terrestrial sections are going to tell us much regarding whether
an asteroid killed the dinos. But I am always ready to look at people's
new data. (In other words, if Sloan or Rigby can show me a
partially-articulated vertebral column from a dinosaur _in situ_
at or above the boundary clay, then I would perk up).