Re: Gary Larson and Purgatorius (was: stego time line, thagomizer)

[chartiem@ERE.UMontreal.CA (Chartier Michel)]

Phil wrote:
>   The evidence for our order (Order Primates) living alongside T. rex comes
> from a single site in Montana (called the Harbicht Hill Locality), where
> screen-washing (i.e., "sieving") of sediments yielded a single tooth (a
> molar) of the probable primate _Purgatorius ceratops_.  This is considered
> by most mammal paleontologists to be the earliest known primate.  The animal
<snip>
> rocks as the Purgatorius molar.  Purgatorius is also found in the
> earliest Tertiary rocks overlying the Harbicht Hill locality in the same
> general area.  Some skeptics have claimed that the Harbicht Hill site is
> actually Tertiary, but they are hard-pressed to explain the presence of
> dinosaur bones at the site (they could possibly be re-deposited).  If
> Purgatorius actually is a primate, and if the rocks that it was found in are
> actually Cretaceous, then members of our own Order had to deal with 
> dinosaurs. 
The Harbicht Hill locality exposes sediments from the Hell Creek Fm, so I
don't think there are any doubts that it is indeed Latest Cretaceous in
age. Plus, the Purgatorius lower molar was found a few feet away from a
partial Triceratops skeleton... It is true that the evidence for this
little guy is problematic, since extensive screening and washing of
sediments of similar age at other localities in the same area by Dave
Archibald and colleagues have never recovered any additional remains
of Purgatorius. One possible explanation for the presence of this fossil
in these deposits is that the screening boxes were used first in sediments
of Early Paleocene age and this mammal tooth, being very small, somehow
got stuck in the mesh, and was recovered only and coincidentally when the
boxes were used to sift latest Cretaceous sediments at the HH locality.
Thus it would be a case of redeposition by (unvoluntary) human action.
These sorts of things happen once in a while when you don't clean your
screening boxes correctly. One of my friends (who also told me this
story), Paul Johnston, can attest to this because it happened to him
when he was collecting Paleocene mammals in Saskatchewan for his master's
thesis. Paul is now curator of invertebrates at the Tyrrell in Alberta,
so he doesn't have to worry about such things anymore. ;-)
Purgatorius unio, the earliest Paleocene species, is known from almost
all of the dentition, and there are no problems with it. One 
unidentified species of Purgatorius is also known from southwestern
Saskatchewan, from sediments of the same age as those in Montana (E. Pal.).
This canadian Purgatorius was described by Paul and Dr. Richard Fox
along with other mammals in the german periodical Palaeontographica
in 1984. Neat stuff for those interested.
Sorry for the digression. I study these critters, so I couldn't shut up.
Feel free to write me off-list for questions or comments.

Michel Chartier

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