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taxonomy is not stratigraphy (was Re: JVP 25(2): New Dinos, Birds, Discoveries)
okay, since we seem to have started on this, I have 2
> > Sampson, S. D. and M. A. Loewen. 2005.
> > *Tyrannosaurus rex* from the Upper
> > Cretaceous (Maastrichtian) North Horn Formation of
> > Utah: Biogeographic and
> > paleoecologic implications. _JVP_ 25(2):469-472.
> Tyrannosaurus is known from all of the intermontane
> units including the Javelina, if Brochu is right
> TMM 41436-1, but preservation is generally poor in
> those units.
No, preservation is comparable to other North American
units, but the nature of erosion and exposure often
leads to collected material looking rather worse than
it would have been fresh from the rock.
In NM, the Naashoibito is not particularly well
indurated (even compared to the underlying Kirtland),
so the matrix readily breaks down, even to subsurface
depths of up to a metre. Thus bones can be
effectively/naturally 'removed from the matrix' before
subaerial exposure. Buried bones tend to act as
moisture traps, which is exploited by plants: alot of
the bones are riddled with rootlets across their
surfaces, and expanding into cracks, damaging the
bones. This does mean that well vegetated areas are
especially worth prospecting; digging the subsurface,
particularly directly under the plants themselves, can
prove a fruitful 'blind' collecting strategy. Bones
that are dug straight out of consolidated matrix are
not nearly so damaged as those dug (more easily!) from
unconsolidated sand. undamaged bone has rather good
preservation, comparable to the underlying kirtland.
Sadly the vast majority of bones are not recovered in
IMO T. rex is underepresented in some
> the Alamosaurus bearing units due to taphonomy.
Not sure what you mean by 'due to taphonomy'. I would
argue that we simply haven't collected enough material
yet. The southern faunas don't receive anywhere near
Do you really mean T.rex here, or just Tyrannosaurus?
be careful as the two should not be considered as
meaning exactly the same thing. (this is a long and
winding road, but hopefully it should all make sense
in the end)
Material assigned to the dustbin taxon 'Alamosaurus'
may or may not represent a single genus or species.
Thus, the occurrence of titanosaurs in the late
cretaceous of southern North America does not
necessarily imply the deposits are the same age. in
fact, 'Alamosaurus' is probably a very bad choice for
a biomarker since the holotype material bears no
Let's say for argument's sake that all 'Alamosaurus'
material is contemporaneous. The best 'Alamosaurus'
material derives from the TX Javelina Fm, dated as
69Ma, give or take, which thus predates the classic
Lancian faunas of the north.
Does it matter that the Lance fm T.rex be considered
the same species as the older pre-Lance/Hell Creek Fm
Let's look at a near identical case: Torosaurus
The collapsing of T.utahensis and T.latus into just
T.latus (thereby creating another monospecific genus)
merely implies a single lineage of Torosaurus, not
that all Torosaurus material is contemporaneous. The
slight variation seen between T.latus and T.utahensis
might not be enough to warrant a different species,
but there are differences, probably representing a
small number of accumulated changes between (and
during) North Horn and Lance Fm time. For southern
faunas, all other published dinosaur material
comparable to Lance Fm taxa is indeterminate (e.g. see
farke ?2002?, or Sullivan/Lucas ?2004?), although new
specimens may change this.
T.rex from the Lance/Hell Creek represents the last
occurrence of a tyrannosaurid in the fossil record.
Tyrannosaurus from the south represents the second
last. This is exactly the same for the ceratopsians,
most notably Torosaurus, but also probably
Triceratops, and might be the same for hadrosaurs and
ankylosaurs pending study of new material. The anomaly
in the south is the presence of titanosaurs.
What 'age' is the Alamosaurus fauna?
If the Javelina is around 69ma, and the
alamosaurus-bearing formations are indeed the same
age, then Alamosaurus faunas represent not a Lancian,
but a pre-Lancian age. Inappropriate assignment of a
'Lancian' age to the Alamosaurus fauna might be seen
to imply contemporaneity with the Lance Fm, which is
incorrect. What we are actually now seeing is the
extension of T.rex down to 69ma, not the cementation
of the Alamosaurus fauna up to the K-T boundary. This
is an important distinction to make, as it serves to
illustrate that sauropods, lambeosaurines, and
centrosaurines all seem to disappear from the north
American record before the lancian. Draw from that
what you will.
So what does this mean? Well <I would argue>, it
suggests that we see alot finer stratigraphic
resolution in the North American late-K dinosaur
record than is currently recognised, and this has very
important implications for dinosaur diversity (turn it
down). The classic 'judithian' 'edmontonian' and
'lancian' mammal-based faunas are either separated by
gaps, or could arguably be split further by the use of
dinosaurian taxa (primarily ceratopsian and
hadrosaurid), and the fact that very few
dinosaur-bearing horizons are actually
contemporaneous. Read the Kirtlandian paper by
Sullivan & Lucas. It makes alot of sense.
back to work...
> should be noted that Tarbosaurus, probably closest
> the ancestry of T. rex, is abundantly preserved in
> Nemegt formation, which had a fair number of
> titanosaurs. That suggests titanosaurs did influence
> tyrannosaur evolution. Tyrannosaurus may have
> larger size to overcome titanosaurs in far inland
> environments, then spread to the lowlands.
> Btw, ankylosaurids may have been present in the
> Leptoceratops habitats but they were hardly among
> dominant taxa there. AFAIK only two A. magniventris
> specimens are known from the Scollard.
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