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TRICERATOPS AND PSITTACOSAURUS ARE NOT OVERSPLIT



Steve Brusatte wrote:
<<Thank you everybody for your insights into my Triceratops question.  It
seems as if there are two prevailing camps here.  I do wish that Dodson would
write a paper on his views, and include a whole lot of character analyses.  I
do agree that Forster's five characters is awful slim...>>

Although five characters may seem slim, they are extremely useful in
delineating the difference between Triceratops horridus and T. prorsus, as
well as Diceratops hatcheri.  The skulls can be identified by the following
characters:

The length of the nose horn: short in T. horridus and long in T. prorsus.

The length of the pre-nasal-horn rostrum: nearly equal to the length between
the orbit and nasal horn in T. horridus, much shorter in T. prorsus (in other
words, T. horridus has a longer beak).

The length and angle of the brow horns: long and at nearly a 90 degree angle
to the maxillary tooth row in T. horridus and shorter and closer to a 60
degree angle in T. prorsus.

The frontal fontanelle: is present in T. horridus and absent in T. prorsus.

The lower temporal fenestra: is bordered dorsally by the jugal in T. horridus
and by the squamosal in T. prorsus.

T. horridus and T. prorsus do overlap somewhat temporally, but there are
periods where the only Triceratops kind in existence was the T. horridus
kind.  Additionally, it seems that when a lot of Triceratops specimens are
found together, the kinds don't mix.

Does this mean that these two kinds were different species or races or sexes?
 I don't know, and neither does anyone else without the help of a time
machine.  The fact that the temporal overlap is only partial, as well as the
fact that the kinds didn't seem to mix, suggests strongly that the
differences we see are specific.

<<I find it hard to believe, in a strict biological sense, that four
different species of the same genera would inhabit the same area.  I do know
that some of the Psittacosaurus species are from different times, but this
notion is still hard to believe.  Dodson, in his book, repeats the fact that
there is only one species of giraffe, one species of hippo, and one species
of rhino in Africa today.  If there were 8 or 10, then all of the species
would be competing for the same general foods and something would go extinct.>
>

This is not entirely correct....  If you mean Asia, by "same area" then
you're correct, but psittacosaurs are known from all over the place.

Psittacosaurus mongoliensis is known from Mongolia; Nei Monggol Zizhiqu
(Inner Mongolia) and Liaoning, in northeastern China.

P. neimongoliensis is known from Nei Monggol Zizhiqu in northern China.

P. xinjiangensis is known from Xinjiang Uygur in China's extreme northwest.  

P. meileyingensis is from Liaoning in the northeast of China.

P. sinensis is from Shandong in China's east, as well as Nei Monggol Zizhiqu
in the north.

P. youngi (which Sereno regards as a synonym of P. sinensis) is also from
Shandong.

P. ordosensis is from Nei Monggol Zizhiqu.

Although all psittacosaurs lived in the Early Cretaceous, it's been difficult
to find actual ages in the literature.  The only two species that actually
lived in the same formation were Psittacosaurus sinensis and P youngi, which
actually differed quite a bit.  

Psittacosaurus mongoliensis and P. meileyingensis both had an odd dentary
ridge, but differed in almost every other feature of the skull, such as the
external anteorbital fenestra, and the rostral length and shape.  P.
mongoliensis
had a much longer skull profile than any other species, and P.
meileyingensis
had the tallest skull profile of any species.

Psittacosaurus sinensis, P. ordosensis and P. xinjiangensis had very
pronounced jugal horns.  P. sinensis and P. xinjiangensis horns were even
more pronounced, so that the rostral edge was nearly perpendicular to the
sagital plane of the skull.

Psittacosaurus youngi and P. neimongoliensis both had shorter, squarer
skulls, as well as exceptionally deep rostral mandibles.  The predentary did
not come to a pointy tip like other ceratopians, but instead looked more like
the end of a robust pair of pliers.

All other species of Psittacosaurus are based on fragments and are of
negligible use at this time.

Pete Buchholz
tetanurae@aol.com