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Re: marginal differences



> Date: Wed, 24 Sep 2003 01:07:24 +1000
> From: zone65@bigpond.com
>
> > Differences are surprisingly few.  Advanced ceratopians
> > (ceratopsids) fall into two groups: Centrosaurinae and
> > Chasmosaurinae (=Ceratopsinae).  My understanding is that its
> > pretty much impossible to tell apart one centrosaur from another
> > from postcranial remains.
> 
> So if it's just a head thing, does this mean they are possibly all
> of the same genus, if not species?

They can't all be the same "genus", no -- there is too much variation.
But it's more than possible that some of the classic centrosaurine
species are really synonymous.  For example, _Monoclonius_ may well be
a subadult _Centrosaurus_.  What makes it even harder to say for sure
is that the distinctive skull features of the various centrosaurines
mostly didn't develop until the animal was fairly mature, so
distinguishing one juvenile from another is all but impossible.

But there are real, unequivocal differences between some of the
centrosaurines.  John Horner's book _Dinosaur Lives_ (pp193-196)
discusses a sequence of four centrosaurines found in adjacent strata
from the top of the Two Medicine formation and the bottom of the
Horseshoe Canyon formation: they look impressively close to an
evolutionary sequence.  The most primitive taxon is was unnamed at the
time of publication in 1997 (he calls it "centrosaurine 1") -- I don't
know if it's since been named, but it looks like a _Styracosaurus_
with only the rearmost pair of frill spikes.  The next is _Einiosaurus
procurvicornis_, then _Achelosaurus horneri_, and finally
_Pachyrhinosaurus_.  It seems clear that these four really are
different taxa.  (And, BTW., the first three have been found in
bonebeds containing juveniles, and Horner says that the juveniles of
these three taxa are completely indistinguishable.)

> Could at least some of the variation just be individuation? I mean,
> people exhibit vastly differing heads, yet we're all the same type
> of critter.

Some of it probably is individual variation.  The number of recognised
_Triceratops_ species has fluctuated between one and nine at different
times, with two currently looking popular.  Several of the old species
were erected on what now seem to be merely unusual individuals of
existing species.

> Have Ceratopsinae perhaps been overclassified?

Maybe.  But this stuff is largely a matter of taste anyway -- what is
a "genus"?  There is no good, objective answer to that question.

 _/|_    _______________________________________________________________
/o ) \/  Mike Taylor  <mike@indexdata.com>  http://www.miketaylor.org.uk
)_v__/\  "You cannot simultaneously have mass adoption and rigor" --
         Clay Shirky.

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