Sexual dimorphism and the quest for the truth (and the failings of said quest)

["Rob Gay" <rob@dinodomain.com>]

Wow! I don't check my e-mail for almost a week, and look what I get! In
chronological order:
HP Mallon wrote:
>Seems like the answer might be obvious, but I have a hard time accepting
the idea that two apparently closely-related forms would have differed so
significantly in such a feature.  _Dilophosaurus wetherilli_ and _D.
breedorum_ aside.<
If _D. breedorum_ is valid (which I believe it is not). Going over the
paper, I find that the characters used to define the species are all either
size or age related (i.e. "crest better developed thatn in UCMP 37302", or
"larger than in UCMP 37302" [These are paraphrases, not direct quotes, I
don't have the paper right in front of me currently]). Furthermore, Ralph
Molnar looked over the paper, and commented to me that it appeared that
Pickering did not actually designate a type species. I haven't looked at the
paper since then, and (as I said) I don't have the paper in front of me
currently, so I can't comment on this. Perhaps someone else for the moment
can.

HP Pickering wrote:
> Dilophosaurus's two species, moreover, may be sexually dimorphic
representatives, crested females to attract males (in this case, males being
the crestless forms known as Liliensternus)<
As others have said, calling age-seperated taxa sexual dimorphs isn't
viable. See literature on on ammonite sexual dimorphism for more comments on
the relevence of stratigrpahy upon claims of sexual dimorphism. The
posibility of _Liliensternus_=_Dilophosaurus_ may exist, sure (though I tend
to doubt it, but I haven't seen the material of _Lilientsernus either), but
the two taxa being sexual dimorphs is I believe is highly unlikely (as other
HPs have stated)

HP Mortimer wrote:
>it's a bit too convenient the only three Liliensternus specimens were found
in Europe and the only known (three plus?) Dilophosaurus specimens were
found in America.<
There are a grand total of 6 described specimens (3 at UCMP, 3 not as nice
ones at MNA), plus a few odds and ends that have been ascribed to
_Dilophosaurus_ at both locations (proximal femur that probably is at MNA,
fragments of verts that may or may not be at UCMP).

>Most importantly however, Liliensternus has numerous morphological
differences from Dilophosaurus, many of which place it closer to
coelophysids.  Such an extreme case of sexual dimorphism, with the added
stratigraphic and phylogenetic problems, is completely implausible<
Well said!

HP Mallon wrote (again!):
>Hmmm... I'm not so sure about this.  Respected listmember Rob Gay has been
looking into sexual dimorphism as it pertains to _Dilophosaurus_ and hasn't
yet found any sign of this condition in this genus.<
Hehehe, thanks for the mention there. Anyways, I will be the first to admit
that my study is not perfect, but its the first look at this genus and the
possibility of sexual dimorphism in a quantitative way. The sample size is
small (as mentioned above), and new methods of analysis have come to light
since I wrote this (Lucas and someone else did a poster at SVP this year
using a new method to examine _Coelophysis_ that is really exciting).
Further work is needed (and will hopefully get done ;)), but other things
need to happen before this work can be finished.

HP Farke wrote:
>It is *extraordinarily* difficult to confirm sexual dimorphism in extinct
animals, especially without a monospecific bonebed accumulation (as Rob
discovered with Dilophosaurus). For instance, I am suspicious of any claims
of sexual dimorphism in Tyrannosaurus. Yes, it looks very convincing when
you place bones side by side. However, the sample is still quite small, and
stratigraphic and temporal data are minimal for many specimens (even
recently collected ones).<
Quite so. There are probably only a handful of extinct dinosaurs for which
we could begin to determine with a high degree of certainty if sexual
dimorphism exists. These are: _Coelophysis bauri_, _C. rhodesiensis_
(maybe), _Allosaurus fragillis_, _Centrosaurus_, _Maiasaura_, and
_Protoceratops_ (though there may be a few I left out). Really, the sample
size on everything else is pitiful (though with Spener's method, the number
needed does come down), and while tests can be done on other taxa (in fact,
I think they should still be done), it only will provide a guideline and a
rough idea, not a hard truth (which of course, also includes my own work.
It's part of science; the results can change if more data is added).
Anyways, there's my thoughts on all of this, for any and all interested
parties. ;)
Peace,
Rob

Student of Geology
Northern Arizona University
P.O. Box 20840
Flagstaff, Az. 86011
http://dinodomain.com
http://www.cafepress.com/robsdinos
"A _Coelophysis_ with feathers?"