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Re: [Gender (was:Re: [Re: Sue(was: Running T. rex)])]
On 29 Dec 00 01:04:44 EST
>Note: I have already read Chris's response and agree that if the placement is
>so variable between extant taxa then it might be best to leave it alone.
>"Steve Brusatte" <email@example.com> wrote:
>> I don't buy the chevron argument. Sure, the crocodylians are close
>relatives of the dinosaurs, but that doesn't necessarily mean anything. There
>are other close dinosaur relatives that do not possess the chevron
>differences, as is true with dinosaur descendants.
>I have trouble accepting that as a viable reason not to use the chevron
>criteria. The closest extant relatives to dinosaurs outside the Crocodylia are
>the lepidosaurs and they all have hemipenes which are a completely separate
>setup. They also have completely different stances, which will effect muscle
>placement. Turtles might be better creatures to look at in this matter, but
>chelonian taxonomy is all over the place as it is so I'm not sure how helpful
>it would be.
Turtles may be better creatures to look at than lepidosaurs, but turtles are
not exactly close relatives to the dinosaurs, so how accurate would a
>> I like the morphotype idea much more. Raath's (1990) work on Syntarsus,
>which appeared in Currie and Carpenter's volume _Dinosaur Systematics_, may be
>the best look at different morphotypes in the same gender. By studying more
>than 30 Syntarsus individuals, Raath concluded that there were two distinct
>body types-robust and gracile. Furthermore, he concluded that the more robust
>form was female.
>Apologies as I haven't read this, but how did Raath come to this conclusion?
>Also, how complete were these skeletons? Would it be possible to try the first
>chevron criteria on them and see how it correlates with the robusticity one?
These Syntarsus individuals were buried in a mass-death assemblages, and were
relatively complete. Raath came to this conclusion by studying each specimen
and comparing them. There were more of the robust forms in the community of
30, and he hypothesized that they were females. Basically, he came to this
conclusion because he believes, in an evolutionary sense, that it is of a
better advantage to a community to have more females present. If the community
was separated, isolated, or broken up then a larger number of females would be
more of an advantage than a larger number of males. In the same volume,
Carpenter came to the robust=female conclusion because he believed that females
(Tyrannosaurus) would need to be larger to accomodate for the passage of eggs.
In the same volume there were two independent studies on morphotypes, both
using different dinosaurs to study, both using different reasons for their
conclusions, and both concluding that robust forms were likely females.
However, that doesn't mean that this is true in all dinosaurs...or even in
Syntarsus or Tyrannosaurus. I simply believe that studying morphotypes would
be a much more accurate way to determine sex in dinosaurs than to study
chevrons. I don't necessarily think that robust=female and gracile=male. I
just think that using morphs is a better and more accurate way to study gender.
>> In the same volume, Carpenter independently suggested that the more robust
>form was female. However, he specifically looked at Tyrannosaurus rex
>individuals. This is more iffy, as 1) there are nowhere near enough T. rex
>individuals to conduct an accurate study (imagine conducting a Presidential
>poll by only talking to eight or ten voters 2) and the T. rex specimens he
>used were from a variety of different locations. Raath's Syntarsus specimens
>were all from the same deathbed assemblage, meaning that it was more than
>likely that they were members of the same community.
>> Regardless, I like the morphotype idea better than the chevron argument
>because the morph studies use the dinosaur fossils themselves, not relatives
>such as crocodiles.
>But crocodiles have more analogous (dare I say homologous) tails with
>theropods than any other extant life form, so it is not like we're really
>shooting far out there.
>Perhaps we should try it on some extinct crocs. Go from a known to an unknown
>and see how it holds up.
I definitely think that this is a good idea, and I'm hoping that somebody will
do this. Maybe Dr. Brochu's Sue paper (monograph?) will drive a researcher to
conduct a study like this. Or, maybe i'll do it someday...
>> However, I don't think that there is a conclusive way to really tell the
>gender of a fossil specimen, unless, of course, it is fossilized giving birth,
>much like that famous Holtzmaden Ichthyosaur is.
>> Just curious...does the chevron argument hold up in any living animals other
>I'd imagine it wouldn't. Birds make bad candidates for theropod penis
>musculature simply because they are too derived. They dropped the use of their
>tails as leg retractor bases and counterbalances. As such, the penis
>musculature probably moved. One couldn't even look in extant emus and
>ostriches which re-evolved their penes, since they still have no tail
>connection. Mammals hardly use their tails for anything and have also
>separated their penes from their rectums. Lepidosaurs have two penes to deal
>with, which could throw the muscle placement off as well.
>In fact the only other animals that we could possibly compare it too would be
>chelonians. AFAIK noone has done this yet.
Then, maybe, would the penis musculature be different between, say,
Cryolophosaurus and Sinosauropteryx?? Sinosauropteryx shares many features
with birds, so then would it have derived musculature features, also?? Maybe,
like metabolism, the penal musculature was much different throughout the
dinosaur lineage. Actually, I would assume that this is so. I would also
think that it may be different in Syntarsus and Tyrannosaurus. So, then, maybe
comparing the chevrons in Syntarsus to those of Tyrannosaurus may not be such a
good idea, after all??
I'm all for comparing dinosaur (especially T. rex) chevrons to chelonians. Of,
course, there's that problem of getting somebody to do it...
Steve Brusatte-DINO LAND PALEONTOLOGY
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