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Re: Comments on Therrien and Henderson's new paper



I must say that I agree in large part with Mickey's analysis, and came to many of the same conclusions in my reading of the paper last night...particularly when it came to a 20 metric ton _Spinosaurus_. I also think it would be great if there was a nice linear relationship between skull length and some body size parameter, but ecologically speaking, I can't think of a good reason why there _would_ be one necessarily, particularly given that skull morphology (like every other anatomical element) is subject to selective pressures acting on its function. Spinosaurid skulls seem to be particularly long and low not because the animals have especially long or heavy bodies, but because it's a morphology well-adapted to (at least in part) piscivory -- alternatively, just because the skull got long and low in order to be an effective fish-snatcher, it doesn't necessarily follow that the body had to become long and/or heavy to somehow compensate (unless there's a good reason that a long and heavy body would also have selective advantage in piscivory...or some other function, I suppose, but then it would _still_ be decoupled from the evolution of the skull morphology!)

I struck me as rather odd that the authors _a priori_ excluded a good number (and diversity!) of theropods from their analysis. Granted, they were looking specifically at _big_ theropods, but as has become rather abundantly clear recently, the big 'uns evolved from small 'uns, and again I can't think of any particular reason that getting big automatically shunts a theropod lineage into some specific allometric pathway -- if that were true, _Therizinosaurus_ ought to have a short neck and big, "carnosaur"-like skull. If the predictive equations don't work well for some of these other theropod lineages that didn't necessarily get big (and note that they _did_ include compsognathids!), then why isn't the gross similarity of many "big" theropods simply convergence (or coincidence)? And it is interesting to note that some big theropods did deviate substantially from the trend (e.g., _Carnotaurus_)...to me, that suggests that the formula they derived has either a very limited range of taxa for which it's accurate (specifically, the standard, rather generic, stereotypical "theropod" Bauplan), or it should really only be used as a very, very gross estimator of body size, particularly if no other data is available.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Jerry D. Harris
Director of Paleontology
Dixie State College
Science Building
225 South 700 East
St. George, UT  84770   USA
Phone: (435) 652-7758
Fax: (435) 656-4022
E-mail: jharris@dixie.edu
and     dinogami@gmail.com
http://cactus.dixie.edu/jharris/

"Trying to estimate the divergence times
of fungal, algal or prokaryotic groups on
the basis of a partial reptilian fossil and
protein sequences from mice and humans
is like trying to decipher Demotic Egyptian with
the help of an odometer and the Oxford
English Dictionary."
-- D. Graur & W. Martin (_Trends
in Genetics_ 20[2], 2004)