Re: Horner's Pachy Lumpin' - Your Thoughts?

["Thomas R. Holtz, Jr." <tholtz@umd.edu>]

David Marjanovic wrote:


> It is absolutely counterintuitive that an animal would first grow horns
> and then shrink them to fairly blunt knobs. But, on a smaller scale, the
> same thing happens to the "epoccipitals" of ceratopsids (which I frankly
> expect to turn out one day to be homologous to pachycephalosaur spikes).
> Perhaps more importantly, the horns of *Triceratops* (even when
> *Torosaurus* is not included) _change curvature_ during ontogeny, from
> backward-pointing to forward-pointing. The only way to change the
> curvature of a bone is to deposit bone on one side and _remove it_ from
> the other. Here we have the large-scale absorption the pachy-lumper
> scenario requires.
>
Another ceratopsid case of metaplasia: the postorbital and nasal horns of
young Pachyrhinosaurus are indeed horns, but as they aged the postorbital
horns get resorbed and remodeled into pits and the nose horn into the
distinctive mound.

-- 
Thomas R. Holtz, Jr.
Email: tholtz@umd.edu   Phone: 301-405-4084
Office: Centreville 1216
Senior Lecturer, Vertebrate Paleontology
Dept. of Geology, University of Maryland
http://www.geol.umd.edu/~tholtz/
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Mailing Address:        Thomas R. Holtz, Jr.
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