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Re: Apatosaurus vs Brontosaurus




Matt Wedel (via Mike Taylor) wrote:

Whoa, hoss.  Kent and I shot down _one_ character that supports A.
louisae as distinct from A. excelsus.  That still leaves 15 or so by
Gilmore's count.

But how many of Gilmore's 15 or so characters are still considered valid? McIntosh (1990), in his chapter on Morrison sauropod taxonomy in the _Dinosaur Systematics_ volume, was less than enthusiastic about most of the characters used by Gilmore (1936) to distinguish _A. louisae_ from _A. excelsus_. For example, McIntosh regarded the three characters based on the shape of the scapula as subject to intraspecific variation, and he dismissed all of Gilmore's hypopsphene characters.


Nevertheless, McIntosh (1990) did regard _A. louisae_ as distinct from _A. excelsus_, and put forward three characters to distinguish the two species: (1) Anterior extension of the cervical ribs (present in _excelsus_, absent from _louisae_); (2) an accessory lamina subdividing the infrapostzygapophysial cavity (try saying that three times!) of the mid- and posterior cervicals (present in _excelsus_, absent from _louisae_); (3) very robust hindlimb bones in _louisae_ (especially the femur).

As discussed previously, (1) has since been refuted as a useful diagnostic character, since the shape of the cervical ribs varies within a species. That leaves only two characters to distinguish _A. louisae_ from _A. excelsus_: the accessory lamina character and the relative robustness of the leg bones. Maybe there are other characters that can be used to diagnose _A. louisae_, and maybe some of the characters proposed by Gilmore are useful after all. However, based on the published literature, Gilmore's 15+ characters have been whittled down to only two, neither of which have been re-examined recently AFAIK.



Tim

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