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RE: What is diagnostic?



Matt Christopher (dinosaurhunter@excite.com) wrote:
 
<The contextual information must also be similar (i.e. age, horizon,
formation, etc.).  It is still very difficult to match most postcranial
elements with much certainty.  To use your Allosaur example, the safest
description would probably be "Theropod indet." (indeterminable), at least
until you can compare it to the same bones from other large theropods of
the Morrison Fm. like ceratosaurs and megalosaurs.>

  The humerus of most theropods are distinct, and it is easy to tell the
difference between *Allosaurus* and similar allosaurs; unlike some
theropods, *Allosaurus*' humerus is very diagnostic, including a small
ridge on the medial and caudal surface, which permits the type of
*Allosaurus* to be compared to more complete material, even though the
humerus of the type is just a portion of the mid-shaft. Only the Japanese
*Fukuiraptor* has a fairly identical humerus, including presence and shape
of fossae, their positions, the relative sizes of crests and condyles ...
that of the other carnosaurs, like *Acrocanthosaurus* or *Sinraptor*, lack
many of these features or are distinct from that of *Allosaurus* to the
point that its humerus is diagnostic. This is not always the case, but it
is possible to refer a humerus much closer than "theropoda incertae sedis"
even though this is the "safe bet" position when it is obviously not a
prosauropod humerus.

  Would like to see an example of where a humerus is not as diagnostic,
where it is possible to distinguish allosaur from "megalosaur" from
ceratosaur humeri, despite some prevalent similarities among them even in
the same formation where they might co-occur (eg., the Morrison and
Shaximiao levels, for instance). There is no real "spinosaur" humeri,
actually, except that *Suchomimus* and *Baryonyx* share nearly identical
hourglass-shaped humeri in aspect.

  Cheers,

=====
Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)

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