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Jordan Mallon wrote:
>What, then, does he say about _Proceratosaurus_, who is also believed to
>have had some type of nasal horn?
"This species is based on a partial skull from the Great Oolite of England,
originally described as _Megalosaurus bradleyi_ by Woodward (1910). Huene
(1926a) removed the species from the genus _Megalosaurus_ and proposed the
new genus _Proceratosaurus_. As indicated by the name, Huene believed this
taxon to be a close relative of the Upper Jurassic genus _Ceratosaurus_,
based on the presence of a dorsal projection on the anterior end of the
nasals, the only part of the skull roof preserved. However, the position of
this projection is unlike that of the horn core in _Ceratosaurus_, and,
because of the incompleteness of the skull roof, it cannot be determined if
it represents a nasal horn or a median cranial crest."
So, _Proceratosaurus_ probably has something on its nose. But whether its
cranial ornamentation more closely resembled the midline crest of
_Monolophosaurus_, or the shorter, more triangular horn of _Ceratosaurus_
(which is placed in a different position relative to the nares) cannot be
determined. We need more _Proceratosaurus_ material - such as a skull with
the roof preserved.
>If the two were so closely related
That's a big "IF". To be sure, they're both Coelurosauria - but, apart from
Paul (1988), I don't think any other author has found support for an
exclusive _Ornitholestes_ + _Proceratosaurus clade. No cladistic analysis
(AFAIK) has found support for such a clade - at least not recently.
> (Seems like the answer might
>be obvious, but I have a hard time accepting the idea that two
>apparently closely-related forms would have differed so significantly in
>such a feature.
Sometimes different sexes of the same species, and different ontogenetic
stages of the same species, vary greatly in the expression of prominent
cranial features. Ceratopsians and hadrosaurs are notorious for this; we're
just scratching the surface for the Theropoda.
>_Dilophosaurus wetherilli_ and _D. breedorum_ aside.)
In the case of _Dilophosaurus wetherilli_ and _D. breedorum_, the latter
"species" is probably referrable to _Dilophosaurus wetherilli_. Also, the
expression of supracranial crests within the Coelophysoidea is highly
variable. _Dilophosaurus_ has a crest (two, actually), and so does
_Syntarsus kayentakatae_; _Syntarsus rhodesiensis_ and _Liliensternus_ spp.
apparently do not. _Coelophysis bauri_ may have a crest of some fashion;
but this feature appears to exist in the eye of the beholder, and/or vary
within the species.