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Ceratopsine phylogeny questions



While browsing the SVP meeting abstracts yesterday, This was,
IMHO, one of the more interesting ones:
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AND THEN THERE WAS ONE: SYNONYMY CONSEQUENCES OF TRICERATOPS
CRANIAL ONTOGENY
SCANNELLA, John, Museum of the Rockies, Montana State University, Bozeman, 
MT,USA

Two years after the initial description of the famous ceratopsid dinosaur 
Triceratops, O.C. Marsh named and described the first specimen of Torosaurus 
latus (Ceratopsidae: Chasmosaurinae) from the same geological formation and 
roughly the same area of Wyoming, USA. Since then, only a handful of specimens 
of T. latus have been recovered whereas Triceratops remains are abundant and 
represented by several different ontogenetic stages. These genera are 
distinguished solely on the basis of their parietal-squamosal frill morphology.
 
Triceratops has been considered an unusual chasmosaurine for possessing a 
short, broad unfenestrated cranial frill, whereas Torosaurus has an expanded, 
fenestrated frill. A study of comparative cranial morphology reveals that the 
major changes which occur
throughout Triceratops ontogeny continue beyond what was previously considered 
the adult growth stage and result in the parietal-squamosal frill morphologies 
which diagnose T. latus. Torosaurus actually represents the mature adult 
morphology of Triceratops.

Osteohistological examination of a Triceratops postorbital horn core growth 
series confirms that large Triceratops which have yet to develop the expanded 
frill morphology previously considered characteristic of Torosaurus are not 
fully mature. Torosaurus horn core osteohistology reveals multigenerational 
Haversian tissue, which is indicative of mature bone. The unexpected degree of 
plasticity found in ceratopsid and other dinosaur skulls throughout growth 
entails that an understanding of ontogeny is critical to our comprehension of 
dinosaur paleobiology and systematics.
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As the abstract notes, _Triceratops_ was named before _Torosaurus_, 
so it would seem _Torosaurus_ would be sunk into _Triceratops_.
However, since the type specimen of _Triceratops_ does not exhibit the adult 
frill morphology of the genus, how, if at all, would this affect the 
nomenclature?
 
In addition, it would seem possible that _Arrhinoceratops_ from the older 
Horseshoe Canyon Fm. might represent the adult morph (and senior synonym) of 
the recently named _Eotriceratops_.  If the latter turns out to be the case, 
are there any real diagnostic differences between the adult morph of 
_Triceratops_ and _Arrhinoceratops_ that would justify separating these taxa at 
the generic level?  To potentially muddy the waters further, where does this 
place the taxonomic status of _Torosaurus_ utahensis?
 
Guy Leahy