RE: Even more concerning the Triceratops/Torosaurus deal

[Jaime Headden <qi_leong@hotmail.com>]

David Marjanovic wrote:

<*Triceratops*, where the epiparietals and -squamosals start as pointed 
(..."acuminate", if you prefer) osteoderms, then fuse to the skull and each 
other, and then become broader and very blunt.>

  I may not have been present for the talk, but it's certainly clear based on 
what else I wrote that you did not read something I wrote. The nodes in 
*Pachycephalosaurus* are distinct from the squamosal in many places, so that 
they bases can be clearly distinguished, especially on the most diagnostic 
specimen which is used in the paper. Thus, like *Dracorex* and *Stygimoloch*, 
they can be measured base to apex. Unlike in *Triceratops* where the authors 
propose that the epiparietal and episquamosal spikes are fused to the frill and 
the sutures are obliterated (which does not also occur in the 
pachycephalosaurs), this material may be collaterally altered while the frill 
is undergoing metaplastic GROWTH, which would subsume the epis rather than 
shrink them (contra Horner and Goodwin, where they proposed this very mechanism 
for the squamosal nodes in pachycephalosaurs). Thus the dimensions of the epis 
are UNKNOWABLE unlike in the spikes, as their limits are now absent. Moreover, 
the determination that the gross dimensions of the epis both broaden 
cirrcumferentially (for want of a term, around the margin of the frill) and 
shrink in basoapical "length" while the metaplastic frill enlargement subsumes 
the bases of the epis coincidentally obliterates the very data used to project 
the dimensions in the first place. These two cases are NOT analogous, even if 
they are both metaplastic.

  Moreover, the authors also project sequential metaplastic growth, then 
erosion, and argued that irregularities in the interface between osteoclastic 
and nonosteoclastic bone support this, but this can also occur with increase in 
invagination of the surface of the bone via nutriating tissues and blood 
vessels (underlying the keratinous sheath) as they themselves argued in 
late-stage metaplasia. A disconformity between the two types of bone, rather 
than regular deposition, is hardly direct evidence of erosion, nor does it 
explain osteoclastic bone erosion while metaplasia was underway, when 
metaplastic transformation is to occur afterward (while the structures were 
still apparently horn-like).

<Or, alternatively, sexual selection is just a bit more complicated than we 
used to think.>

  You know, I recall having discussions that projecting extant analogues was 
the only way to KNOW the constraints under which systems can work. Witmer and 
Holliday studying bone analogues for soft-tissues, Taylor et al studying aniaml 
neck posture, Hutchinson and Gatesy et als on locomotion of various tetrapods. 
Eliminating what doesn't work, to narrow the field to what does. Instead, the 
above quote purports that we can just toss in an idea because it _isn't 
contradictable_; tantamount to asking "why not?" This is science?

<Does "a single taxon must arise from any development" mean anything? Because 
if so, I can't figure out what. Please help me out.>

  I may have missed a "-al projection," which would have alluded to my 
understanding that ontogeny is not a proxy for phylogeny, but must derive from 
it nonetheless (ontogeny in the constraints created by phylogeny, whatever they 
are, while phylogeny will enforce by common descent similar if not identical 
ontogenetic trajectories as the null model). The paper proposes that it can do 
away with the null hypothesis by working itself backwards -- that there is a 
minimal number of taxa, and that metaplasia is the result of distiguishing 
features among age-classes. This is not to say that I prefer taxonomic 
profusion, and am a conservative lumper by concern, rather than practice; but 
simply trying to shoehorn specimens into an ontogenetic scheme and say that the 
ontogenetic scheme supports the shoehorning is certainly not a safe ground.

<I don't think that's a good comparison. We're talking about just one 
ecosystem, one long but narrow strip of forested floodplain. I don't expect 
high diversity of anything there.>

  I would suggest looking up the work Lehman has done to clarify the taxonomic 
disparity among different areas in similar ages of the latest Cretaceous. It 
was certainly not one "narrow strip of forested floodplain," and we are 
cautious about assuming that all taxa recovered where we have originated in 
that region. Simply examining the veldt or Serengheti will provide a diversity 
of Bovidae in what is "just one big savannah," by comparison.

  Cheers,

  Jaime A. Headden

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