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A critique of Sullivan's pachycephalosaur paper,
Sullivan has a new paper out, and the pdf is available on his website at-
It's a long paper, and I don't have time to critique it all. But suffice to
say, I'm not impressed. There are a number of problems in the basic
approach Sullivan takes, which will be illustrated with some examples.
First, Sullivan synonymized Pachycephalosauria with Pachycephalosauridae.
He calls the former taxon redundant and only uses it in quotes. No mention
of their phylogenetic definitions is given at all (one is node-based, the
other branch-based, thus they could only be used as synonyms if Stegoceras
were the most basal pachycephalosaur known). Instead, Sullivan basically
asserts that the traditionally basal 'homalocephalid' grade taxa are nested
within high-domed taxa. He defends this with some spurious reasoning. For
instance, because taxa like Dracorex have flat skull roofs, but otherwise
resemble pachycephalosaurins, the high dome can't be used to help diagnose a
derived pachycephalosaur clade. Uh... er... just because a character
exhibits homoplasy doesn't mean it's impossible to use in diagnoses.
Sullivan seems to have some typographic concept of taxa, where they are
defined by 'key characters', and if that key character reverses, the taxon
is invalid. Also, flat-domed taxa appear later than some high-domed taxa
(an unnamed Santonian form), and there can't be a ghost lineage of
flat-domed taxa because Stygimoloch has a small dome yet lived in the Late
Maastrichtian. Uh.... so because some high-domed taxa later reduced their
dome, other flat-domed taxa can't be more basal, and must have come from
high-domed taxa too?
Similarly, Sullivan rejects Pachycephalosaurinae, because Sereno (2000)
seemingly described it as fully-domed (and domes might reduce or be lost!).
Completely ignoring the fact that Sereno defined the clade phylogenetically,
which doesn't rely on morphology at all.
Stenopelix is relegated to Ornithischia incertae sedis. Not because its
pachycephalosaur characters are invalid, but because the "features are not
readily preserved in most pachycephalosaurid specimens, and their absence
and/or presence in non-pachycephalosaurid taxa are not well-established."
The characters are postcranial (obviously) and easily evaluated in putative
pachycephalosaur outgroups (ceratopsians, heterodontosaurids,
hypsilophodonts, etc.). I don't see how they aren't well established in
outgroups, and the fact they're not preserved in most pachycephalosaurs (due
to incompleteness) doesn't matter. It's like Suzuki et al. (2002) rejecting
arctometatarsalian characters of alvarezsaurids because they could only be
observed in Shuvuuia among alvarezsaurids. Taxa which don't preserve a
character can be ignored when evaluating that character.
Sullivan's diagnosis for Pachycephalosauridae reads like one of those
classic 'diagnoses' from the 1950's that lists symplesiomorphic or variable
characters in addition to synapomorphies. "Ornithischian dinosaurs with
or incipiently to fully-domed frontoparietals." So... the thickening is all
that needs to be mentioned then. "Supratemporal fenestrae absent to
well-developed." Uh... this would exclude which states now?
The author questions marginocephalian monophyly because he views the
low-domed taxa as being derived. Thus, the parietosquamosal shelf would be
convergently developed with ceratopsians'. I don't see the logic here, as
even some high-domed taxa like Stegoceras have parietosquamosal shelves.
Sullivan would have to show taxa like Colepiocephale, which lack the shelf,
are the most basal pachycephalosaurs. And thus hasn't been done yet.
Finally, anyone following pachycephalosaur taxonomy is familiar with the
conflicting interpretations of several taxa between Sullivan (2003) and
Williamson and Carr (2002). Apparently Ryan and Evans (2005) tried to sort
out the conflict between the analyses, though I have yet to see the paper.
I don't have an opinion regarding who's correct, but some of Sullivan's
reasoning is questionable. For instance- "Ryan and Evans (2005) resurrected
Stegoceras breve despite the distinct nature of the strongly down-turned
parietal and paired nodes that clearly indicate it is not Stegoceras.
Contrary to their statement, there is no demonstrable growth series for this
species, and primitive features (i.e., horizontal temporal chamber; frontal
grooved anteriorly) do not support reference to a more ?primitive? taxon
(i.e., Stegoceras sensu stricto). The compelling feature is a single row of
nodes, presumably on the squamosals, that lap onto the strongly downturned
posterior part of the parietal. This character is not seen in Stegoceras but
is also present in Prenocephale edmontonensis."
Why one character should be 'compelling' while other characters aren't isn't
obvious. We're back to the dark ages of key characters and personal
The paper has its good points however. There are excellent photos of
numerous skulls, including Tylocephale and Prenocephale. Also, there are
the only detailed evaluations of "Troodon" bexelli and Heishansaurus I've
Bakker et al.'s description of Dracorex (available on the same website) is
just as phylogenetically useless. After a long spiel about the problems
with assuming characters have equal weight (with some knee-slappers like
"PAUP-fiction" thrown in - guh-hyuk), the authors don't provide an objective
alternative. Instead, we get a 'scenario' (their term) that's basically
what cladistic analyses have shown - the long snout and large spike clusters
of pachycephalosaurins are derived. Thus Dracorex would have a secondarily
reduced dome, like its presumed relative Stygimoloch. Bakker et al. seem to
misunderstand cladistics the same way Charig and Milner (1990) did when they
were trying to place Baryonyx in Gauther's (1986) phylogeny. If a taxon
exhibits homoplasy, the whole phylogeny is claimed to be doomed. They say
"Most recently published phylogenies assume that the large dome evolved once
and was never lost (Sereno, 1986, 2000). Such an assumption makes Dracorex
hogwartsia difficult to interpret." But cladistic analyses assume no such
thing. It's just a result of the data entered at the time. Put Dracorex in
there and the dome may end up most parsimoniously lost.
In all, the two papers didn't affect my opinion of pachycephalosaur
phylogenetics at all because they just contain a bunch of unfounded
assertions. 'Homocephalids' may be derived from high-domed taxa, but a
stratigraphic difference of two stages and an analogy with Dracorex isn't
convincing. Until it is convincing, any affect it has on marginocephalian
monophyly is hypothetical. The rationale behind excluding Stenopelix from
Pachycephalosauria is flawed, and there's no alternative presented in any
case. The rejection of Pachycephalosauria and Pachycephalosaurinae
completely ignores phylogenetic nomenclature, so means nothing. The papers
left me with no idea how the authors think pachycephalosaurs are related to
each other or to other ornithischians, except that pachycephalosaurins are
monophyletic and Sullivan finds a relationship with ankylosaurs more
convincing than one with ceratopsians. Someone needs to enter Dracorex and
Alaskacephale into Williamson and Carr's matrix and get some science done on
its phylogenetic affinities.