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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 thickened, fully-flat
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 evolutionary scenarios.

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 seen.

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.

Mickey Mortimer