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Dracorex's phylogenetic position examined with science



Hmm. This seems not to have made it though the first time. Apologies if it did.

After the big deal made by Sullivan and Bakker et al. about Dracorex making "all previous phylogenetic analyses ... inadequate", what actually happens when Dracorex is included in Williamson and Carr's (2002) matrix? Does the universe implode? Well, let's see.

First, I made a NEXUS file from Williamson and Carr's matrix. I left characters ordered, but altered some of the codings so as to avoid weighting characters. For instance, character 35 is "Parietosquamosal shelf nodes: absent (0); present (1).", but character 36 is "Parietosquamosal shelf nodes: absent (0); in more than two rows (1); in a single row, sometimes accompanied by ventrolateral corner nodes (2)." This weights the presence of parietosquamosal shelf nodes, since their absence gets a state in two characters. So I changed state 0 in character 36 to inapplicable. I also did this for 26 vs. 21, 34 vs. 51, 41 vs. 40, 43 vs. 8, 47 vs. 46, and 53 vs. 52. Finally, I switched the order of states in character 47 (which describes which end of the parietosquamosal bar is taller) to be a logical progression when the character is ordered (medial end taller > ends subequal > lateral end taller).

I added Dracorex, the undescribed Dracorex-like skull with the low dome (referred to here as the Galiano taxon), Alaskacephale and Micropachycephalosaurus. I would add Triebold's specimen ('Sandy'), except that I don't know exactly which parts are real and which are reconstructed. I also added Yinlong as another outgroup, because it is similar to pachycephalosaurs in several ways.

The first 100000 most parsimonious trees had the following strict consensus-

|--Psittacosaurus
`--+--Micropachycephalosaurus
  `--+--Stenopelix
     `--+--Wannanosaurus
        `--+--Yinlong
           `--+--Goyocephale
              `--+--Homalocephale
                 `--+--Ornatotholus
                    `--Pachycephalosauridae
                       |--+--'Stegoceras' breve
                       |  `--+--UCMP 130051
                       |     |--Gravitholus
                       |     |--Stegoceras validum
                       |     |--Hansuessia
                       |     `--Colepiocephale
                       `--Pachycephalosaurinae
                          |--+--Tylocephale
                          |  `--Prenocephale
                          |*-Sphaerotholus? edmontonensis
                          |--+--Sphaerotholus goodwini
                          |  `--Sphaerotholus buchholtzae
                          `--Pachycephalosaurini
                             |--Alaskacephale
                             |--Pachycephalosaurus
                             |--Stygimoloch
                             |--Galiano taxon
                             `--Dracorex

The position of Micropachycephalosaurus is extremely tenuous, as it is based on the original brief description and line drawings. The position of Yinlong should be considered as an indication we need to examine Stenopelix and Wannanosaurus in light of the fact we now know basal ceratopsians are very pachycephalosaur-like. Perhaps these other fragmentary basal pachycephalosaurs are basal ceratopsians instead. Using Yinlong as the outgroup instead of Psittacosaurus leaves a polytomy between Wannanosaurus, Stenopelix and Micropachycephalosaurus.
Wannanosaurus and Ornatotholus are based on immature specimens, and both Sullivan and Williamson and Carr believe the latter to be a juvenile Stegoceras. It's thus possible Wannanosaurus would have a more derived position if adult specimens were known. Excluding these juvenile specimens does not influence tree topology.


The main differences between Williamson and Carr's (2002) and Sullivan's (2003) trees are-
Hansuessia (including Gravitholus and UCMP 130051 in Sullivan's view; both Stegoceras sp. indet. in Williamson and Carr's view) and Colepiocephale are pachycephalosaurines sister to Pachycephalosaurini in Sullivan's tree. Here they are 'stegocerines', as in Williamson and Carr's published tree.
Sullivan considers buchholtzae a synonym of edmontonensis, and places it, goodwini and breve/brevis in Prenocephale. In Williamson and Carr's published tree, buchholtzae and goodwini are sister to pachycephalosaurins, brevis is a basal 'stegocerine', and edmontonensis is a pachycephalosaurid that can be excluded from pachycephalosaurins, Prenocephale+Tylocephale and the present clade of 'stegocerines'. Here, the results are more equivocal. buchholtzae and goodwini could clade with Prenocephale+Tylocephale or pachycephalosaurins. edmontonensis is a pachycephalosaurine in this tree, and can fall into Sphaerotholus (but not Pachycephalosaurini or Prenocephale+Tylocephale). This makes a synonymization with buchholtzae possible, as Sullivan suggests. It should be noted the only reason Tylocephale clades with Prenocephale is because both are Asian.
Note I didn't include Sullivan's characters in the analysis, nor did I check the accuracy of the codings. So this post isn't a test of his ideas on pachycephalosaurid phylogeny. I would recommend against referring to American taxa as Prenocephale though, especially if you keep Tylocephale separate. Similarly, I would recommend referring to Colepiocephale and Hansuessia instead of Stegoceras lambei and S. sternbergi respectively, until their positions are more definitively determined. edmontonensis might be best referred to Sphaerotholus, as this could work in both phylogenies. breve/brevis is a more difficult case.


Most importantly for this post, however, is that Dracorex still clades with pachycephalosaurins, despite its flat skull and large supratemporal fenestrae. Apparently a little homoplasy did not destroy the cladogram. The Galiano taxon and Alaskacephale are also pachycephalosaurins, as predicted. Despite "the conceptual challenges presented by the incongruous combination of skull adaptations" in Dracorex, or the "unwarranted" assumption of "mixing apples and oranges, bowling balls and cannon balls, BB?s and seeds from currants" that Bakker et al. likened to unweighted numerical cladistic analyses, the cladogram places Dracorex where Sullivan's and Bakker's subjective scenario does. If people would just try using cladistics instead of complaining about it, they may be surprised at its effectiveness. But note that the fact Dracorex is secondarily domeless does not mean all domeless pachycephalosaurs (Homalocephale, Goyocephale..) are. Nor does it invalidate Pachycephalosauria or Pachycephalosaurinae, as there are still several taxa outside of (Stegoceras + Pachycephalosaurus), and they're placed there for more reasons than lacking a dome. In fact, besides Pachycephalosauria itself, Pachycephalosaurinae, Pachycephalosauridae and the basal placement of Goyocephale are the best-supported nodes in the cladogram. Indeed, excluding the 'key character' "dome present" leaves the tree intact. 'Key character' indeed.

Mickey Mortimer