Reader Comments

Post a new comment on this article

Alternative Interpretations of Cluster Analysis

Posted by AndyFarke on 07 Mar 2012 at 22:50 GMT

Longrich and Field (2012) provide another critique of the hypothesis that Triceratops and Torosaurus are different growth stages of the same taxon (Scannella and Horner, 2010, 2011; hereafter termed the Ontogenetic Trajectory Hypothesis, or OTH). Although I agree with Longrich and Field in principle (indeed, many of their observations and interpretations confirm those previously described in Farke, 2011), I do not think that the analytical methods are as conclusive as implied by the paper.

Specifically, parsimony-based cluster analysis was used to group specimens by similar patterns of cranial morphology. Certain groups of specimens were then inferred to be juvenile, subadult, or adult ontogenetic stages at the times of their death (Longrich and Field, 2012:fig.5; paralleling a terminology and set of criteria originally applied by Horner and Goodwin, 2006). However, this does not specifically test the hypothesis that Torosaurus is an aged form of Triceratops.

One of the central claims of the OTH is that features such as an increase in epiparietal count, broadening of the parietal, and acquisition of parietal fenestrae occur only late in ontogeny (Scannella and Horner, 2010, 2011). Thus, the features are ontogenetic rather than taxonomic in significance. However, these characters were not included in the cluster analysis, and thus it cannot be considered a conclusive test of the OTH. Indeed, by excluding these characters, the analysis is biased from the outset against the OTH as proposed by Scannella and Horner (2010). Even if such characters were included, the results could arguably support multiple taxonomic interpretations.

I agree that sutural fusion is most likely a reliable indicator of maturity, that this allows identification of potentially "young adult" Torosaurus, and that Torosaurus is taxonomically distinct from Triceratops (Farke, 2011). However, I do not think that cluster analysis as applied here provides adequate support for any particular interpretation. Instead, additional histological analysis, sample collection, and reinterpretation of previously collected specimens will be most informative.

I thank Mark Loewen and John Scannella for discussion of the topic, although all opinions here are my own.

Farke AA (2011) Anatomy and taxonomic status of the chasmosaurine ceratopsid Nedoceratops hatcheri from the Upper Cretaceous Lance Formation of Wyoming, U.S.A. PLoS ONE 6: e16196. doi:10.1371/journal.pone.0016196
Horner JR, Goodwin MB (2006) Major cranial changes during Triceratops ontogeny. Proceedings of the Royal Society B 273: 2757-2761.
Longrich NR, Field DJ (2012) Torosaurus is not Triceratops: ontogeny in chasmosaurine ceratopsids as a case study in dinosaur taxonomy. PLoS ONE 7: e32623. doi:10.1371/journal.pone.0032623
Scannella J, Horner JR (2010) Torosaurus Marsh, 1891 is Triceratops, Marsh, 1889 (Ceratopsidae: Chasmosaurinae): synonymy through ontogeny. Journal of Vertebrate Paleontology 30: 1157–1168.
Scannella JB, Horner JR (2011) ‘Nedoceratops’: an example of a transitional morphology. PLoS ONE 6: e28705. doi:28710.21371/journal.pone.0028705

Competing interests declared: I am a volunteer academic editor and section editor for PLoS ONE, and have published a paper in the journal with similar interpretations of Triceratops/Torosaurus taxonomy.