When Does Morphology Trump Ontogeny?

An outstanding issue among some systematists is the question when, in a given sample, a series of recognized taxa are argued to be an array of successively distant sister taxa versus a reflection of transformation in an ontogenetic series in a more confined concept of a species. Leaving out the question of what a species or how fine the variation should be calibrated, how do we tell when morphological variation is sufficient enough to contradict implied ontogeny? Which of these, would also be the null hypothesis?

A recent paper by Andy Farke, a ceratopsian specialist at the Raymond M. Alf Museum of Paleontology in Claremont, California, USA, seeks to resolve a question I discussed earlier here. In it, he tackles the issue of whether a particular specimen, USNM 2412 (the holotype of Nedoceratops hatcheri; formerly Diceratops hatcheri, then named Diceratus hatcheri followed by Nedoceratops hatcheri when it was found that Diceratops was preoccupied … by an insect {this happens a lot}) is morphologically distinct enough not to be conflated into a variation-rich species complex confined to Triceratops. Again, we’re leaving the question of “what is a species?” out this time around.

Skull of Nedoceratops hatcheri, USNM 2412 (holotype). A-B, right lateral; C-D, left lateral views; A,C, photos of the skull; B,D, line drawings of the same skull, with grey indicating missing (restored) material and yellow remaining unprepared matrix. From Farke (2011:fig.1).

Farke proposes [1] that morphological variation in USNM 2412 renders it distinct enough to warrant not just specific identity, but also generic separation from Triceratops, and at the same time rejects Scannella and Horner [2] in arguing that Triceratops, Nedoceratops and Torosaurus are all congeneric, with all species collapsed into a few (prorsus and horridus).

Several features are used to separate the taxa:

1. The squamosal anteroventral corner is short and rounded, and does not extent in lateral view to the level of the maxillary dentigerous (alveolar) margin — this is also seen in Ojoceratops fowleri [3], a chasmosaurine ceratopsid from New Mexico.
2. Presence of parietal fenestrae — otherwise present only in Torosaurus specimens.
3. A broad jugal notch (the “otic notch” between jugal/quadrate and squamosal) distinguishes it from Torosaurus latus, and also relates to a rounded, indistinct ventral margin of the squamosal, as in Ojoceratops fowleri.
4. Like Triceratops, but unlike Torosaurus, the medial margin of the squamosal is not expanded into a thickened “bar.”
5. Episquamosals are few in number, similar to Triceratops rather than Torosaurus.

Farke distinguishes among Scannella and Horner’s [2] ontogenetic profiles of Triceratops species by noting that at successive stages, both Nedoceratops hatcheri and Torosaurus latus appear to be anomalous:

First, Nedoceratops hatcheri indicates an “old adult” in the possession of an extremely reduce or absent nasal horn (resulting in a round “lump” leading to the original use of “Diceratops“), upright and forward-only curving brow horns, episquamosals and epiparietals expanded into “spool-like” structures nearly fused or fully integrated and indistinguishable from the frill, cranial sutures are nearly completely obliterated while facial and frill vascularization is deeply emarginated into the bone surface without pebbling texture or ventral depressions on the squamosal or parietal as indicated for Triceratops.

Second, Farke finds that the ontogenetic trajectory and argument that Torosaurus latus represents a natural progression in Triceratops ontogeny is not consistent with other ceratopsians: Both Protoceratops and Centrosaurus show incipient parietal fenestra early in ontogeny, rather than very late, and mottling or pebbly texture (associated with resorption according to [2]) occurs without eventual fenestra on the squamosals of the latter taxon. Additionally, specimens of Torosaurus latus indicate an extreme number of epiparietal ossifications (over 4-6 more) versus any Triceratops specimen, which seem inconsistent with the argument of absorption and eventual loss (why develop new epiparietals prior to absorbing them back into the frill?); additionally, centrosaurine ceratopsids with suggested juvenile specimens indicate an epiparietal (and episquamosal) count almost equivalent that seen in adults (as known), implying that a high number of epiparietals are not acquired, contrasting with the implied variation in [2].

This doesn’t mean Scannella and Horner are wrong, however. Farke does not examine histological sections of the specimen (likely due to its unavailability as a display specimen and being a holotype impairing this) to place this within the implied range of growth where Scannella and Horner place the “old adult” morphology implied by Torosaurus-leaning specimens (like Nedoceratops hatcheri).

It should be noted that an additional phylogenetic analysis supports a complex where Nedoceratops and Torosaurus are related to one another in a complex outside of Triceratops prorsus or horridus. Sampson et al. [4], in describing an additional two new taxa (Kosmoceratops richardsoni and Utahceratops gettyi, from the Campanian Kiparowits Formation), sampled the primary specimens (especially holotypes and complete referred skulls) for Triceratops prorsus, horridus, Torosaurus latus, utahensis, and the only known specimen of Nedoceratops hatcheri. Also included were the specimens used to diagnose Ojoceratops fowleri, in which the analysis resolves (along with Eotriceratops xerinsularis [5]) basal to the rest of the Triceratops-Torosaurus group.

Phylogeny of Chasmosaurinae (Ceratopsidae, Ceratopsia) from Sampson et al. (2010).

In this analysis, then, the morphology of Torosaurus seems less derived than that of Triceratops, owing primarily to the absence of parietal fenestrae in the latter (the only autapomorphy found uniting both species of Triceratops relative to other taxa). Some analyses may test for the relative value of juvenile specimens’ affect on the matrix by removing them a posteriori, then recovering a different phylogeny. So far, no phylogenetic analysis has used the subadult or juvenile material recovered from the various upper Maastrichtian horizons, focusing instead on holotypes and primary research specimens. Should juveniles be used, it may test whether the “basal” Triceratops-grade chasmosaurines may in fact be juvenile or adult of other taxa by skewing them (without performing histological analysis).

The next issue affecting this is whether relative ontogeny exists in this clade, or whether the peramorphic trend permits “adult” features in one lineage without being indicative of the entire subgroup in question. Conversely, can the Triceratops condition (re: Sampson et al. and Farke [3,1]) be somewhat paedomorphic in attaining large, adult size while retaining subadult or juvenile characteristics (distinct, large epioccipitals, no parietal fenestrae).

It should be interesting to see what these studies do to promote further analysis of chasmosaurine evolution.

[1] Farke, A. A. 2011. Anatomy and taxonomic status of the chasmosaurine ceratopsid Nedoceratops hatcheri from the Upper Cretaceous Lance Formation of Wyoming, USA. PLoS ONE 6(1):e16196. doi:10.1371/journal.pone.0016196
[2] Scannella, J. B. & Horner, J. H. 2010. Torosaurus Marsh, 1891, is Triceratops Marsh, 1889 (Ceratopsidae: Chasmosaurinae): Synonymy through ontogeny. Journal of Vertebrate Paleontology 30(4):1157–1168.
[3] Sullivan, R. M. & Lucas, S. G. 2010. A New Chasmosaurine (Ceratopsidae, Dinosauria) from the Upper Cretaceous Ojo Alamo Formation (Naashoibito Member), San Juan Basin, New Mexico. pg.169-180 in Ryan, Chinnery-Allgeier & Eberth (eds.) New Perspectives on Horned Dinosaurs: The Royal Tyrrell Museum Ceratopsian Symposium. Indiana University Press (Bloomington).
[4] Sampson, S. D., Loewen, M. A., Farke, A. A., Roberts, E. M., Forster, C. A., Smith, J. A. & Titus, A. L. 2010. New horned dinosaurs from Utah provide evidence for intracontinental dinosaur endemism. PLoS ONE 5(9):e12292 doi:10.1371/journal.pone.0012292
[5] Wu, X-C., Brinkman, D. B., Eberth, D. A. & Braman, D. R. 2007. A new ceratopsid dinosaur (Ornithischia) from the uppermost Horseshoe Canyon Formation (upper Maastrichtian), Alberta, Canada. Canadian Journal of Earth Science — Revue canadienne de Sciences de la Terre 44(9):1243-1265.

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