Sunday, June 15, 2008

Centrosaurus apertus - initially Monoclonius crassus

My attempt at another ceratopsian; this time, Monoclonius crassus - but I realised after reading up on this genus that my drawing is probably more like Centrosaurus. Damn! I really wanted to draw Monoclonius but I guess the picture I found on the Wikipedia that was labeled "Monoclonius" is probably a Centrosaurus...

According to Sampson et al. (1997), "Monoclonius specimens are generally defined on the presence of a thin, scalloped parietal and on the absence of hooks, spikes and horns seen on the posterior transverse ramus of other genera". The only complete skull specimen of Monoclonius, a specimen previously attributed to M. lowei, has a short nasal horn core and a pair of low rounded supraorbital horns (Sampson et al. 1997).

The absence of elaborate cranial ornamentations in Monoclonius and its occurrences in slightly older strata can be indicative of a primitive condition in Monoclonius. On the other hand, these features are also commonly associated with juvenile and subadult centrosaurine specimens, indicating a possible paedomorphosis in Monoclonius. However, it seems more likely that Monoclonius is based on subadult specimens rather than being adults with primitive/juvenile characteristics. Sampson et al. (1997) regard Monoclonius as a numen dubium because 1, diagnostic characters of Monoclonius are present in subadults of other centrosaurines; 2, suture closure and bone surface texture supports subadult status of Monoclonius specimens; and 3, Monoclonius-type cranial elements have been found in bonebeds of other centrosaurines spanning a large time period.

The third point is the most convincing argument against the validity of Monoclonius. Monoclonius-type parietals have been found in association with Centrosaurus, Einiosaurus, and Pachyrhinosaurus bonebeds in Montana and Alberta. These are indistinguishable to isolated parietals usually assigned to Monoclonius. Perhaps, Monoclonius lived alongside other centrosaurines in several different locations over a large time period. Sampson et al. (1997) however think this unlikely and proposes that Monoclonius is likely made up of subadult specimens of Styracosaurus, Centrosaurus, Einiosaurus, and/or Achelousaurus and that all centrosaurines go through a 'Monoclonius stage' through ontogeny (Sampson et al. 1997).

On the other hand, Dodson (1990) argued that the type specimen of Monoclonius is diagnostic and that this genus is valid based on biometric results. Dodson et al. (2004 in The Dinosauria, 2nd edition) also seem to follow this argument as they treat Monoclonius as a valid monotypic genus with the complete skull, M. lowei, included in the type species. Though not discussed in detail, Dodson et al. (2004) citing Tumarkin and Dodson (1998) mention the possibility of paedomorphosis in Monoclonius based on the large adult-sized 'M. lowei' - i.e. retaining juvenile characteristics while attaining adult size.

As much as I love the name Monoclonius, it seems more study is in need in order to either invalidate or validate the genus. As Dodson et al. (2004) mention, aging techniques may perhaps provide more evidences. Bone histology comes to mind...

References:

Dodson, P. 1990. On the status of the ceratopsids Monoclonius and Centrosaurus. In: Carpenter, K. and Currie, P. J. (eds.). Dinosaur Systematics: Approaches and Perspectives. Cambridge University Press, Cambridge. Pp. 231-243.

Dodson, P., Forster, C. A., and Sampson, S. D. 2004. Ceratopsidae. In: Weishampel, D. B., Dodson, P., and Osmóslka, H. (eds.). The Dinosauria (Second Edition). California University Press, London. Pp. 494-513.

Sampson, S. D., Ryan, M. J., and Tanke, D. H. 1997. Craniofacial ontogeny in centrosaurine dinosaurs (Ornithischia: Ceratopsidae): taxonomic and behavioral implications. Zoological Journal of the Linnean Society, 121: 293–337.

Tumarkin, A. R. and Dodson, P. 1998. A heterochronic analysis of enigmatic ceratopsids. Journal of Vertebrate Paleontology, 18(suppl.): 83A.

6 comments:

Traumador said...

Has anyone published on Pachyrhino growth series?

If not than ppl should pressure the Royal Tyrrell to do so.

They've got at LEAST one good skull for every growth phase of Pachyrhino from the Pipestone Creek bonebed. The reason I bring this up is that of course most of the younger to juvie stages resemble normal horned Centrosaurs. It's not till their pretty old that the horn grows inward into the boss.

The University of Alberta has a "Monoclonius" skull that to me looked like a larger version of a juvie Pachyrhino skull just before the ingrowing of the horn. At the time I saw it though I was under the impression that Monos were Centrosaurs so I didn't inquire further as to why this skull was being called Mono (I just assumed the label was out of date and uncorrected... which it could have been. It was the original catolouge tag from the 1940's)

Love the picture. The angle on the head is cool, and not often seen. I feel sorry for the Tyrannosaurid off panel that's about to get it!

Zach said...

I'm convinced that Monoclonius is a "teenage" centrosaurine, just as Brachyceratops is a "juvenile" centrosaurine. Bone reabsorbtion is a big deal in ceratopsians, and the the animals changed their appearance many times over their lifetimes. Bone reabsorbtion may occur in pachycephalosaurs, too (squamosal horns in Pachycephalosaurus).

Very good drawing, though! I have such a hard time with ceratopsians, it's always nice to see somebody get it right.

Mambo-Bob said...

Hey guys, thanks for the comments!

>traumador
I don't know if there is a study on growth series of Pachyrhinosaurus. In fact, I think there really is a lack of studies on this genus, period! - something for the younger ceratopsian enthusiasts out there to consider perhaps?

>Zach,

I agree. There is enough compelling arguments for the subadult status of Monoclonius. It's a shame really, because I really like the name...

Zach said...

Mike Skrepnick tells me that a Pachyrhino monograph is due out this year...cross your fingers!

Mambo-Bob said...

Awr sweet!

Traumador said...

I'll have to give Darren Tanke a bad time about it not being out yet when I see him in August. When he showed me the skull series he was like a proud father. Not that they aren't something to be proud of (he did a lot of the prospecting and prep on them)seriously their TOO cool lined up, its like watching a Pachyrhino grow up in front of you!