More musings on dinosaury things from January 2009. For the back-story you’ll need to see part 1 and part 2, both of which are on theropods (and, specifically, on maniraptorans). This time we look at ornithischians…
One ornithischian in particular has been the subject of much discussion lately. Namely, the Upper Cretaceous Mongolian ankylosaurid
Minotaurasaurus ramachandrani Miles & Miles, 2009. This isn’t such an inappropriate name – after all, the holotype specimen does have bull-like horns that project outwards from above its eyes – but it doesn’t sound very euphonious… and shouldn’t it be Minotaurosaurus anyway? The skull of Minotaurasaurus is pretty neat-looking: it has a broad, squared-off beak, is covered on its dorsal surface by pyramidal osteoderms, sports large horns on the cheeks and rear corners of the skull, and has a series of unusual openings in the nasal region (Miles & Miles 2009) [the drawing above is by Waylon Rowley].
That’s all very nice, but there’s actually nothing new here if you already know ankylosaurs: Minotaurasaurus is similar to another Upper Cretaceous Mongolian ankylosaurid, Pinacosaurus, specifically to P. mephistocephalus Godefroit et al., 1999 [shown here]. Both share those weird accessory openings in the nasal region, among other things (nobody really knows what these were for, but it’s been suggested that they housed glands or erectile tissue). The only real difference between Minotaurasaurus and Pinacosaurus is that the former has that broad, squared-off premaxillary beak whereas Pinacosaurus has far narrower premaxillae (Godefroit et al. 1999, Hill et al. 2003). Minotaurasaurus is also extremely similar to another Mongolian ankylosaurid: Saichania chulsanensis. The two are said to differ in the orientation of the pterygoid, orbit shape and other details. The precise relationship between Minotaurasaurus and/or Pinacosaurus and Saichania needs, I think, some evaluation, but it’s surprising that Miles & Miles (2009) didn’t even cite Godefroit et al.’s (1999) paper on P. mephistocephalus*.
* Incidentally, I once went to a meeting where a poster announced the discovery of P. mephistocephalus. The amusing thing is that a cladogram included only P. mephistocephalus, a salmon, a lungfish and a cow. This is one of those in-jokes again.
It’s not just the phylogenetic affinities of Minotaurasaurus that have been the subject of discussion, but also the provenance and origin of the specimen. Miles & Miles (2009) stated that the specimen was purchased by V. S. Ramachandran (hence the specific name), and we now know that it was, more specifically, purchased at a fossil show. And like lots of specimens sold at fossil shows, it may not have been obtained legally (Dalton 2009). On the one hand, a lack of provenance data and absence of legal documentation doesn’t automatically make a specimen scientifically worthless. On the other hand, gone are the days when it was ok to get fossils out of countries without the appropriate permission. We wait and see what will happen in the case of Minotaurasaurus.
Incidentally, for those of you wondering whether I’ll get round to doing another Ankylosaur Week this year, I have to say it’s looking doubtful.
New news on giant chasmosaurines
Moving now to marginocephalians, ceratopsians have been in the news lately. For starters we have Farke et al.’s (2009) paper on combat in Triceratops. In recent decades it has become increasingly recognised (mostly thanks to comparison with extant horned tetrapods) that the frills and horns of horned dinosaurs predominantly functioned intraspecifically, either in combat or in display. Given the irresistible idea that these dinosaurs grappled with one another over mates or territory or resources or whatever, there has been some interest in testing the possibility of intraspecific combat [adjacent image, from Farke et al. (2009), shows incident rate of cranial lesions on the various skull bones of Triceratops and Centrosaurus].
But how do you test for this in long-dead animals? One possibility is to use models – as in, actual, plastic little dinosaur models – and see how the horns and frills match up when posed in possible combat positions. This is what Farke (2004) did: he concluded that, if Triceratops did fight by locking horns, then injuries on certain regions of the frill, jugal and horncore tip should be present in the fossils. Farke et al. (2009) represents the next logical step in research: the authors looked for, found, and analysed preserved lesions on Triceratops skulls. The lesions in Triceratops were mostly concentrated in the regions where they were previously predicted to be most abundant: their presence in these regions of the skull was statistically significant, ergo the hypothesis that the horns were used as weapons in intraspecific combat is supported (Farke et al. 2009).
The authors also looked at the skull lesions of a ceratopsid that lacked long brow horns (the centrosaurine Centrosaurus). In this taxon, the incidence of lesions was much lower, and those lesions that were present were not concentrated on the frill or cheek region. These data provide further support for the idea that horned dinosaurs with long brow horns fought and injured each other intraspecifically with those horns. In contrast, the forms lacking long brow horns might have mostly relied on display, or may have fought by stabbing or butting the body rather than the head. The latter possibility is in fact supported by the fact that fractured ribs are known for these dinosaurs. Finally on this subject, you’ll know already that Andy Farke has his own excellent blog – The Open Source Paleontologist – and here, of course, he discusses this research further.
Also on the subject of giant chasmosaurine ceratopsids, something else we’ve been discussing a lot this month has been the ‘new’ name for the chasmosaurine ceratopsid Diceratops hatcheri. Diceratops was first named by Richard Swan Lull in 1905 (this was in John Bell Hatcher’s monograph: Hatcher died of typhus before completing the work). Its status as a valid genus has been somewhat controversial: we’ll get to that in a minute. Anyway, it was pointed out by Mateus (2008) that Diceratops was preoccupied by an ichneumon named in 1869. The chasmosaurine therefore needed a new name, so Mateus published Diceratus. It now turns out that Ukrainsky (2007) had already done all of this: Ukrainsky’s name is (in my opinion) rather poorer than Mateus’s but, regardless, it has priority and is therefore the one we have to use. It’s Nedoceratops. ‘Nedo’, Ukrainsky explained, indicates ‘insufficiency’, though quite why Nedoceratops is insufficient was never explained. Maybe its lacks of a prominent nasal horn makes it ‘less sufficient’ than Triceratops.
While Nedoceratops is undoubtedly similar to Triceratops, the only known specimen is different enough from its better known relative (and from the other member of the ‘giant chasmosaurine’ clade, Torosaurus) for at least some horned dinosaur workers to regard it as a distinct taxon. Compared to Triceratops, it has been said to be shorter-faced, and it has more erect brow horns and a rounded stump in place of a nasal horn. It also differs in having a thin parietal with small fenestrae, and in having squamosal fenestrae. In their paper on Eotriceratops, a fourth member of the ‘giant chasmosaurine’ clade, Wu et al. (2008) regarded all of these genera as distinct, and they further found Nedoceratops and Torosaurus to form a clade.
However, Hunt & Lehman (2008) regarded Nedoceratops as synonymous with Triceratops (as have a few previous authors). Furthermore, I learnt recently (thanks to peer-review) that some marginocephalian workers think that Eotriceratops is of questionable status, and that the characters regarded by Wu et al. (2008) as diagnostic for Eotriceratops are instead merely the consequences of ontogenetic variation within Triceratops. These characters include the shape of the epijugal (the horn-like element that projects sideways from the cheek) and the form of the epoccipitals (the small bones that line the edges of the frill). If Eotriceratops does fall within the range of variation (ontogenetic or otherwise) of Triceratops, what does this mean for Nedoceratops and Torosaurus? I realise that ontogenetic and intra-populational variation might mean that at least some alleged dinosaur taxa aren’t ‘taxa’ at all (Aublysodon, Stygivenator, Nanotyrannus and Dinotyrannus come to mind), but in this case I am – for the time being – inclined to regard all four ‘giant chasmosaurines’ as distinct [Hunt & Lehman's (2008) Torosaurus reconstruction is shown here. Like other Lehmanesque ceratopsids, note that it's shown with a freaky-short tail. And, incidentally, many of you will know the aforementioned Hunt as ReBecca Hunt-Foster of Dinochick Blogs].
That’ll have to do for now on the ornithischians, though I also wanted to discuss Holliday & Witmer’s (2008) very neat paper on cranial kinesis in dinosaurs. These authors argue that claims of alleged cranial kinesis in non-avian dinosaurs – such as the maxillary pleurokinesis proposed for iguanodontians and the alleged facial and palatal kinesis reported for some non-avian theropods – don’t stand up to scrutiny, and they show that non-avian dinosaur skulls actually lack the requisite details that might have permitted kinesis. The good news (for me) is that you don’t need me to discuss this research here: Andy Farke provides a good discuss of it here at The Open Source Paleontologist, and a ton of information (and the paper itself) is available here at Casey Holliday’s webpage.
Well, so long Ornithischia (for now). What next?
Refs – -
Dalton, R. 2009. Paper sparks fossil fury. Nature doi:10.1038/news.2009.60
Godefroit, P., Pereda Suberbiola, X., Li, H. & Dong, Z.-M. 1999. A new species of the ankylosaurid dinosaur Pinacosaurus from the Late Cretaceous of Inner Mongolia (P.R. China). Bulletin de L’Institut Royal des Sciences Naturelles de Belgique, Sciences de la Terre 69-Supp. B, 17-36.
Hill, R. V., Witmer, L. M. & Norell, M. A. 2003. A new specimen of Pinacosaurus grangeri (Dinosauria: Ornithischia) from the Late Cretaceous of Mongolia: ontogeny and phylogeny of ankylosaurs. American Museum Novitates 3395, 1-29.
Holliday, C. M. & Witmer, L. M. 2008. Cranial kinesis in dinosaurs: intracranial joints, protractor muscles, and their significance for cranial evolution and function in diapsids. Journal of Vertebrate Paleontology 28, 1073-1088.
Hunt, R. K. & Lehman, T. M. 2008. Attributes of the ceratopsian dinosaur Torosaurus, and new material from the Javelina Formation (Maastrichtian) of Texas. Journal of Paleontology 82, 1127-1138.
Mateus, O. 2008. Two ornithischian dinosaurs renamed: Microceratops Bohlin 1953 and Diceratops Lull 1905. Journal of Paleontology 82, 423.
Ukrainsky, A. S. 2007. A new replacement name for Diceratops Lull, 1905 (Reptilia: Ornithischia: Ceratopsidae). Zoosystematica Rossica 16, 292.
Wu, X.-C., Brinkman, D. B., Eberth, D. A. & Braman, D. R. 2007. A new ceratopsid dinosaur (Ornithischia) from the uppermost Horsehoe Canyon Formation (upper Maastrichtian), Alberta, Canada. Canadian Journal of Earth Sciences 44, 1243-1265.