One of the most distinctive features of ceratopsian dinosaurs is the conspicuous bony frill, formed from the parietal and squamosal bones, that projected backwards (and sometimes upwards too) from the rear margin of the skull.
Typically decorated around their edges by semi-circular bones called epoccipitals, and sometimes sporting horns, spikes or mid-line keels, frills were a fundamental part of the skull: this means that, whenever the animal moved its head up, or down, or to the side, the whole frill moved too. The fact that the ceratopsian occipital condyle is damn-near spherical shows that the head-neck joint was particularly mobile, and that these dinosaurs were well able to dip the snout right down to the ground, to throw the head from side to side, and to point the snout upwards until the frill rested on the shoulders.
Because the supratemporal fenestrae (the openings at the back of the skull that, at their edges, anchor various jaw muscles*) open on the surface of the frill, at least some of the jaw musculature must have extended on to the frill. In normal tetrapods, these muscles are restricted to the areas just around the fenestrae… but ceratopsians aren’t normal tetrapods: the bones at the rear of the skull are horrendously modified. Could, therefore, the massive frill be a specialisation allowing hypertrophied jaw musculature? A number of authors have proposed exactly this, and have imagined ceratopsian jaw musculature extending right across the frill, all the way to the rear margin. As Dodson (1996) explained, the logic behind this super-muscled frill was flawed: animals do not need gigantic, super-long jaw-closing muscles to have a powerful bite, and the surface texture of the frill is not consistent with a covering of musculature. It seems that the musculature was, most likely, restricted to the areas just around the supratemporal fenestrae [adjacent reconstructions of jaw musculature in Chasmosaurus and Styracosaurus from Dodson (1996). Note that Styracosaurus is now known to have a shorter nasal horn than the one shown here].
* Note that the ‘windows’ on the frills of most ceratopsians are not the same thing as the supratemporal fenestrae.
However, back when the hypothesis of extensive frill musculature was considered reasonable, a truly extreme version was proposed by John McLoughlin in his book Archosauria: : A New Look at the Old Dinosaur. McLoughlin (1979) proposed that the frill did not just anchor musculature: it was essentially buried by a massive amount of muscle that extended around the back of the skull, and across the shoulders too. I never saw John’s book until I was in my 20s, and only knew of his ‘alternative’ look for ceratopsians thanks to a picture [shown here] in David Lambert’s Collins Guide to Dinosaurs (one of my favourite childhood books). Some of his reconstructions, which look very strange today, are shown here: his Triceratops is shown above, and his Chasmosaurus is below.
While these reconstructions are improbable for the reasons mentioned above, they can also be considered illogical because they would mean that the frill – and hence the entire head – was virtually immobile, and held firmly in place against the shoulders. The projecting epoccipitals, knobs and other structures on the frill surfaces and edges also show that the frill cannot have been submerged in soft tissue. Furthermore, the frills of some ceratopsid ceratopsians – notable examples include Pentaceratops and Agujaceratops – are so long, and so strongly inclined upwards relative to the long axis of the skull, that there can be no doubt that they stood well up above the shoulders in life, and cannot have been ‘tied’ to the thorax by soft tissues. While some modern lizards sport bony frills that are connected to the thorax by skin webs (the iguanian Corytophanes is perhaps the best example: see the photo I used here), the frills in these animals are narrow, sheet like structures. Certain chameleons do have wider, more ceratopsian-like frills, and these are not connected by soft tissue to the animal’s back.
Well, this post started as an excuse to show some of John McLoughlin’s interesting pictures, but (like so many others) it quickly got out of hand. For previous ceratopsian posts see…
- Tet Zoo picture of the day # 11 (or, speculations on omnivory in ceratopsids)
- Pentaceratops: that’s quite the skull
- SVPCA 2007: dinosaurs attack (includes stuff on Psittacosaurus)
- A month in dinosaurs (and pterosaurs): 3, Minotaurasaurus and giant chasmosaurines
- Udanoceratops tschizhovi, the basics
- No-one talks about Anchiceratops, boo hoo
- Zuniceratops and the early acquisition and alleged dimorphism of ceratopsian brow horns
Refs – –
Dodson, P. 1996. The Horned Dinosaurs. Princeton University Press, Princeton, NJ.
McLoughlin, J. C. 1979. Archosauria: A New Look at the Old Dinosaur. Penguin Books, London.