Another day, another ankylosaur. This time: Silvisaurus condrayi. Known only from the Albian-Cenomanian Dakota Formation of Kansas and described by Theodore H. Eaton in 1960, Silvisaurus is a surprisingly well known, though enigmatic, nodosaurid. Because Eaton provided a life restoration in his paper (albeit it not a very good one – read on. It’s shown here), Silvisaurus is one of the few nodosaurids which has been widely depicted in the popular literature. The production of a life restoration, while not regarded as integral to a scientific description, does mean that your animal will get illustrated forever afterwards, even if the original restoration is bloody awful. I call this the ‘pretty picture syndrome’ – it explains why life restorations of Silvisaurus, Nodosaurus and Polacanthus are in all the kid’s books, while those depicting Hierosaurus, Pawpawsaurus and Hylaeosaurus are not…
The only known specimen of Silvisaurus was discovered by Warren Condray in a streambed on his farm, and cattle trampling and erosion had damaged it. Nevertheless, it yielded a good skull and much of the skeleton except the pectoral girdle, tail, and much of the hindlimbs. The skull, which is pear-shaped when seen from above, exhibits a narrow muzzle and a prominent scute just behind each nostril (Carpenter & Kirkland 1998). A large postorbital boss was also present, as was a small, rounded jugal boss [Eaton’s skull illustrations are shown below. The lateral view is inaccurate in not depicting the laterotemporal fenestra – this opening is in fact entirely visible in the specimen].
Eaton (1960) reconstructed Silvisaurus with armour and beak tissue covering most of the large external nostril, but Carpenter & Kirkland (1998) noted that this probably wasn’t the case. Big nostrils were present in many ankylosaurs, as they were in other ornithischians like ceratopsians and iguanodontians, and at the moment all we can do is wave our arms and say something about CNS cooling (my, how well the evidence for ectothermic dinosaurs is stacking up these days). Soft tissue would have occupied most of the external nostril, but I don’t think anyone has studied the extent, form, and function of such in ankylosaurs. Like other nodosaurids except Edmontonia and Panoplosaurus, Silvisaurus possessed premaxillary teeth and had a higher number (eight or nine) than any other ankylosaur.
Eaton (1960) also reconstructed Silvisaurus with transverse bands of armour across its neck and back, laterally projecting shoulder spikes, a big plate over the hips and tail-base, and a spiky tail. Reinterpretation has shown that Silvisaurus possessed at least three cervical half-rings, it probably had at least one shoulder spike, and it also had flat, rectangular scutes over the pelvic region (Carpenter & Kirkland 1998, Ford 2000). However, nothing is known about its tail armour. If Silvisaurus was closely related to Sauropelta (read on), it may similarly have possessed a lengthy tail consisting of more than 40 vertebrae [image below shows John Conway’s Sauropelta, borrowed from wikipedia. We don’t know how long ankylosaur spikes were in life, as of course we only have the bony cores and not the keratin sheaths that covered them. In this restoration, John has added particularly big sheaths to the neck spikes. This restoration was [I presume] produced before the recent discovery that Sauropelta actually had two parallel rows of neck spikes, one projecting dorsolaterally and the other projecting ventrolaterally].
Exactly how Silvisaurus was related to other nodosaurids has been somewhat problematical. Its premaxillary teeth, poorly developed osseous secondary palate and style of armour suggest that it was a basal nodosaurid, and it shares a few features (including high number of mandibular teeth and form of cervical armour) with the far better known Sauropelta: Ford (2000) united the two in a new nodosaurid subfamily he named Sauropeltinae, Carpenter (2001) found Sauropelta and Silvisaurus to be sister-taxa and deeply nested within Nodosauridae, while Vickaryous et al. (2004) found Pawpawsaurus, Sauropelta and Silvisaurus to group in an unresolved polytomy at the base of Nodosauridae (only Cedarpelta was more basal among the taxa they included: this taxon was originally described as an ankylosaurid perhaps allied to Shamosaurus). All of these basal nodosaurids are from the Lower Cretaceous of the USA. With a skull 33 cm long, Silvisaurus had a total length of around 4 m.
And, tomorrow, we say farewell to the Ankylosauria. Don’t miss the next thrilling instalment.
Refs – –
Carpenter, K. 2001. Phylogenetic analysis of the Ankylosauria. In Carpenter, K. (ed) The Armored Dinosaurs. Indiana University Press (Bloomington and Indianapolis), pp. 455-483.
– . & Kirkland, J. I. 1998. Review of Lower and Middle Cretaceous ankylosaurs from North America. New Mexico Museum of Natural History and Science Bulletin 14, 249-270.
Eaton, T. H. 1960. A new armoured dinosaur from the Cretaceous of Kansas. The University of Kansas Paleontological Contributions, Article 8, 1-24.
Ford, T. L. 2000. A review of ankylosaur osteoderms from New Mexico and a preliminary review of ankylosaur armor. New Mexico of Natural History & Science Bulletin 17, 157-176.
Vickaryous, M. K., Maryanska, T. & Weishampel, D. B. 2004. In Weishampel, D. B., Dodson, P. & Osmólska, H. (eds) The Dinosauria, Second Edition. University of California Press (Berkeley), pp. 363-392.