Ankylosaur week, day 4: Panoplosaurus

Panoplosaurus mirus was a large nodosaurid (reaching 6 m) and a particularly close relative of the even larger Edmontonia (for a quick intro to nodosaurids see the day 2 article). One of several Canadian dinosaurs from the Campanian Dinosaur Park Formation named by Lawrence Lambe, Panoplosaurus was described in 1919 for a skeleton collected by Charles M. Sternberg. As in Edmontonia but unlike other nodosaurids, Panoplosaurus lacked premaxillary teeth and grew an oval scute in the cheek cavity adjacent to its teeth. These nodosaurids must, therefore, have possessed fleshy cheeks (an alternative proposal, informally suggested by Tracy Ford, is that the cheek scute was not embedded in soft tissue, but was instead simply continuous with the lateral surface of the lower jaw - see image below, borrowed from here. We previously looked briefly at ornithischian cheeks in the 2007 article on therizinosauroids).

The skull of Panoplosaurus was broad at the back but narrower at the snout tip. Furthermore, the tooth rows in the upper jaws were set well in from the edges of the skull, so the space inside the animal's mouth was narrow. These features imply that Panoplosaurus was a selective feeder, maybe only nipping off choice parts of low-growing plants. The orientation of the occipital condyle suggested to Lambe (1919) that Panoplosaurus kept its head strongly down-turned in life. Bizarrely for a quadrupedal dinosaur, Panoplosaurus only seems to have had three fingers (Sternberg 1921). While the arrangement of armour over the back of the body is difficult to reconstruct (partly because the type specimen was damaged in a fire), we do know that Panoplosaurus had transverse bands of armour across its neck, elongate keeled plates along the sides of its neck and body, and also keeled scutes running down the forelimb (Carpenter 1990). The illustration shown at the top of this article - produced by Jim Robins - shows Panoplosaurus with anterolaterally projecting shoulder spikes similar to those of Edmontonia, and most other reconstructions of Panoplosaurus depict it similarly. This is a mistake however, as Panoplosaurus seems not to have had these, instead possessing elongate, oval keeled scutes on the sides of the neck and shoulders.

The several Panoplosaurus specimens vary somewhat in skull shape, leading Bakker (1988) to argue that more than one species was present (although he didn't name any species in addition to P. mirus). Carpenter (1990) thought that this variation partly resulted from distortion caused by crushing, but also mentioned the idea that shorter-snouted individuals might be juveniles, or that the variation might represent sexual dimorphism. Bakker, incidentally, treated Panoplosaurus as worthy of its own 'subfamily', Panoplosaurine (first named by Nopcsa in 1928), itself sister to another 'subfamily', Edmontoniinae (first named by Russell in 1940). He further regarded panoplosaurines and edmontoniines as representing a distinct 'family', Edmontoniidae, which was distinct from nodosaurids proper, albeit associated with them in a newly named Nodosauroidea. And - most remarkably - he argued that nodosauroids were not close relatives of ankylosaurids, but were in fact late-surviving stegosaurs (Bakker 1988). None of these contentions have been supported by further study, although Panoplosaurus and Edmontonia are well supported as close relatives within Nodosauridae [adjacent image of P. mirus skull borrowed from WitmerLab Collections here].

More tomorrow!

Refs - -

Bakker, R. T. 1988. Review of the Late Cretaceous nodosauroid Dinosauria. Denversaurus schlessmani, a new armor-plated dinosaur from the latest Cretaceous of South Dakota, the last survivor of the nodosaurians, with comments on the stegosaur-nodosaur relationships. Hunteria 1 (3), 1-23.

Carpenter, K. 1990. Ankylosaur systematics: example using Panoplosaurus and Edmontonia (Ankylosauria: Nodosauridae). In Carpenter, K. & Currie, P. J. (eds) Dinosaur Systematics: Approaches and Perspectives. Cambridge University Press, Cambridge, pp. 281-298.

Lambe, L. M. 1919. Description of a new genus and species (Panoplosaurus mirus) of an armoured dinosaur from the Belly River Beds of Alberta. Transactions of the Royal Society of Canada, Third Series 4, 39-50.

Sternberg, C. M. 1921. A supplementary study of Panoplosaurus mirus. Transactions of the Royal Society of Canada, Third Series 4, 93-102.