Ankylosaur week, day 5: Edmontonia

Welcome to day 5 of ankylosaur week. This time, we look at Panoplosaurus' sister-taxon Edmontonia. Edmontonia was a large (6-7 m long) Campanian-Maastrichtian nodosaurid that lived right across North America...

Two species are presently recognised. The type species, E. longiceps, named by Charles M. Sternberg in 1928 for a specimen from the Horseshoe Canyon Formation of Alberta, is today known from multiple units of both the USA and Alberta [adjacent image shows the Drumheller Edmontonia model].

The second species, E. rugosidens, was first described in 1930 by Charles Gilmore for a specimen discovered by George Sternberg in the Upper Two Medicine Formation of Montana. This species has also proved widespread, being known from various Campanian units of Wyoming, Texas and Alberta. E. longiceps and E. rugosidens differ in skull shape, with E. longiceps having a narrower palate and a smaller boss behind the eye than E. rugosidens. Bakker (1988) argued that E. rugosidens was distinct enough from E. longiceps in its shorter snout and expanded postorbital region to warrant its own 'wide-bossed edmontoniine' subgenus, Chassternbergia (named in honour of Charles Sternberg, obviously. Shades of Florentino Ameghino there*). He also suggested that Chassternbergia specimens from the Judith River Formation might represent a new species or subspecies of this subgenus. Two authors later regarded Chassternbergia as a distinct genus (Olshevsky 1991, Ford 2000), but it has otherwise been thought that E. rugosidens should remain with E. longiceps in Edmontonia.

* The Argentinean palaeontologist Florentino Ameghino enjoyed naming fossil mammals after colleagues, and gave us such wonders as Asmithwoodwardia, Thomashuxleya, Oldfieldthomasia, Ricardolydekkeria, Ricardowenia, Ernestokokenia, Edvardcopeia, Edvardotrouessartia, Henricofilholia, Carolozittelia, Othnielmarshia and Henricosbornia.

Death of Denversaurus

Bakker (1988) also named a new edmontoniine genus and species, Denversaurus schlessmani, for a specimen from the Hell Creek Formation of South Dakota, originally described by Carpenter & Breithaupt (1986) as Edmontonia sp. This animal would have lived alongside Triceratops and Tyrannosaurus, but its position low down in the Hell Creek Formation led Carpenter & Breithaupt (1986) to argue that, while nodosaurids persisted into the Maastrichtian, they apparently disappeared before the end-Maastrichtian event (they didn't base this contention on this specimen alone however, as there are other Maastrichtian nodosaurids). Bakker regarded D. schlessmani as distinct because, compared to other edmontoniines, its orbits were located further posteriorly and the underside of the occipital region was broader. Alas, these putative differences are almost certainly the result of the distortion that the skull had undergone: Carpenter (1990) argued that the diagnosis provided for this species relied on how the skull was reconstructed, and furthermore that the reconstruction was not necessarily accurate anyway [adjacent image is Bakker's iconic 1986 charging Edmontonia. The snout is too short for that genus actually].

Exactly what we're meant to do with D. schlessmani today seems uncertain: everyone agrees that it can't be reliably diagnosed as a distinct species, and should be regarded as part of Edmontonia, but various Hell Creek and Lance Formation specimens are listed by Vickaryous et al. (2004) as representing an 'unnamed Edmontonia species'. Edmontonia specimens from the Aguja Formation of Texas and the Upper Matanuka Formation of Alaska are also regarded as representing an 'unnamed Edmontonia species', but I suspect what's meant here is that we lack remains good enough to diagnose and name these species (Agaju Formation material has more recently been referred to E. longiceps). A third Edmontonia species, E. australis, was named by Ford (2000) for cervical scutes from the Maastrichtian Kirtland Formation of New Mexico. These scutes differ in shape and in the position of their dorsal keels from those of other Edmontonia specimens, but the species has since been listed as a nomen dubium (Vickaryous et al. 2004).

Palates, sinuses and cheeks (again)

In the long-muzzled skull of Edmontonia, the orbits are located far back and are surrounded by bony bosses. These may have protected the eye during combat, but the position of the orbits might also imply that Edmontonia had a particularly extensive field of vision. I can't say any more on this subject, because it's totally unstudied (so far as I know). Edmontonia is also interesting in having a particularly extensive osseous secondary palate (OSP). Some ankylosaurids also had one of these, but because it's absent in both basal ankylosaurids and basal nodosaurids it must have evolved convergently. Edmontonia also resembles OSP-bearing ankylosaurids in possessing sinuses on the lateral sides of its internal nasal cavity, dorsal to the tooth row. Furthermore, the internal nasal cavity was subdivided by a vertical plate of bone, and a bony keel along the palate (formed by fused vomers) would seem to have divided the mouth cavity and decreased the space available for food (Vickaryous et al. 2004, Vickaryous 2006). We now know that Edmontonia was like Panoplosaurus in having a special 'cheek scute', and CT scans show that this wasn't fused to either of the jaws but must have been 'floating' in soft tissue (Vickaryous 2006). All of these features suggest that Edmontonia was doing quite complex things during eating. The OSP and nasal sinuses might have evolved to reinforce the skull (acting as a structural brace); the divided nasal chamber shows that paired streams of air were transmitted during respiration; and the presence of cheeks suggests that food was retained in the mouth while complex chewing was practised [in the adjacent CT-scan, the subdivided nasal chamber is clearly visible, as are cross-sections through the cheek plates. In the image below, the paranasal sinuses are visible below the lateral sides of the nasal chamber, just dorsal to the cheek plates. The deep keel on the palate is also visible. Images kindly provided by Matt Vickaryous and used with permission: for further explanations of these scans see Vickaryous et al. 2004 and Vickaryous 2006].

Edmontonia's most remarkable features were the elongate spikes on the neck and shoulders, the two most anterior of which pointed forwards and sideways. One of the longest spikes had a forked tip, but specimens differ in how pronounced this fork was and it might have been sexually dimorphic. The fact that Edmontonia ranged far and wide across Campanian and Maastrichtian North America suggests that it was an ecological generalist, able to make a living in diverse habitats and to feed on numerous types of plants. These dinosaurs would have had to watch out for giant tyrannosaurids - the dominant predators in their ecosystems - but again I must protest against the textbook wisdom that these dinosaurs squatted on the ground in the vain hope that the lurking 5-ton theropod would get bored and wander off. That is definitely a crap idea.

And - more tomorrow! Credit to puuikibeach for the above.

PS - happy Valentine's Day!

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.

- . & Breithaupt, B. 1986. Latest Cretaceous occurrence of nodosaurid ankylosaurs (Dinosauria, Ornithischia) in western North America and the gradual extinction of the dinosaurs. Journal of Vertebrate Paleontology 6, 251-257.

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.

Olshevsky, G. 1991. A Revision of the Parainfraclass Archosauria Cope, 1869, Excluding the Advanced Crocodylia. Publications Requiring Research, San Diego.

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.

- . 2006. New information of the cranial anatomy of Edmontonia rugosidens Gilmore, a Late Cretaceous nodosaurid dinosaur from Dinosaur Provincial Park, Alberta. Journal of Vertebrate Paleontology 26, 1011-1013.