Edmontonia

Edmontonia
Name	Edmontonia
Order	Ornithischia
Suborder	Ankylosauria
Class	Nodosauridae
Name Translation	Named after the Edmonton Formation
Period	Late Cretaceous (76-65 mya)
Location	Canada
Diet	Plants
Size	23 ft (7 m)

Edmontonia (ed-MON-tone-EE-uh) was a nodosaur ankylosaurid that lived in the Late Cretaceous Period (76.5-65.5 mya). Like other ankylosaurs, it was a heavily armored herbivore that was designed to stay and fight instead of run away.

It lived alongside hadrosaurs like Edmontosaurus, ceratopsians like Pachyrhinosaurus, raptors like Troodon, and tyrannosaurs like Albertosaurus. It was first discovered in 1915 in the Edmonton Formation (now known as the Horseshoe Canyon Formation), and its range included Canada and the northern USA. It was one of the last non-clubbed ankylosaurs to date.

Description

Size and general build

Size comparison Edmontonia was bulky, broad and tank-like. Its length has been estimated at about 6.6 m (22 ft). In 2010, Gregory S. Paul considered both main Edmontonia species, E. longiceps and E. rugosidens, to be equally long at six metres and weigh three tonnes.

Edmontonia had small, oval ridged bony plates on its back and head and many sharp spikes along its sides. The four largest spikes jutted out from the shoulders on each side, the second of which was split into subspines in E. rugosidens specimens. Its skull had a pear-like shape when viewed from above. Its neck and shoulders were protected by three halfrings made of large keeled plates.

Distinguishing traits

Restoration of E. rugosidens In 1990, Kenneth Carpenter established some diagnostic traits for the genus as a whole, mainly comparing it with its close relative Panoplosaurus. In top view, the snout has more parallel sides. The skull armour has a smooth surface. In the palate, the vomer is keeled. The neural arches and neural spines are shorter than those of Panoplosaurus.

Carpenter also indicated in which way the main species differed from each other. The type species, Edmontonia longiceps, is distinguished from E. rugosidens in lacking sideways projecting osteoderms behind the eye sockets; having tooth rows that are less divergent; possessing a more narrow palate; having a sacrum that is wider than long and more robust; and in having shorter spikes at the sides.

Skeleton

The skull of Edmontonia, up to half a metre long, is somewhat elongated with a protruding truncated snout. The snout carried a horny upper beak and the front snout bones, the premaxillae, were toothless.

The cutting edge of the upper beak continued into the maxillary tooth rows, each containing fourteen to seventeen small teeth. In each dentary of the lower jaws, eighteen to twenty-one teeth were present. In the sides of the snout large depressions were present, "nasal vestibules", that each possessed two smaller openings. A study by Matthew Vickaryous in 2006 proved for the first time the presence of multiple openings in a nodosaurid; such structures had already been well established in ankylosaurids. The air tracts are however, much simpler than in the typical ankylosaurid condition, and are not convoluted while lacking bony turbinate bones. Another similarity with Ankylosauridae is the presence of a secondary bone palate, a possible case of parallel evolution. This has been shown too for Panoplosaurus. The AMNH 5381 specimen of E. rugosidens, 1915 (first referred to Palaeoscincus by Matthew in 1922), showing the position of the dermal armour.

The head armour tiles, or caputegulae, are smooth. Details differ between the various specimens but all share a large central nasal tile on the snout, bend large "loreal" tiles at the rear snout edges and a large central caputegula on the skull roof. Contrary to that discovered with Panoplosaurus, it is "free-floating", not fused with the lower jaw bone.

The vertebral column contains about eight neck vertebrae, about twelve "free" back vertebrae, a "sacral rod" of four fused rear dorsal vertebrae, three sacral vertebrae, two caudosacrals and at least twenty, but probably about forty, tail vertebrae. In the neck the first two vertebrae, the atlas and axis, are fused. In the shoulder girdle, the coracoid has a rectangular profile, in contrast to the more rounded shape with Panoplosaurus. Two sternal plates are present, connected to sternal ribs. The forelimb is robust but relatively long. In Edmontonia longiceps and E. rugosidens the deltopectoral crest of the humerus is gradually rounded.

Osteoderms

Skull and neck armor Edmontonia reconstruction in Royal Tyrrell Museum of Paleontology.

Apart from the head armour, the body was covered with osteoderms, skin ossifications. The configuration of the armour of Edmontonia is relatively well known, much of it having been discovered in articulation. The neck and shoulder region was protected by three cervical halfrings, each consisting of fused rounded rectangular, asymmetrically keeled, bone plates. These halfrings did not have a continuous underlying bone band. The first and second halfrings each had three pairs of segments. Below each lower end of the second halfring a side spike was present, a separate triangular osteoderm pointing obliquely forward. In the third halfring over the shoulders, the two pairs of central segments are bordered on each side by a very large forward-pointing spike that is bifurcated, featuring a secondary point above the main one. A third large spike behind it points more sideways; a smaller fourth one, often connected to the third at the base, is directed obliquely to behind. The row of side spikes is continued to the rear but there the osteoderms are much lower, curving strongly to behind, with the point overhanging the rear edge. Gilmore had trouble believing that the shoulder spikes really pointed to the front as this would have greatly hampered the animal while moving through vegetation. He suggested that the points had shifted during the burial of the carcass. However, Carpenter and G.S. Paul, trying to reposition the spikes, found that it was impossible to rotate them without losing conformity with the remainder of the armour. The side spikes have solid, not hollow, bases. The spikes differ in size between E. rugosidens individuals; those of the E. longiceps holotype are relatively small.

Behind the third halfring the back and hip are covered by numerous transverse rows of much smaller oval keeled osteoderms. These are not ordered in longitudinal rows. The front rows have plates oriented along the length of the body, but to the rear the long axis of these osteoderms gradually rotates sideways, their keels ultimately running transversely. Rosettes are lacking. The configuration of the tail armour is unknown. The larger plates of all body parts were connected by small ossicles. Such small round scutes also covered the throat.

Discovery and species

Life restoration of two E. rugosidens from 1922, based on the 1915 AMNH specimen.

In 1915, the American Museum of Natural History obtained the nearly complete, articulated front half of an armoured dinosaur, found the same year by Barnum Brown in Alberta, Canada. In 1922, William Diller Matthew referred this specimen, AMNH 5381, to Palaeoscincus in a popular-science article, not indicating any particular species. It had been intended to name a new Palaeoscincus species in cooperation with Brown but their article was never published. Matthew also referred specimen AMNH 5665, the front of a skeleton found by Levi Sternberg in 1917. In 1930 Charles Whitney Gilmore referred both specimens to Palaeoscincus rugosidens. This species was based on type specimen USNM 11868, a skeleton found by George Fryer Sternberg in June 1928. The specific name is derived from Latin rugosus, "rough", and dens, "tooth". In 1940, Loris Shano Russell referred all three specimens to Edmontonia, as an Edmontonia rugosidens.

Meanwhile, the type species of Edmontonia, Edmontonia longiceps, had been named by Charles Mortram Sternberg in 1928. The generic name Edmontonia refers to Edmonton or the Edmonton Formation. The specific name longiceps means "long-headed" in Latin. Its holotype is specimen NMC 8531, consisting of a skull, right lower jaw and much of the postcranial skeleton, including the armour. It was discovered near Morrin in 1924 by George Paterson, the teamster of the expedition led by C.M. Sternberg.

Edmontonia species include:

• E. longiceps, the type, known from a complete skull, is known from the middle Horseshoe Canyon Formation (Unit 2) which used to be dated to 71.5-71 million years ago. This unit, which straddles the Campanian-Maastrichtian boundary, has since been recalibrated to an age of about 72 million years. Isolated bones and shed teeth from E. longiceps are also known from the upper Judith River Formation in Montana. Left side of E. rugosidens specimen AMNH 5665

• E. rugosidens. This species has been given its own genus, Chassternbergia, first coined as a subgenus by Dr. Robert Thomas Bakker in 1988, as Edmontonia (Chassternbergia) rugosidens and is based on differences in skull proportion from E. longiceps and its earlier time period. It was given its full generic name in 1991 by George Olshevsky. The name Chassternbergia honours Charles, "Chas", M. Sternberg. This subgenus or genus name is rarely applied. E. rugosidens is found in the Campanian lower Dinosaur Park Formation, dating from about 76.5-75 million years ago. Many later finds have been referred to E. rugosidens, among them CMN 8879, the top of a skull found in 1937 by Harold D'acre Robinson Lowe; ROM 433, a forked spine found by Jack Horner in 1986 among Oohkotokia material; ROM 5340, paired medial plates; ROM 1215, a skeleton; RTMP 91.36.507, a skull; RTMP 98.74.1, a possible Edmontonia skull; RTMP 98.71.1, a skeleton; RTMP 98.98.01, a skull and right lower jaw; and RTMP 2001.12.158, a skull.

Edmontonia schlessmani was a renaming in 1992 of Denversaurus schlessmani ("Schlessman's Denver lizard") by Adrian Hunt and Spencer Lucas. This taxon was erected by Bakker in 1988 for a skull from the Late Maastrichtian Upper Cretaceous Lance Formation of South Dakota, specimen DMNH 468 found by Philip Reinheimer in 1922. This type specimen of Denversaurus is in the collections of the Denver Museum of Natural History (now the Denver Museum of Nature and Science), Denver, Colorado for which the genus was named. The specific name honours Lee E. Schlessman, whose Schlessman Family Foundation sponsored the museum. Bakker described the skull as being much wider at the rear than Edmontonia specimens. However, later workers explained this by its being crushed, and considered the taxon a junior synonym of Edmontonia longiceps. The Black Hills Institute has referred a skeleton from the Lance Formation to Denversaurus, nicknamed "Tank". It has the inventory number BHI 127327. New research indicates that it is closely related to Panoplosaurus.

Edmontonia australis was named by Tracy Lee Ford in 2000 on the basis of cervical scutes, the holotype NMMNH P-25063, a pair of medial keeled neck osteoderms from the Maastrichtian Kirtland Formation of New Mexico and the paratype NMMNH P-27450, a right middle neck plate.

• Although later considered to a dubious name, it is now considered a junior synonym of Glyptodontopelta mimus.

The naming history was further complicated in 1971, when Walter Preston Coombs Jr renamed both Edmontonia species, into Panoplosaurus longiceps and Panoplosaurus rugosidens respectively. The latter species, which due to its much more complete material has determined the image of Edmontonia, until 1940 thus appeared under the name of Palaeoscincus, and during the 1970s and 1980s was shown as "Panoplosaurus" until newer research revived the name Edmontonia.

In 2010, G.S. Paul suggested that E. rugosidens was the direct ancestor of Edmontonia longiceps and the latter was again the direct ancestor of E. schlessmani.

Phylogeny

C.M. Sternberg originally did not provide a classification of Edmontonia. In 1930, L.S. Russell placed the genus in the Nodosauridae, which has been confirmed by subsequent analyses. Edmontonia was generally shown to be a derived nodosaurid, closely related to Panoplosaurus. Russell in 1940 named a separate Edmontoniinae. In 1988 Bakker proposed that the Edmontoniinae with the Panoplosaurinae should be joined into Edmontoniidae, the presumed sister group of the Nodosauridae within Nodosauroidea which he assumed not be ankylosaurians but the last surviving stegosaurians. Exact cladistic analysis has not confirmed these hypotheses however, and the concepts of Edmontoniinae and Edmontoniidae are not in modern use.

Paleobiology

Edmontonia would not have made an easy meal for a hungry tyrannosaur. It had a heavily armored body and large, forward-pointing shoulder spines. We know what Edmontonia looked like because two specimens were found with their armor and spikes preserved in the position they had in life. The bodies of these specimens may have dried out because of a drought and then been quickly covered by sediment when the rainy season began. Evidence for these changes in the climate is found in the growth rings of fossil wood.

Edmontonia walked on four legs and was a plant-eater. It had a pear-shaped skull (when viewed from the top). The neck and part of the back were protected by large, flat, keeled (ridged) plates. Smaller keeled plates covered the back, hips, and tail. Spines and large spikes along its sides would have made the animal look short and wide when viewed from the front. This made it look more menacing to an enemy.

An Albertosaurus was not the only animal it needed to protect itself from. A male Edmontonia probably fought with other males for territory and females. The larger males may have used their large shoulder spines for shoving contests. The spines of Edmontonia would have been dangerous to rival males or to an Albertosaurus or Daspletosaurus, if they got too close. Like other ankylosaurs, Edmontonia most likely spent most of its life alone. It was nearly completely covered in armor, with only its soft underbelly left unprotected. If a predator like Albertosaurus came by, it just tucked itself as close as it could to the ground and wait for the tyrannosaur to pass. Along with armor, Edmontonia also had long, sharp, scissor-like spikes along its shoulders, some growing to be nearly a foot and a half (46 cm.) long. It had a thick, tank-like body with stout, slow legs. It didn't have a club on the end of its tail like its more famous cousins Ankylosaurus and Euoplocephalus, but it ended in more like a sharp, bony whip, probably good for whapping tyrannosaur legs and making them back-off.

In Popular Culture

• Edmontonia pronounced as an Ankylosaur is featured in March of the Dinosaurs, where one lives up in the Arctic Circle along with a herd of Edmontosaurus and Pachyrhinosaurus. When the other two herds migrate South to escape the freezing winter, the Edmontonia stays behind and slips in the snow while trying to get some food, and is flipped over onto its back, unable to flip itslef back over. It was nearly killed by a pack of Troodon and a Gorgosaurus, but the tyrannosaur flips it while trying to take the ankylosaur from the Troodon, and the nodosaur mortally wounds it. It then continued on its way and continued to live on as the herds returned.

• An Edmontonia appears in Life After Dinosaurs where it is shown ridiculously to live in South America.
• Edmontonia is featured in Dinosaur King

• Several Edmontonia makes a Big Screen appearance in the 2013 Film Walking with Dinosaurs: The 3D Movie. Like the other one from March of the Dinosaurs, It was also pronounced as an Ankylosaur.