Edmontonia

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Ornithischia

Family: Nodosauridae

Genus: Edmontonia

Species: Edmontonia longiceps

Introduction

Edmontonia has proved to be one of the great finds of the nodosaurs, which are armored dinosaurs that thrived from the Middle Jurassic to the Late Cretaceous periods (174 to 66 million years ago). They were closely related to the more famous ankylosaurs, which included Ankylosaurus (“armored dinosaur”), and which were among the last dinosaurs to survive. Described as “tank-like,” the armored Edmontonia was bulky and covered in armor made from spikes and scutes, or bony scales and plates arranged in rows.

Two definite and accepted species for the genus have been unearthed, each yielding nearly complete skeletons and full skulls: Edmontonia longiceps and Edmontonia rugosidens. These fossils have enabled a thorough reconstruction of the dinosaur. Like other nodosaurs, Edmontonia lacked a characteristic club at the end of its spiked tail, but otherwise appears to be a classic example of an ankylosaurian dinosaur, and was one of the largest armored dinosaurs known. Though Edmontonia longiceps was first identified and assigned to the genus, Edmontonia rugosidens was initially misclassified under the genus Paleoscincus.

Classification

Under the Linnaean system, the two known species of Edmontonia have been classified under the order Ornithischia. One of the orders of dinosaurs, the ornithischians were known for their bird-like hip structure, with their pelvic bones pointed down and back towards their tails. Dinosaurs in this order also tended to be herbivores and to have hoof-like claws on their feet.

Within the ornithischian order, Edmontonia has been further classified as a member of the quadrupedal Thyreophora suborder and the Ankylosauria infraorder. The Thyreophora are known primarily for their armored bodies and included the suborder Stegosauria (of which Stegosaurus is the most famous). This armor varied from long shield-like plates to smaller plates and spikes. In particular, the Ankylosauria had thick armor, often spiked, and comprised two families of dinosaur—the ankylosaurids (Ankylosauridae) and the nodosaurids (Nodosauridae).

The two families have generally been distinguished by the presence of a clubbed tail, which most ankylosaurids possessed but nodosaurids did not. However, some very early specimens of ankylosaurids have been found to lack the clubbed tail as well. In place of the clubbed tail, the spikes and ridges of the nodosaurids’ armor extended down their back and along their tail (while ankylosaurid tails were smoother). Edmontonia, with its spiked but club-less tail, belongs to the Nodosauridae family. Edmontonia took its genus name from the rock formation (which was named after the Canadian city of Edmonton in Alberta) in which it was found and is notable for its distinctive double shoulder spikes.

Scientists also use another classification system, called phylogenetic or cladistic classification. This system classifies animals in family trees grouped by common ancestry. Each split in the tree creates a new group based on shared physical traits. Within cladistics, Edmontonia resides on the Ankylosauria clade, or branch, along the Ornithischia clade of the dinosaur family tree. This branch breaks down into two additional branches—Ankylosauridae and Nodosauridae. As in the Linnaean system, Edmontonia belongs to the Nodosauridae branch, which divides into several smaller clades. The smallest clade to which Edmontonia belongs comprises the two known species, E. longiceps and E. rugosidens, with their next closest relative being Panoplosaurus.

A common ancestor for ornithischians like Edmontonia and theropods like Allosaurus was posited in 2017, and a new clade, Ornithoscelida, was proposed to encompass both groups.

Anatomy

Given its taxonomy, Edmontonia is well-known for its armored body. One of the larger members of the nodosaurid family, Edmontonia had a large torso on four stout legs with wide, four-toed feet. It grew to about six meters (eighteen feet) in length. A long, flat skull sat atop a short, thick neck, framed by distinctive double shoulder spikes. Across its hunched back, rows of triangular spikes enhanced its armor and spilled down along the length of its short tail. Edmontonia's snout curved downward into a toothless, horned beak, and it had flat teeth within its pouched cheeks. Fenestrae, or openings in the skull, indicate eyes set on the sides of the skull.

Dinosaurs have generally been regarded as “cold-blooded,” like modern lizards, though the term itself is deceptive. Modern lizards are ectothermic, meaning that they rely on the sun and other environmental factors to warm their bodies. Though science has traditionally held dinosaurs, specifically non-avian dinosaurs, to be ectothermic, evidence suggests that some might have been endothermic, meaning that they could heat their bodies from within by consuming food. Scientists continue to debate whether nodosaurs such as Edmontonia might have been either ectotherms or endotherms, or possibly something in between.

Intelligence

Slower-moving herbivores tend to fall lower on the EQ scale than their faster, carnivorous, predatory counterparts. The nodosaurids and ankylosaurids, with their large bodies and small heads, are believed to have fallen on the lower end of that scale. Nodosaurids, including Edmontonia, likely ranked around 0.5 on the EQ scale, just above their even larger contemporaries, the sauropods.

Reproduction and Population

Existing evidence indicates that most dinosaurs, including Edmontonia and related nodosaurs and ankylosaurs, were oviparous. This means that they reproduced by laying eggs. However, fossil evidence does not exist to support conclusively whether these dinosaurs laid eggs. Nor does any evidence suggest whether Edmontonia made nests for their eggs (and if so, what type), stayed with their nests, or nested and tended their young collectively as many other herbivorous dinosaurs are believed to have done. Nesting sites and egg fossils for Edmontonia or its close relatives need to be found before further conclusions can be drawn regarding their reproductive habits.

Thus far, skeletal evidence does not suggest any marked distinction between males and females of the species. Some scientists speculate that the prominent shoulder and lateral spikes of Edmontonia served to attract mates and to spar with competitors among their species.

Geographic distribution of nodosaurid and ankylosaurid fossils indicates that they were widespread across the globe and likely existed in great numbers. However, specific demographic information is unavailable. It seems likely, based on the quantity of fossil evidence, that Edmontonia and its relatives were not as numerous as the dominant herbivores of the time, the hadrosaurs and ceratopsians.

Diet

Edmontonia belonged to an order and family of dinosaurs known to have been herbivores. Its weak jaws, beak-like snout, small teeth and large gut support the conclusion that Edmontonia fed on plant life in the Late Cretaceous (99 to 66 million years ago). Given the time period, their diet most likely comprised ferns, gymnosperm (such as conifers and cycads), and perhaps the newly evolved angiosperms. The heavy body structure and short legs of Edmontonia indicate that these dinosaurs likely grazed on low-lying vegetation. Like most herbivores, Edmontonia probably spent a great deal of time looking for and consuming food.

Behavior

Fossil evidence for Edmontonia reveals that it was likely a slow-moving, even sluggish dinosaur who spent its life walking on four legs, low to the ground, in search of food. Its heavily armored, scute-covered body and thick spike tails provided its primary defense. Given its bulk and skeletal structure, Edmontonia and its relatives are not likely to have fled in the face of predatory pursuit. More likely, they made use of their armor—which in Edmontonia, included dangerously sharp spikes—and curled into themselves, crouching protectively with their tough backs exposed and their more-vulnerable undersides protected. The long shoulder and lateral spikes would have afforded a great deal of protection, and its frontal direction indicates that Edmontonia could have attacked or defended itself by lunging forward.

Like other large and slow-moving herbivores, Edmontonia may have been a migratory herd animal. This means that it would have traveled in large groups from place to place in search of food. However, fossil evidence at this time is insufficient to draw firm conclusions about potential herd behavior among nodosaurids and ankylosaurids. Scientists continue to debate whether these dinosaurs were social creatures that traveled in large groups or more solitary wanderers.

Habitat and Other Life Forms

Edmontonia and most of its nodosaurid and ankylosaurid relatives thrived in the western regions of the United States and Canada during the Late Cretaceous, a time of increasing plant and animal diversification. The climate and landscape of this time period differed markedly from that of the same region today. The climate was far warmer, likely subtropical, and the land was likely covered in thick forests and swamps.

By this time, the vast supercontinent of Pangaea had split into two smaller supercontinents, Gondwanaland and Laurasia. Throughout the Late Cretaceous, tectonic change continued, with the breaking apart of Laurasia into smaller continents. These tectonic shifts brought great changes to the climate and landscape. Species became more isolated even as new plants and animals emerged. The Late Cretaceous would prove to be the final period of dinosaur life on earth, during which dinosaurs shared the land with a diverse variety of insects, birds, mammals, amphibians, and flora.

The nodosaurids of western North America shared their landscape with many other great and familiar dinosaurs of the time—including their fellow herbivores, the ceratopsians and hadrosaurs, and their fierce predators, the tyrannosaurs (or tyrannosaurids). Albertosaurus, a close relative of the infamous Tyrannosaurus rex, was likely a predator of Edmontonia and its relatives, either actively hunting or scavenging the dinosaur. Edmontonia may have lived in and migrated through grasslands, woodlands, and wetlands.

Research

Edmontonia longiceps was the first species of Edmontonia unearthed. Discovered by George Paterson in 1924 in the Edmonton Formation (now called the Horseshoe Canyon Formation) in Alberta, Canada, Edmontonia received its classification by Charles M. Sternberg in 1928. Subsequent finds were made in the Judith River Formation in Montana, the Lance Formation in South Dakota, and the Dinosaur Park Formation and St. Mary River Formation near Alberta. Altogether, these excavations yielded five specimens, including complete skulls and nearly complete skeletons.

The second species of this dinosaur, Edmontonia rugosidens, was described by Charles W. Gilmore and classified in 1930 (though some debate exists about the distinction of the rugosidens species from longiceps). Edmontonia rugosidens finds in the Judith River and Upper Two Medicine Formations of Montana, the Aguja Formation of Texas, and the Dinosaur Park Formation have provided four fossils, including complete skulls and a nearly complete skeleton.

Many close relatives of Edmontonia have also been uncovered throughout the western United States and Canada. Among these are Panoplosaurus mirus, discovered in the Dinosaur Park Formation by Lawrence Lambe in 1919, and Silvisaurus condrayi, excavated in the Dakota Formation of Kansas by T. H. Eaton in 1960. Panoplosaurus was a contemporary of Edmontonia, while Silvisaurus predated its relatives by at least ten million years.

Another discovery of a relative of Edmontonia took place in 2008, when the postcranial skeleton (bones below the skull) and partial skull of Peloroplites cedrimontanus, a large nodosaurid ankylosaur, were uncovered in the Cedar Mountain Formation of Utah. The dinosaur was named by American paleontologist Kenneth Carpenter.

In 2017 another nodosaurid, Borealopelta markmitchelli, was discovered in the Clearwater Formation of Canada. Its well-preserved scales and horn sheaths allowed researchers to determine that it exhibited camouflage in its coloration, suggesting predation pressures on even fairly large herbivore.

A 2013 comparative analysis of ankylosaur long bones suggests that slower growth and remodeling in the norosaurid Edmontonia than in ankylosaurids.

In 2015 paraglossalia—bony throat structures common in modern birds—were reported to have been found in both Edmontonia and its relative Pinacosaurus. The relatively large paraglossalia with notable muscle scars suggest that those ankylosaurs had powerful, fleshy tongues.

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