Euoplocephalus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Ornithischia

Family: Ankylosauridae

Genus: Euoplocephalus

Species: Euoplocephalus tutus

Introduction

Euoplocephalus tutus is most remarkable for the abundant number of potential fossil finds from three fossil sites that have given paleontologists a great deal of material to examine and evaluate. The quantity and condition of the specimens has made Euoplocephalus one of the most well-studied ankylosaurs, contributing greatly to scientific understanding of the Ankylosauridae family as well as being the first ankylosaur to yield significant fossilized skull evidence.

Classification

Based on the Linnaean system, Euoplocephalus is classified under the order Ornithischia, one of two orders of dinosaurs. Unlike the more lizard-like saurischian order, ornithischians were known for their bird-like hip structure, with its pelvic bone pointed down and back towards the tail. Dinosaurs in this order also tended to be herbivores and to have hoof-like claws on their feet. At the same time, the traditional division into the two orders of Ornithischia and Saurischia was a subject of scientific debate by the third decade of the twenty-first century.

The ornithischian order breaks down into more specific suborders and infraorders, of which Euoplocephalus belongs to the suborder Thyreophora, which comprises dinosaurs with body armor, spikes or plates, and the infraorder Ankylosauria, which comprises quadrupedal herbivores with heavy body armor. The infraorder Ankylosauria has traditionally been split into two distinct families—the Ankylosauridae and the Nodosauridae. These two families have generally been distinguished by the presence of a clubbed tail in ankylosaurids.

Euoplocephalus, with the characteristic clubbed but otherwise smooth tail (formed from fused vertebrae), belongs to the Ankylosauridae family. Its genus has traditionally been distinguished from other ankylosaurs by its unique pattern of armored plates and spikes, particularly the bony horns that extend from its eyelids. It was this heavy skull armor that gave the genus its name, which means “well-armored head.” Likewise, its species name, tutus, means “safe” or “protected.”

In addition to the Linnaean system, scientists may use another classification system, called phylogenetic or cladistic classification. This system classifies animals in family trees, called cladograms, grouped by common ancestry. Each split in the tree creates a new group based on shared physical traits. A cladistic analysis of Euoplocephalus places it in the Ankylosauria clade, or branch, of the dinosaur family tree. This branch breaks down into two additional branches—Ankylosauridae and Nodosauridae. As in the Linnaean system, Euoplocephalus belongs to the Ankylosauridae (clubbed tail) branch, which divides into several smaller clades. The most specific clade to which Euoplocephalus belongs includes Euoplocephalus and Ankylosaurus.

Anatomy

Euoplocephalus exhibited the distinctive armor and clubbed tail of the ankylosaurids. A mid-sized member of the family, Euoplocephalus possessed a thick torso stacked on short, stubby, legs with hoof-clawed feet. Its posture was low to the ground, with a short, plated neck and a longer tail, smooth except for its heavy club. Bands of armor, horned spikes and studs spanned the dinosaur's back and extended up to the head. The armor was composed of collagen and layers of bone called osteoderm that were not fused together, permitting flexibility.

A small head, relative to its body size, sloped downward from a high brow in a spade shape, with added protection in the form of spiked eyelids set above its eyes. Euoplocephalus also had a large nasal cavity, indicating a strong sense of smell, and small, weak teeth.

Dinosaurs have generally been regarded as cold-blooded animals, like modern lizards, though the term “cold-blooded” 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 self-regulating metabolisms. Scientists continue to debate whether ankylosaurids such as Euoplocephalus might have been ectotherms, endotherms, or something in between.

Intelligence

The Nodosaurids and Ankylosaurids, with their large bodies and small heads, are believed to have fallen on the lower end of that scale. Ankylosaurs, including Euoplocephalus, likely ranked around 0.5 on the EQ scale.

Reproduction and Population

Fossil evidence suggests that ankylosaurids, and Euoplocephalus in particular, were populous herbivores during the Late Cretaceous. Geographic distribution of ankylosaurids and the closely related nodosaurids also indicates that they were widespread across the globe.

Based on existing studies, paleontologists generally believe that most dinosaurs, including Euoplocephalus and related ankylosaurs, were oviparous, meaning 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 Euoplocephalus 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 Euoplocephalus or its close relatives need to be found before further conclusions can be drawn regarding their reproductive behavior.

Diet

Ankylosaurs such as Euoplocephalus were herbivores. The jaw structure, small and leaf-shaped teeth, and large gut of Euoplocephalus support conclusions that it fed predominantly, if not exclusively, on plant life in the Late Cretaceous. Given the time period, their diet most likely comprised gymnosperms (such as ferns, conifers, and cycads), and perhaps the newly evolved flowering angiosperms. The heavy body structure and short legs of Euoplocephalus indicate that it grazed on low-lying vegetation. Some scientists believe that the flexibility of Euoplocephalus' forelimbs would have allowed it to dig, possibly allowing it to consume tubers as well. Like most herbivores, Euoplocephalus probably spent a great deal of time looking for and consuming food.

Behavior

Though Euoplocephalus has yielded numerous potential fossils, the specimens were not discovered in a densely populated find that would suggest they were highly social or herd animals. The isolated nature of the finds suggests a more solitary lifestyle for this populous species. Likewise, comparable finds of its relatives among the ankylosaurs and nodosaurs does not suggest that the families were herding animals like their fellow herbivores, the hadrosaurs and ceratopsians.

Though relatively small in size and stocky, Euoplocephalus had well-designed defenses and is believed to have been both strong and agile. Its heavy body armor, which included horned spikes and boney studs, afforded it the defensive shield mechanism of modern armadillos—when attacked, Euoplocephalus could retreat into a defensive crouch that would have made it difficult for predators to get past its extensive armor. Euoplocephalus also possessed a large club at the end of its tail. This club would have proved a dangerous weapon when swinging toward an opponent. Scientists suggest that Euoplocephalus used its clubbed tail not only for defense, but also in competitive displays with male members of its species.

Habitat and Other Life Forms

All fossil evidence for Euoplocephalus has been unearthed in the western United States and Canada, indicating that the species thrived in this region during the Late Cretaceous. Many other finds suggest that other species of ankylosaurids and nodosaurids also existed in North America. Additional fossil evidence for ankylosaurs places them in Asia, Europe, and South America.

The Late Cretaceous climate was warmer than it is today, most likely sub-tropical, encouraging lush forest and wetland growth across the landscape. By this time, the vast supercontinent of Pangaea had split into two smaller supercontinents, Gondwana and Laurasia, and 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, and dinosaurs shared the land with a diverse variety of insects, birds, mammals, amphibians and flora.

The ankylosaurids of 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. Euoplocephalus most likely lived in a woodland habitat, with ample vegetation to support its diet. It may have also thrived in wetland areas.

Research

In 1902, Canadian paleontologist Lawrence Lambe discovered the first fossils of Euoplocephalus in the Dinosaur Park Formation near Alberta, Canada. Lambe originally named the fossil—a partial skull—Stereocephalus (a name that was already used for an insect). Lambe would change the name in 1910. Subsequent ankylosaurid finds in the Dinosaur Park and Horseshoe Canyon Formations in Canada and the Upper Two Medicine Formation in Montana (in 1924 and 1929) yielded numerous other skulls and a nearly complete skeleton. These were initially identified as other species, including Dyoplosaurus acutosquameus, Scolosaurus cutleri, and Anodontosaurus lambei. Decades would pass before American paleontologist Walter Coombs would observe similarities in the fossils and group them into one distinct species, Euoplocephalus tutus. However, some scientists continued to debate just how related these various fossils are and questioned the validity of the Euoplocephalus tutus taxon. After studying a large sample of fossils from the Alberta formations based on the hypothesis that, particularly according to more developed stratigraphic knowledge of the areas, different species of ankylosaurids had likely been incorrectly lumped into Euoplocephalus, researcher Victoria Arbour argued in a 2013 paper that several specimens from the Horseshoe Canyon Formation should be reassigned to Anodontosaurus lambei. Arbour cited differences in skulls and tail clubs as evidence. As part of this study, it was additionally argued by the researchers involved that there was material distinct enough to consider Scolosaurus cutleri separate from Euoplocephalus. Earlier, in 2009, Arbour had argued that Dyoplosaurus acutosquameus should also be resurrected as its remains showed differences in the hips and vertebrae.

The closest known relative to Euoplocephalus was unearthed in the Scollard Formation near Alberta in 1908 by Barnum Brown and identified as the species Ankylosaurus magniventris. Ankylosaurus was a successor to Euoplocephalus, living in the final millennia of the dinosaur age. Many other close relations of Euoplocephalus were discovered in Mongolia and China throughout the twentieth century, including Tsagantegia, Saichania, and Pinacosaurus.

Additional ankylosaurids and nodosaurids found in North America include Panoplosaurus and Silviasaurus, uncovered in the early to mid-twentieth century, and Cedarpelta, Pawpawsaurus, and Nodocephalosaurus. By the end of the second decade of the twenty-first century, researchers had also dedicated time to studying the complex structure of the nasal passages of ankylosaurids such as Euoplocephalus to learn more about them.

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