Ankylosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Ornithischia

Family: Ankylosauridae

Genus:Ankylosaurus

Species:Ankylosaurus magniventris

Introduction

Ankylosaurus was a genus of large, heavily armored herbivores, part of a highly successful group of dinosaurs in Late Cretaceous North America. It is among the best-known dinosaurs in popular culture, often representing armored dinosaurs in general, although it is in fact somewhat unusual compared to related members of that group. The study of Ankylosaurus and related species has helped paleontologists to better understand the distribution of life during the Jurassic and Cretaceous periods.

During the Triassic period, the continents of the Earth were united in a single land mass known as Pangaea, which split apart during the Jurassic to yield two supercontinents, Laurasia in the north and Gondwana in the south. Dinosaurs evolved and spread during the Triassic, and then became isolated after the breakup of Pangaea and radiated into a variety of species.

Ankylosaurs, a group of armored herbivores, were found in the portion of Laurasia that became North America, and on every other continent except Africa. Paleontologists believe the ancestral species spread across land bridges during the Jurassic and then diversified into a number of species. The absence of ankylosaurs in Africa suggests that the radiation occurred after the land bridge connecting Africa to Laurasia was severed.

Classification

Scientists use two primary methods to group animals into different categories. Linnaean taxonomy organizes animals into categories based on overall physical similarity, while an alternate system called cladistics creates categories that better reflect the evolutionary relationships between species.

The Linnaean system places Ankylosaurus within the Ornithischia or “bird-hipped” dinosaur order, because of the organization of the animal's pelvic girdle (although the longstanding tradition of dividing dinosaurs into Ornithischia and Saurischia has been challenged by some scientists). Within this group, Ankylosaurus belongs to the infraorder Ankylosauria, which is characterized by a quadrupedal stance and heavily armored backs and tails. Ankylosaurus was the original model for the family Ankylosauridae, which contains more than twenty other species. Ankylosaurus magniventris is the type species and only known species within its genus.

From a cladistic perspective, Ankylosaurus is in the clade Thyreophora, which is also generally considered a suborder of the Ornithischia. Dinosaurs in the Thyreophora clade were quadrupedal and all developed some form of bony armor. The clade Ankylosauria contains dinosaurs with extensive “osteoderms,” or bony skin plates, as defensive armor. The clade Ankylosauridae, also recognized as a family in Linnaean classification, contains species with clubbed tails and unique skull features. However, exact taxonomic relationships are often disputed between scientists, especially as new evidence emerges.

Anatomy

Ankylosaurus was a large dinosaur, reaching an adult length of 6 to 9 meters (20–30 ft) and height between 1.6–3.4 m (5–11 ft). The animal could weigh as much as 4,500 to 6,000 kilograms (10,000–13,000 lbs) with short, stocky legs terminating in five-toed feet and blunt, hoof-like nails. The animal's large body could be nearly 1.7 meters (5 ft) wide. It is thought to be one of the largest known ankylosaurs.

All ankylosaurs’ backs were heavily armored with bony plates, called osteoderms or “scutes.” Ankylosaurus is often depicted with horns along portions of its body, but no dorsal horns have been discovered on Ankylosaurus fossils. Paleontologists believe Ankylosaurus's bony armor was reinforced with sheets of keratin. Its long, muscular tail, tipped with a bony club, may have been used for defense as well as interspecies combat and/or displays. The tail was powered with stiff tendons that were partially covered in bony tissue to increase their tensile strength.

Ankylosaurus had a wide, triangular head, a defining characteristic of the family, covered in bony plates and with small, triangular horns above the brows, and small, recessed eyes. It had short jaws with leaf-shaped, crushing teeth. Interestingly, Ankylosaurus's teeth were unusually small for its size, one of several features that make it somewhat unusual compared to similar species. It also had nostrils arranged on the sides of its head, rather than the front as on other ankylosaurs.

Intelligence

Animal intelligence can be estimated by a measurement known as encephalization quotient (EQ), based on relative size of the brain and body. Ankylosaurus and the other thyreophorans had some of the lowest EQ values of all dinosaurs, generally estimated at between 0.5 and 0.9. Thus, Ankylosaurus may have been significantly less intelligent than modern animals living similar lifestyles.

Ankylosaurs may have had little need for cognitive abilities because their heavy defensive armor allowed adults to forage with little fear of predators. Many of the dinosaurs that fall on the extreme low end of the EQ spectrum, like the stegosaurids and the sauropods, were similarly able to avoid predation due to armor or increased size.

Reproduction and Population

Ankylosaurus was most likely oviparous, meaning that it laid eggs, rather than giving live birth. There have been no discoveries of Ankylosaurus nests. Nests of other ornithischian dinosaurs could contain ten or more eggs and were generally covered in vegetation to heat the eggs as they developed.

Ankylosaurus may have laid its eggs in forested areas where the young could hide from predators until they reached a sufficient size. Paleontologists have no evidence as to whether adult Ankylosaurus provided any protection or parental care to their young.

Ankylosaurus is known from only a few isolated specimens, suggesting it was rare compared to many herbivores in its environment. Paleontologists have insufficient evidence to determine whether Ankylosaurus displayed any physical differences between males and females.

Diet

Judging from its dentition, paleontologists believe that Ankylosaurus was an herbivore. The small, leaf-shaped teeth indicate that the animal browsed vegetation, like ferns, cycads, and conifers. The dinosaur's teeth were not efficient for chewing and they most likely swallowed large chunks of plant matter that then fermented in their large stomachs before passing through the intestines.

Behavior

Some ankylosaurs, like Pinacosaurus, have been found in fossil beds containing multiple adults, indicating that some species congregated, either in breeding herds or around a common food source. The lack of fossil groupings of Ankylosaurus has led paleontologists to assume that the species was a solitary forager. Juvenile ankylosaurs may have remained in groups after hatching for mutual defense, as they would have been far more vulnerable to predators as juveniles. Some researchers also suggest that the relative lack of Ankylosaurus fossils may indicate the species lived mainly away from watery areas, or that it was simply rare even in its day.

The tail armaments of Ankylosaurus may have been used for defense, and fossils of predators from the area occasionally reveal blunt force fractures consistent with an Ankylosaurus's tail club. In addition, the tail of Ankylosaurus may have aided in mating or mate competition, allowing males to duel over access to females, as in many modern mammals.

Habitat and Other Life Forms

In the Late Cretaceous, the area that would become North America was warmer and much of the central portion of the continent was covered by shallow inland seas. Ankylosaurus fossils have been found in sediment that used to be part of shallow, tropical flood plains and scrub forest surrounding the inland seas. Gymnosperms, like conifers and cycads, were the dominant type of vegetation, along with ferns and other spore-bearing plants. The first angiosperms had appeared by this time and were spreading across the continent.

A variety of reptiles, amphibians, and small mammals lived in Late Cretaceous North America, many resembling modern species. While the first birds had evolved during the Late Jurassic, the skies were dominated by flying reptiles called pterosaurs that lived alongside the dinosaurs in the Mesozoic.

Ankylosaurus lived close to a few other species of ankylosaurs, which were smaller and may have specialized in the tropical forest environments. A variety of hadrosaurs, or “duck-billed dinosaurs,” like Maiasaura and Edmontosaurus also lived in areas where Ankylosaurus fossils were found. The ceratopsians, or “horned dinosaurs,” were another common group of herbivores that included Triceratops. The area's predators included large bipedal carnivores like Tyrannosaurus and Albertosaurus and a variety of smaller predators such as Troodon and Dromaeosaurus.

Research

Ankylosaurus was first discovered by American paleontologist Barnum Brown from a formation in the Hell's Creek area of Montana in 1906. The specimen was described by Brown in 1908. Several additional specimens and bone fragments were found over the next decade, indicating that Ankylosaurus extended into Canada. The species is generally known from isolated specimens and fragmentary fossil collections. While Ankylosaurus was a model for the entire group of armored dinosaurs now called the Ankylosauria, relatively little is known about the species because of the limited number of fossils discovered. This has led to frequent revisions in reconstructions of how Ankylosaurus looked, with later models tending to favor wider spacing of the armor plates than seen in classic dinosaur illustrations.

Aside from fossilized bones, a number of fossil footprints believed to belong to ankylosaurs have been discovered. One such find in Canada was announced in 2015, and a study of similar tracks in Mexico was published in 2018.

A scientific study published in the journal Biology Letters in 2022 provided further evidence to the theory that the tail of the ankylosaurus was primarily used for intraspecies combat. The study, which examined an extremely well-preserved fossil of a previously unknown species of ankylosaur that was discovered at a Montana dig site in 2014, found that the creature's fossil had evidence of missing spikes and damaged plating consistent with wounds from crushing blows, causing researchers to conclude that the damage was likely caused by the tail of another ankylosaur. The lack of other defensive wounds on the specimen, including bite and claw marks, led researchers to suggest that the primary use of the tail of the Ankylosaurus was likely for ritualized combat between other ankylosaurs instead of defense from predators.

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