Ankylosaurus

Ankylosaurus is a genus of armored dinosaur. It is not to be confused with Ankylosuchus. Its fossils were found in geological formations from the very end of the Cretaceous Period, about 70 to 66 million years ago, in western North America. This makes it one of the last non-avian dinosaurs. It was named by Barnum Brown in 1908. It is monotypic, which means it has only one species, A. magniventris.

The generic name means "fused" or "bent lizard," and the specific name means "great belly." Only a few specimens have been dug up so far, and a complete skeleton has not been found yet. Other members of Ankylosauria are known from much more fossil material. Still, Ankylosaurus is often seen as the archetypal member of its group, even though it has some unusual features.

Possibly the largest known ankylosaurid, Ankylosaurus was about 6 to 8 meters (20 to 26 feet) long. It may have weighed between 4.8 and 8 tonnes (5.3 and 8.8 short tons). It walked on four legs and had a broad, sturdy body covered in armor plates. Its wide, low skull had horns on the back of the head, and it had a beak at the front of its jaws. Behind the beak were rows of small, leaf-shaped teeth. A large club at the end of its tail made it unique. Its bones were fused together, which made them stronger. This is where the genus name comes from.

Ankylosaurus belongs to the family Ankylosauridae. Its closest relatives seem to be Anodontosaurus and Euoplocephalus. It was probably a slow-moving animal that could still make quick movements when needed. It likely ate plants, and its tail club may have been used to defend against predators. Its fossils have been found in several rock layers, but it seems to have been rare in its environment. It lived alongside dinosaurs such as Tyrannosaurus, Triceratops, and Edmontosaurus.

History of discovery

In 1906, an American Museum of Natural History expedition led by American paleontologist Barnum Brown found the type specimen of Ankylosaurus magniventris (AMNH 5895) in the Hell Creek Formation, near Gilbert Creek, Montana. A collector named Peter Kaisen made the find. It included part of a skull, two teeth, parts of the shoulder and backbone, ribs, and more than thirty osteoderms (armor plates). Brown described the animal in 1908. The name comes from Greek words meaning 'bent' or 'crooked' and 'lizard'. The first part is related to the medical term ankylosis, which means stiffness caused by fused bones. The name can be translated as "fused lizard," "stiff lizard," or "curved lizard." The species name, magniventris, comes from Latin words meaning 'great' and 'belly', which describes the animal's wide body.

Brown's 1908 drawing filled in the missing parts in a way similar to Stegosaurus. He also compared the animal to the armored mammal Glyptodon. The drawing showed strong front legs, a strongly arched back, and a short, drooping tail. It did not have a tail club, because that was unknown at the time. The drawing was very influential, and restorations based on it were published as late as the 1980s. The American paleontologist Samuel Wendell Williston disagreed with it in 1908. He said it was based on too few bones and that Ankylosaurus was just a version of Stegopelta. Other researchers did not agree, and today the two are seen as separate animals. Brown had also collected 77 osteoderms in 1900 while digging up a Tyrannosaurus in the Lance Formation of Wyoming. He thought they belonged to that animal. Later study showed they were similar to those of Ankylosaurus. Brown had compared them with Euoplocephalus plates that had been wrongly labeled as Ankylosaurus.

Description

Ankylosaurus was the largest known ankylosaurine dinosaur. It may also have been the largest ankylosaurid. In 2004, Carpenter estimated that the largest known skull belonged to an animal about 6.25 m (20 ft 6 in) long, with a hip height of about 1.7 m (5 ft 7 in). The smallest known skull belonged to an animal about 5.4 m (17 ft 9 in) long. The skull was low and triangular, and it was wider than it was long. It had a broad beak at the front and orbits that were almost round. The three known skulls differ in some details. Scientists think this may be because of changes that happened as the skulls decayed and turned into fossils, or because of individual variation. The snout was unlike that of any other ankylosaur. Its nostrils faced downward and outward, and its sinuses spread out to the sides. The teeth were very small and shaped like leaves.

Its body was held up by four legs, and its back legs were longer than its front legs. Scientists do not yet know much of the skeleton, including most of the pelvis, tail, and feet. A special feature was its armor. It was made of bony knobs and plates called osteoderms that were set into the skin. The osteoderms ranged from 1 cm to 35.5 cm in size and came in different shapes. The neck also had armor plates, but scientists have only found fragments of them, so their exact arrangement is not certain. Researchers have disagreed about how these plates fit together. The tail ended in a club made of two large osteoderms, with smaller ones along its middle and tip. The last seven tail bones formed the handle of the club. They could not bend because they were fused together.

Classification

Brown thought Ankylosaurus was so different from other dinosaurs that he made it the type genus of a new family, Ankylosauridae. Its members had large, triangular skulls, short necks, stiff backs, broad bodies, and bony plates called osteoderms. Brown also placed Palaeoscincus and Euoplocephalus in this family. The fossils were incomplete, so he could not fully tell Euoplocephalus and Ankylosaurus apart. He thought the group belonged to the suborder Stegosauria. In 1923, Osborn named Ankylosauria, which gave the ankylosaurids their own suborder.

Today, Ankylosauria and Stegosauria are grouped together in the Thyreophora. This group first appeared in the Sinemurian age and lived for 135 million years. It disappeared in the Maastrichtian. Ankylosaurus belongs to the subfamily Ankylosaurinae within Ankylosauridae. It is most closely related to Anodontosaurus and Euoplocephalus. Scientists have changed their ideas about how these dinosaurs are related as new discoveries were made. One cladogram, based on a phylogenetic analysis from 2015, helps show these relationships.

Paleobiology

Like other plant-eating dinosaurs, Ankylosaurus was herbivorous. Its wide muzzle helped it crop low-growing plants. It probably ate many ferns and low shrubs. It may also have eaten tough leaves and soft fruit. It may have dug for tubers and roots, so it could have been omnivorous (eating both plants and animals).

Scientists have studied how Ankylosaurus moved, sensed the world, and grew. Its nose and sinus spaces may have helped it balance heat and water, make low sounds, or sense smells. Its forelimbs carried much of its weight. It was probably slow-moving but could move quickly when needed. As it grew, its bones and armor changed shape. Its armor was light but strong. Its tail club was a powerful tool for defending itself.

Paleoenvironment

Ankylosaurus lived between 68 and 66 million years ago, during the last stage of the Late Cretaceous Period. It was one of the last kinds of dinosaurs to appear before the Cretaceous–Paleogene extinction event. Its fossils are rare in the sediments where they are found, so it was probably not very common. It may have lived in the higher lands rather than the coastal lowlands. Another ankylosaur, a nodosaur called Edmontonia sp., lived in the same areas. According to Carpenter, the two did not seem to share the same range. The nodosaur seems to have lived in the lowlands and had a narrower muzzle, which suggests it ate a more selective diet. This shows further ecological separation.

Ankylosaurus had a low center of gravity, so it could not knock down trees like modern elephants do. It also could not chew bark. As an adult, it does not seem to have lived in groups, so it probably did not change its landscape the way elephants do. Hadrosaurids may have played that "ecosystem engineer" role instead. Its home was the western shore of the Western Interior Seaway, a sea that split western and eastern North America. A wide coastal plain stretched west to the newly formed Rocky Mountains. The rocks there are mostly sandstone and mudstone, formed in floodplain areas. The climate was warm, subtropical/temperate, and monsoonal, with some rain, tropical storms, and forest fires. Most plants were angiosperms, with fewer conifers, ferns, and cycads. Much of the area was covered by small trees. Ankylosaurus shared this land with the ceratopsids Triceratops and Torosaurus, the hypsilophodont Thescelosaurus, the hadrosaurid Edmontosaurus, the pachycephalosaurian Pachycephalosaurus, and the theropods Struthiomimus, Ornithomimus, Pectinodon, and Tyrannosaurus.

Cultural significance

In 2004, Carpenter said that Ankylosaurus is the archetypal member of its group. It is also the best-known ankylosaur in popular culture. A life-sized model of the animal was shown at the 1964 World's Fair in New York City, which may be one reason it became so well known. In 2017, Arbour and Mallon called Ankylosaurus an "iconic" dinosaur. They noted that the World's Fair model, the 1947 mural The Age of Reptiles by American artist Rudolph Zallinger, and other later popular pictures showed it with a tail club. The tail club was first discovered in 1910.

Many older pictures show Ankylosaurus crouching, with a big tail club dragging on the ground. Newer reconstructions show it standing with its legs more upright and its tail held off the ground. Many older pictures also show large spines sticking out from the sides of its body. These spines are like those of nodosaurid ankylosaurs, but scientists have not found them on Ankylosaurus itself. Its armor has often been mixed up with that of Edmontonia (earlier called Palaeoscincus). Edmontonia has also been shown with an Ankylosaurus-like tail club, even though nodosaurids did not have one. An example is a 1930 mural by American artist Charles R. Knight. Ankylosaurus also appears in the Jurassic Park series, where it attacks with its tail and runs. Paleontologists have said these abilities are unlikely.