Ankylosaurus

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

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

Possibly the largest known ankylosaurid, Ankylosaurus is estimated to have been between 6 and 8 meters (20 and 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, with 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 helped make it unique. Its bones were fused together, which made them stronger, and this is where the genus name comes from.

Ankylosaurus belongs to the family Ankylosauridae. Its closest relatives appear to be Anodontosaurus and Euoplocephalus. It is thought to have been 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. The find, made by collector Peter Kaisen, 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 the Greek words for 'bent' or 'crooked' (related to the medical term ankylosis, the stiffness caused by fused bones) and 'lizard', so it can be translated as "fused lizard," "stiff lizard," or "curved lizard." The species name, magniventris, comes from Latin words meaning 'great' and 'belly', referring to the animal's wide body.

Brown's 1908 drawing filled in the missing parts in a way similar to Stegosaurus, and he compared the result to the armored mammal Glyptodon. It showed strong front legs, a strongly arched back, and a short, drooping tail without a tail club, which was unknown at the time. His drawing was very influential, and restorations based on it were published as late as the 1980s. The American paleontologist Samuel Wendell Williston criticized it in 1908, saying it was based on too few bones and that Ankylosaurus was just a version of Stegopelta. Other researchers did not accept this, and the two are now seen as separate. Brown had also collected 77 osteoderms in 1900 while digging up a Tyrannosaurus in the Lance Formation of Wyoming, and he thought they belonged to that animal. Later study showed they were similar to those of Ankylosaurus, as Brown had compared them with Euoplocephalus plates that had been wrongly labeled as Ankylosaurus.

Description

Ankylosaurus was the largest known ankylosaurine dinosaur and possibly the largest ankylosaurid. In 2004, Carpenter estimated that the animal with the largest known skull was 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 estimated at 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 details, which is thought to be due to changes during decay and fossilization and to individual variation. The snout was unlike that of any other ankylosaur, with nostrils that faced downward and outward and sinuses that spread to the sides. The teeth were very small and leaf-shaped.

Its body was supported on four legs, and its hind limbs were longer than its forelimbs. Much of the skeleton, including most of the pelvis, tail, and feet, is still unknown. A prominent feature was its armor, made of bony knobs and plates called osteoderms that were set into the skin. These osteoderms ranged from 1 cm to 35.5 cm in size and varied in shape. The neck had armor plates as well, but they are known only from fragments, so their exact arrangement is uncertain. 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 club's handle, and 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. Because the fossils were incomplete, 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 survived for 135 million years, disappearing in the Maastrichtian. Ankylosaurus belongs to the subfamily Ankylosaurinae within Ankylosauridae, and 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, and it probably ate plenty of ferns and low shrubs. It may also have eaten tough leaves and soft fruit, and it may have dug for tubers and roots, which means 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, and it was probably slow-moving but could move quickly when needed. As it grew, its bones and armor changed shape. Its armor was lightweight but strong, and its tail club was a powerful weapon that could break the bones of an attacker.

Paleoenvironment

Ankylosaurus existed between 68 and 66 million years ago, during the final, or Maastrichtian, stage of the Late Cretaceous Period. It was among the last dinosaur kinds to appear before the Cretaceous–Paleogene extinction event. Its fossils are rare in the sediments where they are found, which suggests it was uncommon and may have lived in the uplands rather than the coastal lowlands. Another ankylosaur, a nodosaur called Edmontonia sp., lived in the same formations, but according to Carpenter the two did not seem to share the same range. The nodosaur appears to have lived in the lowlands and had a narrower muzzle, which suggests a more selective diet, further showing ecological separation.

With its low center of gravity, Ankylosaurus could not knock down trees like modern elephants do, and it could not chew bark. As an adult, it does not appear to have gathered in groups, so it was unlikely to 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, which divided western and eastern North America, with a broad coastal plain stretching 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 occasional rain, tropical storms, and forest fires. Plants were mostly angiosperms, with fewer conifers, ferns, and cycads, and the area was largely forested by small trees. Ankylosaurus shared this environment 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 noted that Ankylosaurus has become the archetypal member of its group, and the best-known ankylosaur in popular culture. This may be because a life-sized model of the animal was shown at the 1964 World's Fair in New York City. 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, a feature first discovered in 1910.

Many older pictures show Ankylosaurus crouching, with a huge tail club dragged along the ground. Newer reconstructions show it standing with its legs more upright and its tail held off the ground. Large spines sticking out from the sides of the body, like those of nodosaurid ankylosaurs, appear in many older pictures, but are not known from 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, which nodosaurids did not have, such as in 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.