Ankylosaurusmagniventris

Overview. An animal's ability to defend itself from predators is important, and few animals throughout the earth's history have invested as heavily in defense as the ankylosaurs, and among the ankylosaurs, few were as heavily defended as Ankylosaurus, discovered in the early 20th Century by the great Barnum Brown. One of the last of its kind, it lived at the tail-end of the Age of Dinosaurs where it would have needed its defenses to keep itself safe from the apex predator of its environment, the most fearsome predator of all, Tyrannosaurus rex. With its wide, squat build, barrel shaped body, heavy armor, and formidable tail club, it was built to withstand even the toughest of predators. But even this was not enough to save it from the devastating mass extinction that would spell this animal’s doom 66 million years ago, along with all the other non-avian dinosaurs. Known from several specimens, Ankylosaurus is one of the best known of its kind, though a lack of intensive research on these specimens has meant that our understanding of this “living tank” remains incomplete.

Discovery. The first bones of Ankylosaurus, a set of 77 armor plates called osteoderms, were discovered by the legendary paleontologist Barnum Brown in 1900 in the Lance Formation of Wyoming in association with a specimen of Tyrannosaurus rex. At the time, Brown thought these osteoderms belonged to T. rex, and they were included in his description of the T. rex specimen (published as Dynamosaurus) in 1905. It wasn’t until 1904, four years after the initial discovery of the osteoderms, that an Ankylosaurus skeleton was discovered. This skeleton was discovered in the Hell Creek Formation near Gilbert Creek, Montana, by Peter Kaisen and Barnum Brown on an expedition with the American Museum of Natural History (AMNH). The skeleton consisted of a partial skull, two teeth, part of the shoulder girdle, cervical, dorsal, and caudal vertebrae, ribs, and over thirty osteoderms. Brown described and named this specimen in 1908, calling it Ankylosaurus magniventris. Ankylosaurus is named after the medical term ankylosis, which is stiffness caused by the fusion of bones. The literal translation of Ankylosaurus in Greek is “bent/crooked lizard”. However, it is more accurately translated as "stiff lizard" or “fused lizard”. The species name magniventris is Latin, combining the words magnus ('great') and venter ('belly'), referencing Ankylosaurus’s very wide body. A second specimen, this one including a complete skull and the first and only tail club known from the genus, was discovered by Barnum in 1910. Since then, additional specimens of Ankylosaurus have been discovered, adding to our understanding of this fascinating creature.

Evolution and Description. Ankylosaurus was an ornithischian (“bird hipped”) dinosaur in the clade Thyreophora, which includes the Ankylosauria and the Stegosauria. Ankylosaurs can be divided into two main families, the nodosaurids and the ankylosaurids. Nodosaurids, generally, were spikier, and lacked tail clubs, whereas many ankylosaurids are known to have had tail clubs. Where the two kinds of Ankylosaurs overlapped in habitat they likely differed ecologically. Both groups survived into the latest Cretaceous, and Ankylosaurus, which lived between 68-66 Ma in the Maastrichtian Age of the Late Cretaceous, was not only among the last of its kind, but also the largest, measuring 6-8 m (20-26 ft) in length, and weighing about 4.8-8 tonnes (5.3-8.8 tons). Typical of ankylosaurs, Ankylosaurus was quadrupedal with a robust build, short legs, a wide barrel-shaped body, short neck, and a relatively small head. The skull of Ankylosaurus is rather triangular, with two short, stout backward-pointing squamosal horns on the sides of the back of the skull, and two additional jugal horns beneath those which point down and to the side. The skull is ornamented with a scale-like pattern called "caputegulae" which results from fusion and remodeling of the cranial bones. The snout of Ankylosaurus differs from its kin by being more arched and shortened at the front, and in having nostrils which faced downward and outward, unlike all other known ankylosaurids which had nostrils facing forward or upward. The muzzle of Ankylosaurus was rather wide, and its mouth contained numerous leaf-shaped teeth with large denticles for slicing vegetation. Additionally, like many ornithischians, Ankylosaurus probably had cheeks, which would have aided it in processing its food. The most iconic feature of Ankylosaurus is undoubtedly its armor. Preserved osteoderms range in size from about 1 cm to 35.5 cm (14 in) in length and would have covered the whole dorsal side of the animal. Its neck, its most vulnerable spot, was protected by two armored half-rings. The osteoderms would have been somewhat flat with low keels, which differed from nodosaurs like Edmontonia, which lived alongside Ankylosaurus, and have very high keels, making the osteoderms more spike-like. While Ankylosaurus is often depicted with a row of spikes running down each of its sides, this was actually a nodosaurid trait, not a trait which ankylosaurus possessed. The reason for the confusion regarding this trait likely stems from the 1910 World’s Fair which featured a reconstruction of Ankylosaurus with nodosaurid-style spikes lining its sides. This image became firmly rooted in the public consciousness and has reappeared in numerous forms of media. In addition to large armor plates, numerous smaller osteoderms and ossicles would have occupied the space between the larger osteoderms so that the animal had almost no gaps in its defense. It could be likened to a medieval knight wearing plate armor over a set of chainmail, the plate providing the most robust protection and the chainmail protecting the gaps in the plates. The sturdy build and armor of ankylosaurs have also resulted in them being described as “living tanks”. To top off this tank’s defenses, Ankylosaurus had a large club at the end of its tail, the last seven vertebrae of which are fused, forming a “handle” for the club. The club itself was composed of two exceptionally large osteoderms plus a row of smaller osteoderms down the midline, and two more at the tip of the club. The club was large, some 60 cm, or nearly two ft long, and 49 cm (19.5 in) wide, and 19 cm (7.5 in) tall. Not something that any animal would like to be on the wrong end of.

Behavior and Ecology. Ankylosaurus lived in what is now Montana, Wyoming, and Alberta. To the west were the Rocky Mountains, at that time taller than they are now, and still rising. To the east lay the remnants of the receding Western Interior Seaway. Most fossils from North America during the Maastrichtian Age of the Late Cretaceous are from the region between the mountains and the sea. Despite being rather far north, the warm global climate of the Late Cretaceous resulted in the region enjoying a moist subtropical environment, with swamp environments dominating the lowlands, and open forests that included conifers, Ginkgo, Magnolia, cycads, and palm trees. Like nearly all ornithischians, Ankylosaurus was an herbivore, and it would have fed low to the ground as its squat build and short neck would not have allowed for browsing more than a few feet off the ground. It likely ate mostly ferns and low growing shrubs, and probably had a varied diet which included leaves, tough vegetation, and fruit. Assuming a warm-blooded metabolism, which is likely, Ankylosaurus would have probably needed to consume about 130 lbs of food per day, about the same as a modern elephant. The snout of Ankylosaurus was wider than its nodosaur cousin Edmontonia, which shared its environment, indicating that they probably differed in their food preferences. While the narrower snout of Edmontonia may have indicated a more selective feeder, Ankylosaurus may have been less choosey. Additionally, it is thought that Ankylosaurus may have been more common in the highlands, whereas Edmontonia primarily inhabited the lowlands, which would also explain Ankylosaurus’s relative rarity compared to other animals in its ecosystem, since fossilization is much more common in lowlands where deposition is the dominant sedimentary process, vs highlands, where erosion is dominant.

Besides Edmontonia, Ankylosaurus shared its environment with other well-known dinosaurs including the large duck-billed herbivore Edmontosaurus, the iconic horned herbivore Triceratops, and apex predator, and most famous dinosaur of all, Tyrannosaurus rex. The latter likely would have been the greatest threat to Ankylosaurus, and the armor of Ankylosaurus was likely a critical defense against the massive predator. T. rex was capable of biting through bone, so Ankylosaurus’s armor may not have been 100% effective, but that is where the tail club came into play. Ankylosaurus could have swung its club with devastating effect against predators. A hit to the leg or jaw could have severely injured, crippled, or even outright killed a tyrannosaur that wasn’t careful. Ankylosaurus may have also used these clubs on each other in competition for mates. While juveniles, which would have been unable to swing their tail clubs effectively enough to ward off predators, may have lived in herds, the adults seem to have been solitary, probably only coming together to mate. During this time, males may have competed for mates by bashing each other with their clubs. A violent and brutal form of combat but one which would guarantee the perpetuation of only the strongest and most durable, essential traits for a species living in the shadow of T-rex.

Extinction and Legacy. Despite its armor, Ankylosaurus was no match for a mass extinction. 66 million years ago, an asteroid twice the size of Mt. Everest smashed into the edge of the Gulf of Mexico, causing the Cretaceous-Paleogene (K-Pg) Extinction which resulted in at least 75% of all species vanishing in a geologic instant. Most famous of the casualties were the non-avian dinosaurs, including the mighty Ankylosaurus and its kin. Likely unable to find cover, most if not all Ankylosaurus probably perished in a catastrophic firestorm that cooked them where they stood just minutes after impact. If any did somehow survive the initial destruction, they would have quickly starved to death as the heat of the impact and the ejected material falling back to earth also ignited forests and left the world, and especially North America where Ankylosaurus lived, a scorched wasteland. The Age of Dinosaurs was over, and with it, the time of some of the most heavily armored animals to ever walk the earth. Now they live on in our imaginations as we visit museums, and in our media. Ankylosaurus remains are held in the Canadian Museum of Nature in Ottawa, Ontario, Canada, the Carter County Museum in Ekalaka, Montana, and the American Museum of Natural History in New York City, New York, USA. Replica Ankylosaurus can also be found at other museums including the Museum of the Rockies in Boseman, Montana, USA. Ankylosaurus has been featured in the Jurassic Park Franchise, albeit with incorrect nodosaur spikes along its body. Specifically, it features in Jurassic Park III (2001), Jurassic World (2015), Jurassic World: Fallen Kingdom (2018), Jurassic World: Dominion (2022), and the spinoff series Camp Cretaceous, which features an individual Ankylosaurus named Bumpy in a starring role. An Ankylosaurus named Earl appears in Disney’s Dinosaur (2000), also inaccurately portrayed with side spikes. Ankylosaurus also appeared in the BBC documentary Walking with Dinosaurs, with more accurate armor, though it was portrayed as much larger and taller than it would have been in reality.