Temporal range: Late Cretaceous, 68–66 Ma
|Cast of Ankylosaurus skull (AMNH 5214) in front view, Museum of the Rockies|
Ankylosaurus (// ANG-ki-lo-SAWR-əs or // ang-KY-lo-SAWR-əs, meaning "fused lizard") is a genus of ankylosaurid dinosaur. Fossils of Ankylosaurus are found in geologic formations dating to the very end of the Cretaceous Period, between about 68–66 million years ago, in western North America. Ankylosaurus was named by Barnum Brown in 1908, and the only classified species in the genus is A. magniventris. Five specimens have been escavated to date, but a complete skeleton has not been discovered. Though other ankylosaurs are represented by more extensive fossil material, Ankylosaurus is often considered the archetypal member of the group.
Ankylosaurus is the largest known ankylosaurid, and measured up to 6.25 m (20.5 feet) in length. It had a long, low skull, with backwards pointed horns. The front part of the jaws were covered in a beak, with rows of small teeth further behind it. It was covered in armor plates, or osteoderms, with a bony half ring covering part of its neck and shoulders, and had a large club on the end of its tail. The tail club is thought to have been used in defense against predators or in intraspecific combat.
Ankylosaurus is a member of the subfamily Ankylosaurinae, and its closest relatives appear to be Euoplocephalus and Anodontosaurus. Ankylosaurus has been found in the Hellcreek, Lance, and Scollard formations, but appears to have been rare in its environment. Although it lived alongside another ankylosaur, Edmontonia, their ranges and ecological niches do not appear to have overlapped, and Ankylosaurus may have inhabited upland areas. Ankylosaurus lived alongside dinosaurs such as Tyrannosaurus, Triceratops, and Edmontosaurus.
Ankylosaurus is the largest known ankylosaurid dinosaur, estimated to have been up to 6.25 m (20.5 feet) long, 1.5 m (4.9 feet) wide, and 1.7 m (5.6 feet) tall at the hip. This length is based on the largest known skull (specimen NMC 8880), which is 64.5 cm (25.4 inches) long and 74.5 cm (29.3 inches) wide. The smallest known skull (specimen AMNH 5214) is 55.5 cm long and 64.5 cm wide. The animal has also been estimated to be 9 m (30 ft) long, and weighing up to 6 tonnes (13,000 lb).
The body was low-slung and quite wide. It was quadrupedal, with the hind limbs longer than the forelimbs. Although the form of its feet is still unknown, comparisons with other ankylosaurs suggest Ankylosaurus probably had five toes on each foot. The skull was low and triangular in shape, wider than it was long. Like other ankylosaurs, Ankylosaurus had small, leaf-shaped teeth, which were the smallest relative to the body size among ankylosaurid species. Ankylosaurus lacked the tooth batteries of the contemporaneous ceratopsid and hadrosaurid dinosaurs. Bones in the skull and other parts of the body were fused, increasing their strength. 
A prominent feature of Ankylosaurus is its armor, consisting of massive knobs and plates of bone known as osteoderms or scutes embedded in the skin. Osteoderms are also found in the skin of crocodilians, armadillos and some lizards. The bone was probably covered by a tough, horny layer of keratin. The osteoderms varied greatly in shape, from wide, flat plates to small, round nodules. The plates were aligned in regular horizontal rows down the animal's neck, back, and hips, with the many smaller nodules protecting the areas between the large plates. Smaller plates may have been arranged on the limbs and tail. Compared to the slightly older ankylosaurid Euoplocephalus, the plates of Ankylosaurus were smoother in texture, and without the high keels found on the armor of the contemporaneous nodosaurid Edmontonia. A row of flat, triangular spikes may have protruded laterally along each side of the tail. Tough, rounded scales protected the top of the skull, while four large pyramidal horns projected outwards from its rear corners.
The famous tail club of Ankylosaurus was also composed of several large osteoderms, which were fused to the last few tail vertebrae. It was heavy and supported by the last seven tail vertebrae, which interlocked to form a stiff rod at the base of the club. Thick tendons which attached to these vertebrae have been preserved in the fossil record.
History of discovery
In 1906, an American Museum of Natural History expedition led by paleontologist Barnum Brown discovered the type specimen of Ankylosaurus magniventris (AMNH 5895) in the Hell Creek Formation, near Gilbert Creek, Montana. The specimen (found by collector Peter Kaisen) consisted of the upper part of a skull, two teeth, part of the shoulder girdle, cervical, dorsal, and caudal vertebrae, ribs, and more than thirty osteoderms. Brown scientifically described the animal 1908; the genus name is derived from the Greek words 'αγκυλος/ankulos ('bent' or 'crooked'), referring to the medical term ankylosis, the stiffness produced by the fusion of bones in the skull and body, and σαυρος/sauros ('lizard'). The name can be translated as "fused lizard", "stiff lizard", or "curved lizard". The type species name magniventris is derived from the Latin magnus ('great') and venter ('belly'), referring to the great width of the animal's body.
The skeletal reconstruction accompanying the 1908 description restored the missing parts in a fashion similar to Stegosaurus, and Brown likened the result to the extinct armored mammal Glyptodon. In a 1908 review of Brown's Ankylosaurus description, Samuel Wendell Williston criticised the skeletal reconstruction as being based on too scanty remains, and claimed that Ankylosaurus was merely a synonym of the genus Stegopelta, which Williston had named in 1905. Williston also stated that a skeletal reconstruction of the related Polacanthus by Franz Nopcsa was a better example of how ankylosaurs would have appeared in life. The claim of synonymy was not accepted by other researchers, and the two genera are now considered distinct.
Brown had collected seventy-seven osteoderms while excavating a Tyrannosaurus specimen in the Lance Formation of Wyoming in 1900. He mentioned these osteoderms (AMNH 5866) in his description of Ankylosaurus, but thought they belonged to the Tyrannosaurus instead. Henry Fairfield Osborn also expressed this view when he described the Tyrannosaurus specimen as the now invalid genus Dynamosaurus in 1905. Later examination has shown them to be similar to those of Ankylosaurus, and that Brown had compared them with some Euoplocephalus osteoderms, which had been erroneously catalogued as belonging to Ankylosaurus at the AMNH.
In 1910, another AMNH expedition led by Brown discovered an Ankylosaurus specimen (AMNH 5214) in the Scollard Formation by the Red Deer River in Alberta, Canada. This specimen included a complete skull, mandibles, and the first and only tail club known of this genus, as well as ribs, vertebrae, limb bones, and armor. In 1947, Charles M. Sternberg and T.P. Channey collected a skull and mandible (NMC 8880), a kilometre north of where the 1910 specimen was found. This is the largest known Ankylosaurus skull, but is badly preserved. A section of caudal vertebrae (CCM V03) was discovered in the 1960s, in the Powder River drainage, Montana, also part of the Hell Creek Formation. In addition, many other isolated osteoderms and teeth have been found. In 1990, Walter P. Coombs pointed out that the teeth of two skulls referred to A. magniventris differed from those of the holotype specimen in some details, and though he expressed a "considerate temptation" to name a new species of Ankylosaurus for these, he refrained from doing so, as the range of variation in the species was not completely documented.
Most of the five known incomplete Ankylosaurus specimens were not described at length, though several palaeontologists planned to do so, until American palaeontologist Kenneth Carpenter redescribed the genus in 2004. Carpenter noted that Ankylosaurus has become the archetypal ankylosaur, and the best known member of the group in popular culture, perhaps due to a life-sized reconstruction of the animal being featured at the 1964 World's Fair in New York City. The structure of much of the skeleton, including the pelvis and most of the tail and feet, is still unknown.
Brown considered Ankylosaurus so distinct that he made it the type genus of a new family, Ankylosauridae (members of which are called ankylosaurids), typified by massive, triangular skulls, stiff backs, broad bodies, and osteoderms. He also classified Palaeoscincus (only known from teeth), and Euoplocephalus (then only known from a partial skull and osteoderms) as part of the family. Due to the fragmentary remains, Brown was unable to fully distinguish between Euoplocephalus and Ankylosaurus. The genus Troodon was included as well, at the time only known from a single tooth, which Brown found to be similar to those of the ankylosaurs. Due to the few remains known of this family, he believed the group was part of the suborder Stegosauria. In 1923, Osborn coined the name Ankylosauria (members of which are called ankylosaurs), thereby giving the ankylosaurids their own suborder.
Ankylosauria and Stegosauria are now grouped together within the clade Thyreophora, consisting of armored dinosaurs. Ankylosauromorphs first appeared in the Sinemurian age, and survived for 135 million years, until disappearing in the Maastrichtian. They were widespread and inhabited a broad range of environments. As more complete specimens and new taxa have been discovered, theories about ankylosaurian interrelatedness have become more complex, and hypotheses have often changed between studies. In addition to Ankylosauridae, Ankylosauria has been divided into the families Nodosauridae, and sometimes Polacanthidae (these families lacked tail clubs). Ankylosaurus is considered part of the subfamily Ankylosaurinae within Ankylosauridae. Ankylosaurus and Euoplocephalus are sometimes thought to be sister taxa, but have also been found in different positions. The following cladogram is based on a 2013 phylogenetic analysis of the Ankylosaurinae conducted by Victoria M. Arbour and Philip J. Currie:
Like other ornithischians, Ankylosaurus was herbivorous. Its wide muzzle was adapted for non-selective low-browse cropping. Paleontologist Georg Haas (1969) concluded that despite the large size of the skulls, the associated musculature was relatively weak. He also thought jaw movement was limited to up and down movements. Extrapolating from this, Haas suggested that ankylosaurids ate relatively soft non-abrasive vegetation. However, later research on Euoplocephalus indicates that forward and sideways jaw movement was possible. Ankylosaurus may have had a hindgut fermentation system like modern herbivorous lizards, based on the features of the ribcage.
Nasal passages of Ankylosaurus indicate that airflow was unidirectional, (looping through the lungs during inhalation and exhalation), although it may also have been bidirectional in the posterior nasal chamber. The complex air passages may have acted as a chamber for vocal resonance but this hypothesis was rejected by Carpenter (2004). A 2011 study of the air passages of the related genus Euoplocephalus suggested this function. In addition, the researchers suggested that ankylosaurids had well-developed olfactory systems and ears adapted to hearing at low frequencies, such as the low-tuned resonant sounds produced by the nasal passages. The position of the orbits of Ankylosaurus suggest some stereoscopic vision.
Reconstructions of the forelimb musculature made by Coombs in 1978 suggest that the forelimbs bore the majority of the animal's weight, and were adapted for high force delivery of the front feet, possibly for food gathering. In addition, Coombs suggested that ankylosaurids may have been capable diggers, though the hoof-like structure of the manus would have limited fossorial activity. Ankylosaurids were likely to have been slow-moving and sluggish animals.
The osteoderms of ankylosaurids were thin in comparison to those of other ankylosaurs, and appear to have been strengthened by randomly distributed cushions of collagen fibers. These were structurally similar to Sharpey's fibers, and were embedded directly into the bone tissue, a feature unique to ankylosaurids. This would have provided the ankylosaurids with an armor covering which was both lightweight and highly durable, being resistant to breakage and penetration by the teeth of predators. In addition to protection, Carpenter (1982) suggests that the heavily-vascularized armor may have had a role in thermoregulation as in modern crocodilians.
The tail club of Ankylosaurus seems to have been an active defensive weapon, capable of producing enough of an impact to break the bones of an assailant. The tendons of the tail were partially ossified (or bony) and were not very elastic, allowing great force to be transmitted to the club when it was swung. Coombs suggested in 1979 that the large hindlimb muscles would have controlled the swinging of the tail, and that violent thrusts of the club would have been able to break the metatarsal bones of large theropods. A 2009 study found that large ankylosaurid tail clubs were capable of breaking bones, but medium and small clubs were not. Despite the feasibility of tail swinging, the researchers could not determine whether ankylosaurids used their clubs for defense against potential predators, in intraspecific combat or both. Thulborn (1993) has proposed that the tail club acted as a decoy for the head, although this idea is disputed.
Ankylosaurus existed between 68 to 66 million years ago, in the final, or Maastrichtian, stage of the Late Cretaceous Period. It was among the last dinosaur species that appeared before the Cretaceous–Paleogene extinction event. The type specimen is from the Hell Creek Formation of Montana, while other specimens have been found in the Lance Formation of Wyoming and the Scollard Formation in Alberta, Canada, all of which date to the end of the Cretaceous. Fossils of Ankylosaurus are rare in these sediments, and the distribution of its remains suggest that it was restricted to the uplands of the formations, rather than the coastal lowlands. Another ankylosaur, the nodosaurid Edmontonia, is also found in the same formations, but the range of the two genera does not seem to have overlapped. Their remains have so far not been found in the same localities, and Edmontonia appears to have inhabited the lowlands. The narrow muzzle of Edmontonia suggests it had a more selective diet than of Ankylosaurus, further indicating ecological separation.
The Hell Creek, Lance and Scollard Formations represent different sections of the western shore of the shallow sea that divided western and eastern North America during the Cretaceous. They represent a broad coastal plain, extending westward from the seaway to the newly formed Rocky Mountains. These formations are composed largely of sandstone and mudstone, which have been attributed to floodplain environments. The Hell Creek is the best studied of these ancient environments. At the time, this region was subtropical, with a warm and humid climate. Many plant species were supported, primarily angiosperms, with less common conifers, ferns and cycads. An abundance of fossil leaves found at dozens of different sites indicates that the area was largely forested by small trees. Ankylosaurus shared its environment with dinosaurs including the ceratopsids Triceratops and Torosaurus, the hypsilophodont Thescelosaurus, the hadrosaurid Edmontosaurus, Edmontonia, the pachycephalosaurian Pachycephalosaurus, and the theropods Struthiomimus, Ornithomimus, Troodon, and Tyrannosaurus.
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