Ankylosaurus

''Ankylosaurus'' is a genus of armored dinosaur. Its fossils have been found in geological formations dating to the very end of the Cretaceous Period, about 68–66 million years ago, in western North America, making it among the last of the non-avian dinosaurs. It was named by Barnum Brown in 1908; it is monotypic, containing only ''A. magniventris''. The generic name means "fused" or "bent lizard", and the specific name means "great belly". A handful of specimens have been excavated to date, but a complete skeleton has not been discovered. Though other members of Ankylosauria are represented by more extensive fossil material, ''Ankylosaurus'' is often considered the archetypal member of its group, despite having some unusual features.

Possibly the largest-known ankylosaurid, ''Ankylosaurus'' is estimated to have been between long and to have weighed between . It was quadrupedal, with a broad, robust body. It had a wide, low skull, with two horns pointing backward from the back of the head, and two horns below these that pointed backward and down. Unlike other ankylosaurs, its nostrils faced sideways rather than towards the front. The front part of the jaws was covered in a beak, with rows of small, leaf-shaped teeth farther behind it. It was covered in armor plates, or osteoderms, with bony half-rings covering the neck, and had a large club on the end of its tail. Bones in the skull and other parts of the body were fused, increasing their strength, and this feature is the source of the genus name.

''Ankylosaurus'' is a member of the family Ankylosauridae, and its closest relatives appear to be '' Anodontosaurus'' and ''Euoplocephalus''. ''Ankylosaurus'' is thought to have been a slow-moving animal, able to make quick movements when necessary. Its broad muzzle indicates it was a non-selective browser. Sinuses and nasal chambers in the snout may have been for heat and water balance or may have played a role in vocalization. The tail club is thought to have been used in defense against predators or in intraspecific combat. Specimens of ''Ankylosaurus'' have been found in the Hell Creek, Lance, Scollard, Frenchman, and Ferris formations, but appears to have been rare in its environment. Although it lived alongside a nodosaurid ankylosaur, their ranges and ecological niches do not appear to have overlapped, and ''Ankylosaurus'' may have inhabited upland areas. ''Ankylosaurus'' also 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 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 (armor plates). Brown scientifically described the animal in 1908; the generic name is derived from the Greek words ' ('bent' or 'crooked'), referring to the medical term ankylosis, the stiffness produced by the fusion of bones in the skull and body, and ' ('lizard'). The name can be translated as "fused lizard", "stiff lizard", or "curved lizard". The type species name, ''magniventris,'' is derived from the la, magnus ('great') and la, 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 contrast to modern depictions, Brown's stegosaur-like reconstruction showed robust forelimbs, a strongly arched back, a pelvis with prongs projecting forwards from the ilium and pubis, as well as a short, drooping tail without a tail club, which was unknown at the time. Brown also reconstructed the armor plates in parallel rows running down the back; this arrangement was purely hypothetical. Brown's reconstruction became highly influential, and restorations of the animal based on his diagram were published as late as the 1980s. In a 1908 review of Brown's ''Ankylosaurus'' description, the American paleontologist Samuel Wendell Williston criticised the skeletal reconstruction as being based on too few 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 Hungarian paleontologist 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 77 osteoderms while excavating a ''Tyrannosaurus'' specimen in the Lance Formation of Wyoming in 1900. He mentioned these osteoderms (specimen AMNH 5866) in his description of ''Ankylosaurus'' but thought they belonged to the ''Tyrannosaurus'' instead. Paleontologist Henry Fairfield Osborn also expressed this view when he described the ''Tyrannosaurus'' specimen as the now synonymous genus ''Dynamosaurus'' in 1905. More recent examination has shown them to be similar to those of ''Ankylosaurus''; it seems that Brown had compared them with some ''Euoplocephalus'' osteoderms, which had been erroneously cataloged 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, the first and only tail club known of this genus, as well as ribs, vertebrae, limb bones, and armor. In 1947 the American fossil collectors Charles M. Sternberg and T. Potter Chamney collected a skull and mandible (specimen CMN 8880, formerly NMC 8880), north of where the 1910 specimen was found. This is the largest-known ''Ankylosaurus'' skull, but it is damaged in places. A section of caudal vertebrae (specimen CCM V03) was discovered in the 1960s in the Powder River drainage, Montana, part of the Hell Creek Formation. In addition to these five incomplete specimens, many other isolated osteoderms and teeth have been found.

In 1990, American paleontologist Walter P. Coombs pointed out that the teeth of two skulls assigned 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. He also raised the possibility that the two teeth associated with the holotype specimen perhaps did not belong to it, as they were found in matrix within the nasal chambers. The American paleontologist Kenneth Carpenter accepted the teeth as belonging to ''A. magniventris'' in 2004, and that all the specimens belonged to the same species, noting that the teeth of other ankylosaurs are highly variable.

Most of the known ''Ankylosaurus'' specimens were not scientifically described at length, though several paleontologists planned to do so until Carpenter redescribed the genus in 2004. Carpenter noted that ''Ankylosaurus'' has become the archetypal member of its group, and the best-known ankylosaur 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. That sculpture, as well as the American artist Rudolph Zallinger's 1947 mural ''The Age of Reptiles'' and other later popular depictions, showed ''Ankylosaurus'' with a tail club, following the first discovery of the feature in 1910. In spite of its familiarity, it is known from far fewer remains than its closest relatives. In 2017 the Canadian paleontologists Victoria M. Arbour and Jordan Mallon redescribed the genus in light of newer ankylosaur discoveries, including elements of the holotype that had not been previously mentioned in the literature (such as parts of the skull and the cervical half-rings). They concluded that though ''Ankylosaurus'' is iconic and the best-known member of its group, it was bizarre in comparison to related ankylosaurs, and therefore not representative of the group.

Many traditional popular depictions show ''Ankylosaurus'' in a squatting posture and with a huge tail club being dragged over the ground. Modern reconstructions show the animal with a more upright limb posture and with the tail held off the ground. Likewise, large spines projecting sideways from the body (similar to those of nodosaurid ankylosaurs) are present in many traditional depictions, but are not known from ''Ankylosaurus'' itself. The armor of ''Ankylosaurus'' has often been conflated with that of ''Edmontonia'' (earlier referred to as '' Palaeoscincus''); in addition to ''Ankylosaurus'' being depicted with spikes, ''Edmontonia'' has also been depicted with an ''Ankylosaurus''-like tail club (a feature nodosaurids did not have), including in a mural by the American artist Charles R. Knight from 1930.

Description

''Ankylosaurus'' was the largest-known ankylosaurine dinosaur and possibly the largest ankylosaurid. In 2004 Carpenter estimated that the individual with the largest-known skull (specimen CMN 8880), which is long and wide, was about long and had a hip height of about . The smallest-known skull (specimen AMNH 5214) is long and wide, and Carpenter estimated that it measured about long and about tall at the hips. The American paleontologist Roger B. J. Benson and colleagues estimated the weight for AMNH 5214 at in 2014.

In 2017, based on comparisons with more complete ankylosaurines, Arbour and Mallon estimated a length of for CMN 8880, and for AMNH 5214. Though the latter is the smallest specimen of ''Ankylosaurus'', its skull is still larger than those of any other ankylosaurins. A few other ankylosaurs reached about in length. Because the vertebrae of AMNH 5214 are not significantly larger than those of other ankylosaurines, Arbour and Mallon considered their upper range estimate of nearly for large ''Ankylosaurus'' too long, and suggested a length of instead. Arbour and Mallon estimated a weight of for AMNH 5214, and tentatively estimated the weight of CMN 8880 at .

Skull

The three known ''Ankylosaurus'' skulls differ in various details; this is thought to be the result of taphonomy (changes happening during decay and fossilization of the remains) and individual variation. The skull was low and triangular in shape, and wider than it was long; the back of the skull was broad and low. The skull had a broad beak on the premaxillae. The orbits (eye sockets) were almost round to slightly oval and did not face directly sideways because the skull tapered towards the front. The braincase was short and robust, as in other ankylosaurines. Crests above the orbits merged into the upper squamosal horns (their shape has been described as "pyramidal"), which pointed backwards to the sides from the back of the skull. The crest and horn were probably separate elements originally, as seen in the related ''Pinacosaurus'' and ''Euoplocephalus''. Below the upper horns, jugal horns were present, which pointed backward and down. The horns may have originally been osteoderms that fused to the skull. The scale-like cranial ornamentation on the surfaces of ankylosaurs skulls is called "", and were the result of remodeling of the skull itself. This obliterated the sutures between skull elements, which is common for adult ankylosaurs. The caputegulum pattern of the skull was variable between specimens, though some details are shared. The caputegulae are named according to their position on the skull, and those of ''Ankylosaurus'' include a relatively large, hexagonal (or diamond-shaped) nasal caputegulum at the front of the snout between the nostrils, which had a loreal caputegulum on each side, an anterior and posterior supraorbital caputegulum above each orbit, and a ridge of nuchal caputegulae at the back of the skull.

The snout region of ''Ankylosaurus'' was unique among ankylosaurs, and had undergone an "extreme" transformation compared to its relatives. The snout was arched and truncated at the front, and the nostrils were elliptical and were directed downward and outward, unlike in all other known ankylosaurids where they faced obliquely forward or upward. Additionally, the nostrils were not visible from the front because the sinuses were expanded to the sides of the premaxilla bones, to a larger extent than seen in other ankylosaurs. Large loreal caputegulae—strap-like, side osteoderms of the snout—completely roofed the enlarged opening of the nostrils, giving a bulbous appearance. The nostrils also had an intranarial septum, which separated the nasal passage from the sinus. Each side of the snout had five sinuses, four of which expanded into the maxilla bone. The nasal cavities (or chambers) of ''Ankylosaurus'' were elongated and separated by a septum at the midline, which divided the inside of the snout into two mirrored halves. The nasal chambers had two openings, including the choanae (internal nostrils), and the air passage was looped. The maxillae expanded to the sides, giving the impression of a bulge, which may have been due to the sinuses inside. The maxillae had a ridge that may have been the attachment site for fleshy cheeks; the presence of cheeks in ornithischians is controversial, but some nodosaurs had armor plates that covered the cheek region, which may have been embedded in the flesh.

Specimen AMNH 5214 has 34–35 dental alveoli (tooth sockets) in the maxilla. The tooth rows in the maxillae of this specimen are about long. Each alveolus had a foramen (opening) near its side where a replacement tooth could be seen. Compared to other ankylosaurs, the mandible of ''Ankylosaurus'' was low in proportion to its length, and, when seen from the side, the tooth row was almost straight instead of arched. The mandibles are completely preserved only in the smallest specimen (AMNH 5214) and are about long. The incomplete mandible of the largest specimen (CMN 8880) is the same length. AMNH 5214 has 35 dental alveoli in the left dentary and 36 in the right, for a total of 71. The predentary bone of the tip of the mandibles has not yet been found. Like other ankylosaurs, ''Ankylosaurus'' had small, phylliform (leaf-shaped) teeth, which were compressed sideways. The teeth were mostly taller than they were wide, and were very small; their size in proportion to the skull meant that the jaws of ''Ankylosaurus'' could accommodate more teeth than other ankylosaurines. The teeth of the largest ''Ankylosaurus'' skull are smaller than those of the smallest skull in the absolute sense. Some teeth from behind in the tooth row curved backwards, and tooth crowns were usually flatter on one side than the other. ''Ankylosaurus'' teeth are diagnostic and can be distinguished from the teeth of other ankylosaurids based on their smooth sides. The denticles were large, their number ranging from six to eight on the front part of the tooth, and five to seven behind.

Postcranial skeleton

The structure of much of the skeleton of ''Ankylosaurus'', including most of the pelvis, tail, and feet, is still unknown. It was quadrupedal, and its hind limbs were longer than its forelimbs. In the holotype specimen, the scapula (shoulder blade) measures long and was fused with the coracoid (a rectangular bone connected to the lower end of the scapula). It also had entheses (connective tissue) for various muscle attachments. The humerus (upper arm bone) of AMNH 5214 was short, very broad and about long. The femur (thigh bone), also from AMNH 5214, was long and very robust. While the feet of ''Ankylosaurus'' are incompletely known, the hindfeet probably had three toes, as is the case in advanced ankylosaurids.

The cervical vertebrae had broad neural spines that increased in height towards the body. The front part of the neural spines had well-developed entheses, which was common among adult dinosaurs, and indicates the presence of large ligaments, which helped support the massive head. The dorsal vertebrae had centra (or bodies) that were short relative to their width, and their neural spines were short and narrow. The dorsal vertebrae were tightly spaced, which limited the downwards movement of the back. The neural spines had ossified (turned to bone) tendons, which also overlapped some of the vertebrae. The ribs of the last four back vertebrae were fused to the and (the structures that articulated the ribs with the vertebrae), and the ribcage was very broad in this part of the body. The caudal vertebrae had centra that were slightly amphicoelous, meaning they were concave on both sides.

Armor

A prominent feature of ''Ankylosaurus'' was its armor, consisting of knobs and plates of bone known as osteoderms, or scutes, embedded in the skin. These have not been found in articulation, so their exact placement on the body is unknown, though inferences can be made based on related animals, and various configurations have been proposed. The osteoderms ranged from in diameter to in length, and varied in shape. The osteoderms of ''Ankylosaurus'' were generally thin walled and hollowed on the underside. Compared to ''Euoplocephalus'', the osteoderms of ''Ankylosaurus'' were smoother. Many smaller osteoderms and ossicles probably occupied the space between the larger ones, as in other ankylosaurids. The osteoderms covering the body were very flat, though with a low keel at one margin. In contrast, the nodosaurid ''Edmontonia'' had high keels stretching from one margin to the other on the midline of its osteoderms. ''Ankylosaurus'' had some smaller osteoderms with a keel across the midline.

Like other ankylosaurids, ''Ankylosaurus'' had (armor plates on the neck), but these are known only from fragments, making their exact arrangement uncertain. Carpenter suggested that when seen from above, the plates would have been paired, creating an inverted V-shape across the neck, with the midline gap probably being filled with small ossicles (round bony scutes) to allow for movement. He believed the width of this armor belt was too wide to have fitted solely on the neck, and that it covered the base of the neck and continued onto the shoulder region. Arbour and the Canadian paleontologist Philip J. Currie disagreed with Carpenter's interpretation in 2015 and pointed out that the cervical half-ring fragments of the holotype specimen did not fit together in the way proposed by Carpenter (though this could be due to breakage). They instead suggested that the fragments represented the remains of two cervical half-rings, which formed two semi-circular plates of armor around the upper part of the neck, as in the closely related '' Anodontosaurus'' and ''Euoplocephalus''. Arbour and Mallon elaborated on this idea, describing the shape of these half-rings as "continuous U-shaped yokes" over the upper part of the neck, and suggested that ''Ankylosaurus'' had six keeled osteoderms with oval bases on each half-ring.

The first osteoderms behind the second cervical half-ring would have been similar in shape to those in the first half-ring, and the osteoderms on the back probably decreased in diameter hindwards. The largest osteoderms were probably arranged in transverse and longitudinal rows across most of the body, with four or five transverse rows separated by creases in the skin. The osteoderms on the flanks would probably have had a more square outline than those on the back. There may have been four longitudinal rows of osteoderms on the flanks. Unlike some basal ankylosaurs and many nodosaurs, ankylosaurids do not appear to have had co-ossified pelvic shields above their hips. Some osteoderms without keels may have been placed above the hip region of ''Ankylosaurus'', as in ''Euoplocephalus''. ''Ankylosaurus'' may have had three or four transverse rows of circular osteoderms over the pelvic region, which were smaller than those on the rest of the body, as in ''Scolosaurus''. Smaller, triangular osteoderms may have been present on the sides of the pelvis. Flattened, pointed plates resemble those on the sides of the tail of ''Saichania'', and may have been distributed similarly on ''Ankylosaurus''. Osteoderms with oval keels could have been placed on the upper side of the tail or the side of the limbs. Compressed, triangular osteoderms found with ''Ankylosaurus'' specimens may have been placed on the sides of the pelvis or the tail. Ovoid, keeled, and teardrop-shaped osteoderms are known from ''Ankylosaurus'', and may have been placed on the forelimbs, like those known from ''Pinacosaurus'', but it is unknown whether the hindlimbs bore osteoderms.

The tail club (or tail knob) of ''Ankylosaurus'' was composed of two large osteoderms, with a row of small osteoderms at the midline, and two small osteoderms at the tip; these osteoderms obscured the last tail vertebra. As only the tail club of specimen AMNH 5214 is known, the range of variation between individuals is unknown. The tail club of AMNH 5214 is long, wide, and tall. The club of the largest specimen may have been wide. The tail club of ''Ankylosaurus'' was semicircular when seen from above, similar to those of ''Euoplocephalus'' and ''Scolosaurus'' but unlike the pointed club osteoderms of ''Anodontosaurus'' or the narrow, elongated club of ''Dyoplosaurus''. The last seven tail vertebrae formed the "handle" of the tail club. These vertebrae were in contact, with no cartilage between them, and were sometimes co-ossified, which made them immobile. Ossified tendons attached to the vertebrae in front of the tail club, and these features together helped strengthen it. The interlocked zygapophyses (articular processes) and neural spines of the handle vertebrae were U-shaped when seen from above, whereas those of most other ankylosaurids are V-shaped, which may be due to the handle of ''Ankylosaurus'' being wider. The larger width may indicate that the tail of ''Ankylosaurus'' was shorter in relation to its body length than those of other ankylosaurids, or that it had the same proportions but with a smaller club.

Classification

Brown considered ''Ankylosaurus'' so distinct that he made it the type genus of a new family, Ankylosauridae, typified by massive, triangular skulls, short necks, 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 condition of the remains, Brown was unable to fully distinguish between ''Euoplocephalus'' and ''Ankylosaurus''. Having for comparison only a few, incomplete members of the family, he believed the group was part of the suborder Stegosauria. In 1923 Osborn coined the name Ankylosauria, thereby placing the ankylosaurids in their own suborder.

Ankylosauria and Stegosauria are now grouped together within the clade Thyreophora. This group 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 genera 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 (members of which are called ankylosaurines) within Ankylosauridae. ''Ankylosaurus'' appears to be most closely related to ''Anodontosaurus'' and ''Euoplocephalus''. The following cladogram is based on a 2015 phylogenetic analysis of the Ankylosaurinae conducted by Arbour and Currie:

Because ''Ankylosaurus'' and other Late Cretaceous North American ankylosaurids were grouped with Asian genera (in a tribe the authors named Ankylosaurini), Arbour and Currie suggested that earlier North American ankylosaurids had gone extinct by the late Albian or Cenomanian ages of the Middle Cretaceous. Ankylosaurids thereafter recolonized North America from Asia during the Campanian or Turonian ages of the Late Cretaceous, and there diversified again, leading to genera such as ''Ankylosaurus'', ''Anodontosaurus'', and ''Euoplocephalus''. The theory explains a 30-million-year gap in the fossil record of North American ankylosaurids between the ages.

Paleobiology

Feeding

Like other ornithischians, ''Ankylosaurus'' was herbivorous. Its wide muzzle was adapted for non-selective low- browse cropping, although not to the extent seen in some related genera, especially ''Euoplocephalus''. Though ankylosaurs may not have fed on fibrous and woody plants, they may have had a varied diet, including tough leaves and pulpy fruits. ''Ankylosaurus'' probably fed on abundant ferns and low-growing shrubs. Assuming it was endothermic, ''Ankylosaurus'' would have eaten of ferns per day, similar to the amount of dry vegetation a large elephant would consume. The requirements for nutrition could have been more effectively met if ''Ankylosaurus'' ate fruit, which its small, cusp-like teeth and the shape of its beak seem well adapted for, compared to for example ''Euoplocephalus''. Certain invertebrates, which the small teeth may have been adapted for handling, could also have provided supplemental nutrition.

Fossils of ''Ankylosaurus'' teeth exhibit wear on the face of the crown rather than on the tip of the crown, as in nodosaurid ankylosaurs. In 1982 Carpenter ascribed to baby ''Ankylosaurus'' two very small teeth that originate from the Lance and Hell Creek Formations and measure in length, respectively. The smaller tooth is heavily worn, leading Carpenter to suggest that ankylosaurids in general or at least the young did not swallow their food whole but employed some sort of chewing. Since adult ''Ankylosaurus'' did little chewing of its food, it would have spent less time in the day foraging than an elephant. Based on the broadness of the ribcage, the digestion of unchewed food may have been facilitated by hindgut fermentation

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