Ankylosaurus

South Dakota, North America

Wyoming, North America

***Ankylosaurus*** (/ˌæŋkəloʊˈsɔːrəs/ ***ang**-kə-lo-**sawr**-əs*) is a genus of armored dinosaur. Fossils of *Ankylosaurus* have been found in geological formations dating to the very end of the Cretaceous Period, between 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, and the only species classified in the genus is ***A. magniventris***. The genus name means "fused 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.

The largest known ankylosaurid, *Ankylosaurus* measured up to 6.25 m (20.5 feet) in length, 1.7 m (5.6 feet) in height, and weighed 6 tonnes (13,000 lb). It was a quadrupedal animal, with a broad, robust body. It had a wide, low skull, with two horns pointing backwards from the back of the head, and two horns below these that pointed backwards and down. The front part of the jaws were covered in a beak, with rows of small, leaf-shaped teeth further 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 played a role in vocalization. The tail club is thought to have been used in defense against predators or in intraspecific combat. *Ankylosaurus* has been found in the Hell Creek, Lance, and Scollard 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*.

## Description

*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 has been proposed by American palaeontologist Kenneth Carpenter, and 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 (21.9 inches) long and 64.5 cm (25.4 inches) wide, and this specimen is estimated to have been 5.4 m (17.7 feet) long and around 1.4 m (4.6 feet) tall. Other authors have proposed a body length of 7 m (23 feet), 8–9 m (26.2–29.5 ft), or more than 9 m (29.5 feet). The weight of the animal has been estimated at 6 tonnes (13,000 lb).

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 the forelimbs. The scapula (shoulder blade) and coracoid (a rectangular bone connected to the lower end of the scapula) of specimen AMNH 5895 were fused, and had entheses (connective tissue) for various muscle attachments. The scapula was 61.5 cm (24.2 inches) long. The humerus (upper arm bone) was short and very broad, and about 54 cm (21 inches) long in specimen AMNH 5214. The femur (thigh bone) was very robust, and 67 cm (26 inches) long in AMNH 5214. While the feet of *Ankylosaurus* are incompletely known, the hindfeet probably had three toes, as is the case in related animals.

The cervical vertebrae of the neck 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 of the back 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 them, and the ribcage was very broad in this part of the body. The ribs had scars that show where muscles attached to them. The caudal vertebrae of the tail had centra that were slightly amphicoelous, meaning they were concave on both sides. The interlocked zygapophyses (articular processes) of the caudal vertebrae formed a V-shape when seen from above.

### **Skull**

The three known *Ankylosaurus* skulls differ in various details, but this is thought to be the result of taphonomy (changes happening during fossilisation of the remains) and individual variation. The skull was low and triangular in shape, wider than it was long. It 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. 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 backwards and down. The horns may have originally been osteoderms (armor plates) that fused to the skull. However, the scale pattern on the skull surface was instead the result of remodelling of the skull. This obliterated the suturesbetween skull elements, which is common for adult ankylosaurs. The scale pattern of the skull was variable between specimens, though some details are shared; it had a diamond-shaped scale (internarial scale) at the font of the snout, two squamosal osteoderms above the orbit, and a ridge of scales at the back of the skull.

The snout of *Ankylosaurus* was arched and truncated at the front, and the nostrils were ellipse-shaped and faced sideways, unlike in many other ankylosaurids where they faced the front or upwards. This was because the sinuses were expanded to the sides of the premaxilla bone, to a larger extent than seen in other ankylosaurs. 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 in to 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 in two mirrored halves. The septum had two openings, including the choanae (internal nostrils).

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 which may have been the attachment site for fleshy cheeks; the presence of cheeks in ornithischians is controversial, but some nodosaurid ankylosaurs 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, which was more teeth than any other ankylosaurid. The tooth rows in the maxillae of this specimen are about 20 cm (8 inches) long. Each aveoli had a foramen (opening) near its side wherein a replacement tooth could be seen. The back of the skull was broad and low.

Compared to other ankylosaurs, the mandible of *Ankylosaurus* was low in proportion to its length, and the tooth row was almost straight instead of arched, when seen from the side. The mandibles are only completely preserved in the smallest specimen (AMNH 5214) and are about 41 cm (16 inches) long. The incomplete mandible of the largest specimen (NMC 8880) is the same length. AMNH 5214 has 35 dental alveoli in the left dentary and 36 in the right. 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 in relation to the size of the skull. Some teeth from behind in the tooth row curved backwards, and tooth crownswere 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.

### **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. The osteoderms ranged from 1 cm (0.4 inches) in diameter to 35.5 cm (14.0 inches) in length, and also 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 in texture. 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. Some osteoderms without keels may have been placed above the hip region, as in *Euoplocephalus*. Flattened, pointed plates resemble those on the sides of the tail of *Saichania*. Osteoderms with oval keels could have been placed on the upper side of the tail or the side of the limbs. Small osteoderms and ossicles likely occupied the space between the larger ones.

Like other ankylosaurids, *Ankylosaurus* had cervical half-rings (armor plates on the neck), but these are only known from fragments, making their exact arrangement uncertain. Carpenter suggested that when seen from above, the plates would have been paired, and created 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. Paleontologists Victoria M. Arbour and Philip J. Currie disagreed with Carpenter's interpretation, and pointed out that the cervical half-ring fragments of specimen AMNH 5895 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*.

The tail club of *Ankylosaurus* was composed of two large osteoderms, with a row of small osteoderms at the midline, and two small osteoderms at the tip. 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 450 mm (18 in) wide. The last seven tail vertebrae formed the "handle" of the tail club. These vertebrae were in contact, with no cartilage between them, and 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.

## 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 in 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*. Contrary to modern depictions, Brown's reconstruction showed a strongly arched back, while the tail club was missing as it was unknown at the time. Brown also reconstructed the armor plates in parallel rows running down the back; this arrangement, however, was purely hypothetical. In a 1908 review of Brown's *Ankylosaurus* description, American palaeontologist 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 Hungarian palaeontologist 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 (specimen AMNH 5866) in his description of*Ankylosaurus*, but thought they belonged to the *Tyrannosaurus* instead. Palaeontologist 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, the first and only tail club known of this genus, as well as ribs, vertebrae, limb bones, and armor. In 1947, fossil collectors Charles M. Sternberg and T.P. Channey collected a skull and mandible (specimen NMC 8880), a kilometre (0.6 miles) north of where the 1910 specimen was found. This is the largest known *Ankylosaurus* skull, but is badly preserved. A section of caudal vertebrae (specimen CCM V03) was discovered in the 1960s, in the Powder River drainage, Montana, also 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 palaeontologist 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. 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. Kenneth Carpenter accepted the teeth as belonging to *A. magniventris* and that all 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 palaeontologists 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. Many traditional popular depictions show *Ankylosaurus* in a squatting posture and with a huge tail club being dragged over the ground. Modern reconstructions, however, show the animal with a more upright limb posture and with the tail being held clear off the ground. Likewise, large spines projecting sidewards from the body are present in many traditional depictions, but are actually only known in nodosaurids while being unknown in ankylosaurids.

## Classification

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, 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 remains, Brown was unable to fully distinguish between *Euoplocephalus* and *Ankylosaurus*. Only having few, incomplete members of the family to compare with, he believed the group was part of the suborder Stegosauria. In 1923, Osborn coined the name Ankylosauria (members of which are called ankylosaurs or ankylosaurians), 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*.