''Ankylosaurus'' is a
genus
Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of
armored dinosaur. Its
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...
s have been found in
geological formation
A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics (lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock expo ...
s dating to the very end of the
Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
Period, about 68–66 million years ago, in western North America, making it among the last of the non-avian
dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...
s. It was named by
Barnum Brown
Barnum Brown (February 12, 1873 – February 5, 1963), commonly referred to as Mr. Bones, was an American paleontologist. He discovered the first documented remains of ''Tyrannosaurus'' during a career that made him one of the most famous fossil ...
in 1908; it is
monotypic
In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unisp ...
, 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
Ankylosauria is a group of herbivorous dinosaurs of the clade Ornithischia. It includes the great majority of dinosaurs with armor in the form of bony osteoderms, similar to turtles. Ankylosaurs were bulky quadrupeds, with short, powerful limbs ...
are represented by more extensive fossil material, ''Ankylosaurus'' is often considered the
archetypal
The concept of an archetype ( ) appears in areas relating to behavior, History of psychology#Emergence of German experimental psychology, historical psychology, philosophy and literary analysis.
An archetype can be any of the following:
# a stat ...
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
Quadrupedalism is a form of Animal locomotion, locomotion in which animals have four legs that are used to weight-bearing, bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four l ...
, 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
Paranasal sinuses are a group of four paired air-filled spaces that surround the nasal cavity. The maxillary sinuses are located under the eyes; the frontal sinuses are above the eyes; the ethmoidal sinuses are between the eyes and the sphenoi ...
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
The Hell Creek Formation is an intensively studied division of mostly Upper Cretaceous and some lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana. The formation stretches over portions o ...
,
Lance
The English term lance is derived, via Middle English '' launce'' and Old French '' lance'', from the Latin '' lancea'', a generic term meaning a wikt:lancea#Noun">lancea'', a generic term meaning a spear">wikt:lancea#Noun">lancea'', a generi ...
,
Scollard,
Frenchman, and
Ferris formations, but it appears to have been rare in its environment. Although it lived alongside ''
Denversaurus'', a
nodosaurid
Nodosauridae is a family of ankylosaurian dinosaurs known from the Late Jurassic to the Late Cretaceous periods in what is now Asia, Europe, North America, and possibly South America. While traditionally regarded as a monophyletic clade as the s ...
ankylosaur, their ranges and
ecological niche
In ecology, a niche is the match of a species to a specific environmental condition.
Three variants of ecological niche are described by
It describes how an organism or population responds to the distribution of Resource (biology), resources an ...
s do not appear to have overlapped, and ''Ankylosaurus'' may have inhabited upland areas. ''Ankylosaurus'' also lived alongside dinosaurs such as ''
Tyrannosaurus
''Tyrannosaurus'' () is a genus of large theropod dinosaur. The type species ''Tyrannosaurus rex'' ( meaning 'king' in Latin), often shortened to ''T. rex'' or colloquially t-rex, is one of the best represented theropods. It lived througho ...
'', ''
Triceratops
''Triceratops'' ( ; ) is a genus of Chasmosaurinae, chasmosaurine Ceratopsia, ceratopsian dinosaur that lived during the late Maastrichtian age of the Late Cretaceous Period (geology), period, about 68 to 66 million years ago on the island ...
'', and ''
Edmontosaurus
''Edmontosaurus'' ( ) (meaning "lizard from Edmonton"), with the second species often colloquially and historically known as ''Anatosaurus'' or ''Anatotitan'' (meaning "duck lizard" and "giant duck"), is a genus of hadrosaurid (duck-billed) din ...
''.
History of discovery

In 1906, an
American Museum of Natural History
The American Museum of Natural History (AMNH) is a natural history museum on the Upper West Side of Manhattan in New York City. Located in Theodore Roosevelt Park, across the street from Central Park, the museum complex comprises 21 interconn ...
expedition led by American
paleontologist
Paleontology, also spelled as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure geolo ...
Barnum Brown
Barnum Brown (February 12, 1873 – February 5, 1963), commonly referred to as Mr. Bones, was an American paleontologist. He discovered the first documented remains of ''Tyrannosaurus'' during a career that made him one of the most famous fossil ...
discovered the
type specimen
In biology, a type is a particular wikt:en:specimen, specimen (or in some cases a group of specimens) of an organism to which the scientific name of that organism is formally associated. In other words, a type is an example that serves to ancho ...
of ''Ankylosaurus magniventris'' (AMNH 5895) in the
Hell Creek Formation
The Hell Creek Formation is an intensively studied division of mostly Upper Cretaceous and some lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana. The Formation (stratigraphy), formation s ...
, near Gilbert Creek,
Montana
Montana ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Idaho to the west, North Dakota to the east, South Dakota to the southeast, Wyoming to the south, an ...
. 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
Osteoderms are bony deposits forming scales, plates, or other structures based in the dermis. Osteoderms are found in many groups of Extant taxon, extant and extinct reptiles and amphibians, including lizards, crocodilians, frogs, Temnospondyli, ...
(armor plates). Brown scientifically
described the animal in 1908; the generic name is derived from the
Greek
Greek may refer to:
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group
*Greek language, a branch of the Indo-European language family
**Proto-Greek language, the assumed last common ancestor of all kno ...
words ' ('bent' or 'crooked'), referring to the medical term
ankylosis
Ankylosis () is a stiffness of a joint due to abnormal adhesion and rigidity of the bones of the joint, which may be the result of injury or disease. The rigidity may be complete or partial and may be due to inflammation of the Tendon, tendinous ...
, 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
In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...
name, ''magniventris,'' is derived from the ('great') and ('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
''Stegosaurus'' (; ) is a genus of herbivorous, four-legged, armored dinosaur from the Late Jurassic, characterized by the distinctive kite-shaped upright plates along their backs and spikes on their tails. Fossils of the genus have been fo ...
'', 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
In zoology, a tail club is a bony mass at the end of the tail of some dinosaurs and of some mammals, most notably the Ankylosauridae, ankylosaurids and the glyptodonts, as well as meiolaniid turtles. It is thought that this was a form of defensive ...
, 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
Samuel Wendell Williston (July 10, 1852 – August 30, 1918) was an American educator, entomologist, and Paleontology, paleontologist who was the first to propose that birds developed flight Origin of birds#Origin of bird flight, cursorially (by ...
criticized 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
''Polacanthus'', deriving its name from the Ancient Greek polys-/πολύς- "many" and akantha/ἄκανθα "thorn" or "prickle", is an early armoured, spiked, plant-eating ankylosaurian dinosaur from the early Cretaceous period of England.
In ...
'' by Hungarian paleontologist Franz Nopcsa Franz may refer to:
People
* Franz (given name)
* Franz (surname)
Places
* Franz (crater), a lunar crater
* Franz, Ontario, a railway junction and unorganized town in Canada
* Franz Lake, in the state of Washington, United States – see Fran ...
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
''Tyrannosaurus'' () is a genus of large theropod dinosaur. The type species ''Tyrannosaurus rex'' ( meaning 'king' in Latin), often shortened to ''T. rex'' or colloquially t-rex, is one of the best represented theropods. It lived througho ...
'' specimen in the Lance Formation
The Lance (Creek) Formation is a division of Late Cretaceous (dating to about 69–66 Ma) rocks in the western United States. Named after Lance Creek, Wyoming, the microvertebrate fossils and dinosaurs represent important components of the lates ...
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
A synonym is a word, morpheme, or phrase that means precisely or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are a ...
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
The Red Deer River is a river in Alberta and a small portion of Saskatchewan, Canada. It is a major tributary of the South Saskatchewan River and is part of the larger Saskatchewan / Nelson River, Nelson system that empties into Hudson Bay.
T ...
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
Matrix (: matrices or matrixes) or MATRIX may refer to:
Science and mathematics
* Matrix (mathematics), a rectangular array of numbers, symbols or expressions
* Matrix (logic), part of a formula in prenex normal form
* Matrix (biology), the m ...
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. 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 the best-known member of its group, it was bizarre in comparison to related ankylosaurs, and therefore not representative of the group. In spite of its familiarity, it is known from far fewer remains than its closest relatives.
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 English 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
Taphonomy is the study of how organisms decay and become fossilized or preserved in the paleontological record. The term ''taphonomy'' (from Greek language, Greek , 'burial' and , 'law') was introduced to paleontology in 1940 by Soviet scientis ...
(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
The beak, bill, or rostrum is an external anatomical structure found mostly in birds, but also in turtles, non-avian dinosaurs and a few mammals. A beak is used for pecking, grasping, and holding (in probing for food, eating, manipulating and ...
on the premaxilla
The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammals h ...
e. The orbit
In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...
s (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
The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone.
In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestra ...
horns (their shape has been described as "pyramid
A pyramid () is a structure whose visible surfaces are triangular in broad outline and converge toward the top, making the appearance roughly a pyramid in the geometric sense. The base of a pyramid can be of any polygon shape, such as trian ...
al"), 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
''Pinacosaurus'' (meaning "Plank lizard") is a genus of ankylosaurid thyreophoran dinosaur that lived in Asia during the Late Cretaceous (Campanian, roughly 75 to 71 million years ago), mainly in Mongolia and China.
The first remains of the genu ...
'' 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
Renovation (also called remodeling) is the process of improving broken, damaged, or outdated structures. Renovations are typically done on either commercial or residential buildings. Additionally, renovation can refer to making something new, o ...
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
Paranasal sinuses are a group of four paired air-filled spaces that surround the nasal cavity. The maxillary sinuses are located under the eyes; the frontal sinuses are above the eyes; the ethmoidal sinuses are between the eyes and the sphenoi ...
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
In biology, a septum (Latin language, Latin for ''something that encloses''; septa) is a wall, dividing a Body cavity, cavity or structure into smaller ones. A cavity or structure divided in this way may be referred to as septate.
Examples
Hum ...
, which separated the nasal passage from the sinus. Each side of the snout had five sinuses, four of which expanded into the maxilla
In vertebrates, the maxilla (: maxillae ) is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The two maxil ...
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
The choanae (: choana), posterior nasal apertures or internal nostrils are two openings found at the back of the nasal passage between the nasal cavity and the pharynx, in humans and other mammals (as well as crocodilians and most skinks). They ...
(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
In anatomy and osteology, a foramen (; : foramina, or foramens ; ) is an opening or enclosed gap within the dense connective tissue (bones and deep fasciae) of extant and extinct amniote animals, typically to allow passage of nerves, artery, ...
(opening) near its side where a replacement tooth could be seen. Compared to other ankylosaurs, the mandible
In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla).
The jawbone i ...
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 bone () and 36 in the right, for a total of 71. The predentary
Ornithischia () is an extinct clade of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds. The name ''Ornithischia'', or "bird-hipped", reflects this similarity and is derived from the Ancient ...
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
Diagnosis (: diagnoses) is the identification of the nature and cause of a certain phenomenon. Diagnosis is used in a lot of different academic discipline, disciplines, with variations in the use of logic, analytics, and experience, to determine " ...
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
The pelvis (: pelves or pelvises) is the lower part of an Anatomy, anatomical Trunk (anatomy), trunk, between the human abdomen, abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also c ...
, tail, and feet, is still unknown. It was quadruped
Quadrupedalism is a form of locomotion in which animals have four legs that are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four legs is said to be a quadruped (fr ...
al, and its hind limbs were longer than its forelimbs. In the holotype specimen, the scapula
The scapula (: scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on either side ...
(shoulder blade) measures long and was fused with the coracoid
A coracoid is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is present as part of the scapula, but this is n ...
(a rectangular bone connected to the lower end of the scapula). It also had entheses
The enthesis (plural entheses) is the connective tissue which attaches tendons or ligaments to a bone.
There are two types of entheses: ''Fibrous tissue, fibrous entheses'' and ''fibrocartilaginous entheses''.
In a fibrous enthesis, the collage ...
(connective tissue) for various muscle attachments. The humerus
The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...
(upper arm bone) of AMNH 5214 was short, very broad and about long. The femur
The femur (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the knee. In many quadrupeds, four-legged animals the femur is the upper bone of the hindleg.
The Femo ...
(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
In tetrapods, cervical vertebrae (: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In saurop ...
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 ligament
A ligament is a type of fibrous connective tissue in the body that connects bones to other bones. It also connects flight feathers to bones, in dinosaurs and birds. All 30,000 species of amniotes (land animals with internal bones) have liga ...
s, which helped support the massive head. The dorsal
Dorsal (from Latin ''dorsum'' ‘back’) may refer to:
* Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism
* Dorsal, positioned on top of an aircraft's fuselage
The fus ...
vertebra
Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spina ...
e 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
Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...
(turned to bone) tendon
A tendon or sinew is a tough band of fibrous connective tissue, dense fibrous connective tissue that connects skeletal muscle, muscle to bone. It sends the mechanical forces of muscle contraction to the skeletal system, while withstanding tensi ...
s, 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
In biological taxonomy, the type genus (''genus typica'') is the genus which defines a biological family and the root of the family name.
Zoological nomenclature
According to the International Code of Zoological Nomenclature, "The name-bearin ...
of a new family
Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...
, 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
Order () is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and recognized ...
Stegosauria
Stegosauria is a group of Herbivore, herbivorous ornithischian dinosaurs that lived during the Jurassic and early Cretaceous Period (geology), periods. Stegosaurian fossils have been found mostly in the Northern Hemisphere (North America, Europe a ...
. In 1923 Osborn coined the name Ankylosauria
Ankylosauria is a group of herbivorous dinosaurs of the clade Ornithischia. It includes the great majority of dinosaurs with armor in the form of bony osteoderms, similar to turtles. Ankylosaurs were bulky quadrupeds, with short, powerful limbs ...
, thereby placing the ankylosaurids in their own suborder.
Ankylosauria and Stegosauria are now grouped together within the clade Thyreophora
Thyreophora ("shield bearers", often known simply as "armored dinosaurs") is a group of armored ornithischian dinosaurs that lived from the Early Jurassic until the end of the Cretaceous.
Thyreophorans are characterized by the presence of bod ...
. This group first appeared in the Sinemurian
In the geologic timescale, the Sinemurian is an age (geology), age and stage (stratigraphy), stage in the Early Jurassic, Early or Lower Jurassic epoch (geology), Epoch or series (stratigraphy), Series. It spans the time between 199.5 ±0.3 annu ...
age, and survived for 135 million years until disappearing in the Maastrichtian
The Maastrichtian ( ) is, in the International Commission on Stratigraphy (ICS) geologic timescale, the latest age (geology), age (uppermost stage (stratigraphy), stage) of the Late Cretaceous epoch (geology), Epoch or Upper Cretaceous series (s ...
. 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
In biological classification, a subfamily (Latin: ', plural ') is an auxiliary (intermediate) taxonomic rank, next below family but more inclusive than genus. Standard nomenclature rules end botanical subfamily names with "-oideae", and zo ...
Ankylosaurinae (members of which are called ankylosaurines) within Ankylosauridae. ''Ankylosaurus'' appears to be most closely related to ''Anodontosaurus'' and ''Euoplocephalus''. The following cladogram
A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...
is based on a 2015 phylogenetic analysis
In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical data ...
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 term tribe is used in many different contexts to refer to a category of human social group. The predominant worldwide use of the term in English is in the discipline of anthropology. The definition is contested, in part due to conflict ...
the authors named Ankylosaurini), Arbour and Currie suggested that earlier North American ankylosaurids had gone extinct by the late Albian
The Albian is both an age (geology), age of the geologic timescale and a stage (stratigraphy), stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early Cretaceous, Early/Lower Cretaceous epoch (geology), Epoch/s ...
or Cenomanian
The Cenomanian is, in the International Commission on Stratigraphy's (ICS) geological timescale, the oldest or earliest age (geology), age of the Late Cretaceous epoch (geology), Epoch or the lowest stage (stratigraphy), stage of the Upper Cretace ...
ages of the Middle Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
. Ankylosaurids thereafter recolonized North America from Asia during the Campanian
The Campanian is the fifth of six ages of the Late Cretaceous epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campa ...
or Turonian
The Turonian is, in the International Commission on Stratigraphy, ICS' geologic timescale, the second age (geology), age in the Late Cretaceous epoch (geology), Epoch, or a stage (stratigraphy), stage in the Upper Cretaceous series (stratigraphy), ...
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
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...
record of North American ankylosaurids between the ages.
Paleobiology
Feeding
Like other ornithischians, ''Ankylosaurus'' was herbivorous
A herbivore is an animal anatomically and physiologically evolved to feed on plants, especially upon vascular tissues such as foliage, fruits or seeds, as the main component of its diet. These more broadly also encompass animals that eat n ...
. 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
Fiber (spelled fibre in British English; from ) is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often inco ...
and woody plant
A woody plant is a plant that produces wood as its structural tissue and thus has a hard stem. In cold climates, woody plants further survive winter or dry season above ground, as opposed to Herbaceous plant, herbaceous plants that die back to t ...
s, they may have had a varied diet, including tough leaves and pulpy fruits. ''Ankylosaurus'' probably fed on abundant fern
The ferns (Polypodiopsida or Polypodiophyta) are a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular, i.e., having specialized tissue ...
s and low-growing shrub
A shrub or bush is a small to medium-sized perennial woody plant. Unlike herbaceous plants, shrubs have persistent woody stems above the ground. Shrubs can be either deciduous or evergreen. They are distinguished from trees by their multiple ...
s. Assuming it was endotherm
An endotherm (from Greek ἔνδον ''endon'' "within" and θέρμη ''thermē'' "heat") is an organism that maintains its body at a metabolically favorable temperature, largely by the use of heat released by its internal bodily functions inst ...
ic, ''Ankylosaurus'' would have eaten of ferns per day, similar to the amount of dry vegetation a large elephant
Elephants are the largest living land animals. Three living species are currently recognised: the African bush elephant ('' Loxodonta africana''), the African forest elephant (''L. cyclotis''), and the Asian elephant ('' Elephas maximus ...
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
Hindgut fermentation is a digestive process seen in monogastric herbivores (animals with a simple, single-chambered stomach). Cellulose is digested with the aid of symbiotic microbes including bacteria, archaea, and eukaryotes. The microbial fe ...
like in modern herbivorous lizards, which have several chambers in their enlarged colon.
In 1969, paleontologist Georg Haas concluded that despite the large size of ankylosaur 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 ankylosaurs ate relatively soft non-abrasive vegetation. Later research on ''Euoplocephalus'' indicates that forward and sideways jaw movement was possible in these animals, the skull being able to withstand considerable forces.[ A 2016 study of the dental occlusion (contact between the teeth) of ankylosaur specimens found that the ability for backwards (palinal) jaw movement evolved independently in different ankylosaur lineages, including Late Cretaceous North American ankylosaurids like ''Ankylosaurus'' and ''Euoplocephalus''.]
The retracted position of the nostrils of ''Ankylosaurus'' were compared to those of fossorial
A fossorial animal () is one that is adapted to digging and which lives primarily (but not solely) underground. Examples of fossorial vertebrates are Mole (animal), moles, badgers, naked mole-rats, meerkats, armadillos, wombats, and mole salamand ...
(digging) worm lizards and blind snakes by Arbour and Mallon in 2017, and though it was probably not a burrowing animal, the snout of ''Ankylosaurus'' may indicate earth-moving behavior. These factors, as well as the low rate of tooth formation in ankylosaurs compared to other ornithischians, indicate that ''Ankylosaurus'' may have been omnivorous
An omnivore () is an animal that regularly consumes significant quantities of both plant and animal matter. Obtaining energy and nutrients from plant and animal matter, omnivores digest carbohydrates, protein, fat, and fiber, and metabolize ...
(eating both plant and animal matter). It may also (or alternatively) have dug in the ground for roots and tuber
Tubers are a type of enlarged structure that plants use as storage organs for nutrients, derived from stems or roots. Tubers help plants perennate (survive winter or dry months), provide energy and nutrients, and are a means of asexual reproduc ...
s. A 2023 study by paleontologist Antonio Ballell and colleagues found that North American ankylosaurids from the latest Cretaceous (including ''Ankylosaurus'') had jaws with low mechanical advantage, whereas those of earlier relatives were high to moderate. These late ankylosaurids also had tooth occlusion and complex biphasal jaw mechanisms, features shared with some Late Cretaceous nodosaurids, but those instead have jaws with high mechanical advantage. This indicates that while the two groups converged in some features, the nodosaurs had higher relative bite force, which suggests diverging jaw mechanics and dietary partitioning between the two.
Airspaces and senses
In 1977, paleontologist Teresa Maryańska proposed that the complex sinuses and nasal cavities of ankylosaurs may have lightened the weight of the skull, housed a nasal gland
A gland is a Cell (biology), cell or an Organ (biology), organ in an animal's body that produces and secretes different substances that the organism needs, either into the bloodstream or into a body cavity or outer surface. A gland may also funct ...
, or acted as a chamber for vocal resonance.[ Carpenter rejected these hypotheses, arguing that ]tetrapod
A tetrapod (; from Ancient Greek :wiktionary:τετρα-#Ancient Greek, τετρα- ''(tetra-)'' 'four' and :wiktionary:πούς#Ancient Greek, πούς ''(poús)'' 'foot') is any four-Limb (anatomy), limbed vertebrate animal of the clade Tetr ...
animals make sounds through the larynx
The larynx (), commonly called the voice box, is an organ (anatomy), organ in the top of the neck involved in breathing, producing sound and protecting the trachea against food aspiration. The opening of larynx into pharynx known as the laryngeal ...
, not the nostrils, and that reduction in weight was minimal, as the spaces only accounted for a small percent of the skull volume. He also considered a gland unlikely and noted that the sinuses may not have had any specific function. It has also been suggested that the respiratory passages were used to perform a mammal-like treatment of inhaled air, based on the presence and arrangement of specialized bones.[
A 2011 study of the nasal passages of ''Euoplocephalus'' by paleontologist Tetsuto Miyashita and colleagues supported their function as a heat and water balancing system, noting the extensive blood vessel system and an increased surface area for the mucosa membrane (used for heat and water exchange in modern animals). The researchers also supported the idea of the loops acting as a resonance chamber, comparable to the elongated nasal passages of saiga antelope and the looping trachea of cranes and ]swan
Swans are birds of the genus ''Cygnus'' within the family Anatidae. The swans' closest relatives include the goose, geese and ducks. Swans are grouped with the closely related geese in the subfamily Anserinae where they form the tribe (biology) ...
s. Reconstructions of the inner ear suggest adaptation to hearing at low frequencies, such as the low-toned resonant sounds possibly produced by the nasal passages. They disputed the possibility that the looping is related to olfaction
The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste.
In humans, ...
(sense of smell) as the olfactory region is pushed to the sides of the main airway.
According to Carpenter, the shape of the nasal chambers 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, with air directed past the olfactory lobes. The enlarged olfactory region of ankylosaurids indicates a well-developed sense of smell. Though hindwards retraction of the nostrils is seen in aquatic animals and animals with a proboscis
A proboscis () is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate. In invertebrates, the term usually refers to tubular arthropod mouthparts, mouthparts used for feeding and sucking. In vertebrates, a pr ...
, it is unlikely either possibility applies to ''Ankylosaurus'', as the nostrils tend to be reduced or the premaxilla extended. In addition, though the widely separated nostrils may have allowed for stereo-olfaction (where each nostril senses smells from different directions), as has been proposed for the moose
The moose (: 'moose'; used in North America) or elk (: 'elk' or 'elks'; used in Eurasia) (''Alces alces'') is the world's tallest, largest and heaviest extant species of deer and the only species in the genus ''Alces''. It is also the tal ...
, little is known about this feature. The position of the orbits of ''Ankylosaurus'' suggest some stereoscopic vision
Binocular vision is seeing with two eyes, which increases the size of the visual field. If the visual fields of the two eyes overlap, binocular depth can be seen. This allows objects to be recognized more quickly, camouflage to be detected, spa ...
.
Limb movements
Reconstructions of ankylosaur 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 on the front feet, possibly for food gathering. In addition, Coombs suggested that ankylosaurs may have been capable diggers, though the hoof-like structure of the manus would have limited fossorial activity. Ankylosaurs were likely to have been slow-moving and sluggish animals, though they may have been capable of quick movements when necessary.
Growth
The squamosal horns of the largest ''Ankylosaurus'' specimen are blunter than those of the smallest specimen, which is also the case in ''Euoplocephalus'', and this may represent ontogenetic
Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the egg to adult. The term can also be used to refer to the stu ...
variation (related to growth development). Studies of specimens of ''Pinacosaurus'' of different ages found that during ontogenetic development, the ribs of juvenile ankylosaurs fused with their vertebrae. The forelimbs strongly increased in robustness while the hindlimbs did not become larger relative to the rest of the skeleton, further evidence that the arms bore most of the weight. In the cervical half-rings, the underlying bone band developed outgrowths connecting it with the underlying osteoderms, which simultaneously fused to each other. On the skull, the middle bone plates first ossified at the snout and the rear rim, with ossification gradually extending towards the middle regions. On the rest of the body, ossification progressed from the neck backward in the direction of the tail.
Defense
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
Collagen () is the main structural protein in the extracellular matrix of the connective tissues of many animals. It is the most abundant protein in mammals, making up 25% to 35% of protein content. Amino acids are bound together to form a trip ...
fibers. Structurally similar to Sharpey's fibres
Sharpey's fibres (bone fibres, or perforating fibres) are a matrix of connective tissue consisting of bundles of strong predominantly type I collagen fibres connecting periosteum to bone. They are part of the outer fibrous layer of periosteum ...
, they were embedded directly into the bone tissue, a feature unique to ankylosaurids. This would have provided the ankylosaurids with an armor covering that was both lightweight and highly durable, being resistant to breakage and penetration by the teeth of predators. The palpebral bones over the eyes may have provided additional protection for them.[ Carpenter suggested in 1982 that the heavily vascularized armor may also have had a role in ]thermoregulation
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature ...
as in modern crocodilian
Crocodilia () is an Order (biology), order of semiaquatic, predatory reptiles that are known as crocodilians. They first appeared during the Late Cretaceous and are the closest living relatives of birds. Crocodilians are a type of crocodylomorp ...
s.[
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 and were not very elastic, allowing great force to be transmitted to the club when it was used as a weapon.] Coombs suggested in 1979 that several 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
The metatarsal bones or metatarsus (: metatarsi) are a group of five long bones in the midfoot, located between the tarsal bones (which form the heel and the ankle) and the phalanges ( toes). Lacking individual names, the metatarsal bones are ...
of large theropods
Theropoda (; from ancient Greek , (''therion'') "wild beast"; , (''pous, podos'') "foot"">wiktionary:ποδός"> (''pous, podos'') "foot" is one of the three major groups (clades) of dinosaurs, alongside Ornithischia and Sauropodom ...
.
A 2009 study estimated that ankylosaurids could swing their tails at 100 degrees laterally, and the mainly cancellous clubs would have had a lowered moment of inertia
The moment of inertia, otherwise known as the mass moment of inertia, angular/rotational mass, second moment of mass, or most accurately, rotational inertia, of a rigid body is defined relatively to a rotational axis. It is the ratio between ...
and been effective weapons. The study also found that while adult ankylosaurid tail clubs were capable of breaking bones, those of juveniles 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.[ Other studies have found evidence of ankylosaurids using their tail clubs for intraspecific combat. One specimen of '']Tarchia
''Tarchia'' (meaning "brainy one") is a genus of herbivorous ankylosauridae, ankylosaurid dinosaur from the late Cretaceous of Mongolia.
Discovery and naming
In 1970, a Polish-Mongolian expedition discovered an ankylosaurian skull near Khulsan. ...
'' showed signs of injury on both the pelvic and tail area and the club was found to be asymmetrical, a sign of being worn down by the strikes.
In 1993, Tony Thulborn proposed that the tail club of ankylosaurids primarily acted as a decoy for the head, as he thought the tail too short and inflexible to have an effective reach; the "dummy head" would lure a predator close to the tail, where it could be struck. Carpenter has rejected this idea, as tail club shape is highly variable among ankylosaurids, even in the same genus.
Paleoenvironment
''Ankylosaurus'' existed between 68 and 66 million years ago, in the final, or Maastrichtian, stage of the Late Cretaceous
The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cre ...
Period. It was among the last dinosaur genera that appeared before the Cretaceous–Paleogene extinction event
The Cretaceous–Paleogene (K–Pg) extinction event, also known as the K–T extinction, was the extinction event, mass extinction of three-quarters of the plant and animal species on Earth approximately 66 million years ago. The event cau ...
. The type specimen is from the Hell Creek Formation of Montana, while other specimens have been found in the Lance and Ferris Formations in Wyoming, the Scollard Formation in Alberta, and the Frenchman Formation in Saskatchewan, all of which date to the end of the Cretaceous.
Fossils of ''Ankylosaurus'' are rare in the sediments
Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...
it is known from, and the distribution of its remains suggests that it was ecologically rare, or restricted to the uplands of the formations, where it would have been less likely to fossilize, rather than the coastal lowlands. Another ankylosaur, a nodosaur referred to as ''Edmontonia'' sp., is also found in the same formations, but according to Carpenter, 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 the nodosaur appears to have inhabited the lowlands. The narrower muzzle of the nodosaur suggests it had a more selective diet than ''Ankylosaurus'', further indicating ecological separation, whether their range overlapped or not.
With its low center of gravity, ''Ankylosaurus'' would have been unable to knock down trees like modern elephants do. It was also incapable of chewing bark and thus unlikely to have practiced bark stripping. As an adult, ''Ankylosaurus'' does not appear to have congregated in groups (though some ankylosaurs appear to have congregated when young). It is therefore improbable that ''Ankylosaurus'' was able to modify the landscape of its ecosystem in the way elephants do; hadrosaurid
Hadrosaurids (), also hadrosaurs or duck-billed dinosaurs, are members of the ornithischian family Hadrosauridae. This group is known as the duck-billed dinosaurs for the flat duck-bill appearance of the bones in their snouts. The ornithopod fami ...
s may instead have had such an "ecosystem engineer
An ecosystem engineer is any species that creates, significantly modifies, maintains or destroys a habitat. These organisms can have a large impact on species richness and landscape-level heterogeneity of an area. As a result, ecosystem engine ...
" role.
The formations where ''Ankylosaurus'' fossils have been found represent different sections of the western shore of the Western Interior Seaway
The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, or the Western Interior Sea) was a large inland sea (geology), inland sea that existed roughly over the present-day Great Plains of ...
dividing western and eastern North America during the Cretaceous, a broad coastal plain extending westward from the seaway to the newly formed Rocky Mountains
The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in great-circle distance, straight-line distance from the northernmost part of Western Can ...
. These formations are composed largely of sandstone
Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...
and mudstone
Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...
, which have been attributed to floodplain
A floodplain or flood plain or bottomlands is an area of land adjacent to a river. Floodplains stretch from the banks of a river channel to the base of the enclosing valley, and experience flooding during periods of high Discharge (hydrolog ...
environments. The regions where ''Ankylosaurus'' and other Late Cretaceous ankylosaurs have been found had a warm subtropical
The subtropical zones or subtropics are geographical zone, geographical and Köppen climate classification, climate zones immediately to the Northern Hemisphere, north and Southern Hemisphere, south of the tropics. Geographically part of the Ge ...
/temperate climate
In geography, the temperate climates of Earth occur in the middle latitudes (approximately 23.5° to 66.5° N/S of the Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ra ...
, which was monsoonal, had occasional rainfall, tropical storms
A tropical cyclone is a rapidly rotating storm system with a low-pressure area, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depending on its locat ...
, and forest fires A forest fire
A wildfire, forest fire, or a bushfire is an unplanned and uncontrolled fire in an area of combustible vegetation. Depending on the type of vegetation present, a wildfire may be more specifically identified as a bushfire ( in Au ...
. In the Hell Creek Formation, many types of plants were supported, primarily angiosperm
Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed within a fruit ...
s, with less common conifer
Conifers () are a group of conifer cone, cone-bearing Spermatophyte, seed plants, a subset of gymnosperms. Scientifically, they make up the phylum, division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a sin ...
s, ferns and cycad
Cycads are seed plants that typically have a stout and woody (ligneous) trunk (botany), trunk with a crown (botany), crown of large, hard, stiff, evergreen and (usually) pinnate leaves. The species are dioecious, that is, individual plants o ...
s. 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 other dinosaurs that included the ceratopsids ''Triceratops
''Triceratops'' ( ; ) is a genus of Chasmosaurinae, chasmosaurine Ceratopsia, ceratopsian dinosaur that lived during the late Maastrichtian age of the Late Cretaceous Period (geology), period, about 68 to 66 million years ago on the island ...
'' and ''Torosaurus
''Torosaurus'' (meaning "perforated lizard", in reference to the large openings in its frill) is a genus of herbivorous Chasmosaurinae, chasmosaurine Ceratopsia, ceratopsian dinosaur that lived during the late Maastrichtian age of the Late Cret ...
'', the hypsilophodont ''Thescelosaurus
''Thescelosaurus'' ( ) is a genus of Ornithischia, ornithischian dinosaur that lived during the Late Cretaceous period (geology), period in western North America. It was named and described in 1913 by the Paleontology, paleontologist Charles W. G ...
'', the hadrosaurid ''Edmontosaurus
''Edmontosaurus'' ( ) (meaning "lizard from Edmonton"), with the second species often colloquially and historically known as ''Anatosaurus'' or ''Anatotitan'' (meaning "duck lizard" and "giant duck"), is a genus of hadrosaurid (duck-billed) din ...
'', an indeterminate nodosaur, the pachycephalosaurian ''Pachycephalosaurus
''Pachycephalosaurus'' (; meaning "thick-headed lizard", from Greek ''pachys-/'' "thickness", ''kephalon/'' "head" and ''sauros/'' "lizard") is a genus of pachycephalosaurid ornithischian dinosaur. The type species, ''P. wyomingensis'', ...
'', and the theropods '' Struthiomimus'', '' Ornithomimus'', '' Pectinodon'', and ''Tyrannosaurus''.[
]
Cultural significance
Carpenter noted in 2004 that ''Ankylosaurus'' has become the archetypal
The concept of an archetype ( ) appears in areas relating to behavior, History of psychology#Emergence of German experimental psychology, historical psychology, philosophy and literary analysis.
An archetype can be any of the following:
# a stat ...
member of its group, and the best-known ankylosaur in popular culture
Popular culture (also called pop culture or mass culture) is generally recognized by members of a society as a set of cultural practice, practices, beliefs, artistic output (also known as popular art f. pop art
F is the sixth letter of the Latin alphabet.
F may also refer to:
Science and technology Mathematics
* F or f, the number 15 (number), 15 in hexadecimal and higher positional systems
* ''p'F'q'', the hypergeometric function
* F-distributi ...
or mass art, sometimes contraste ...
, perhaps due to a life-sized reconstruction of the animal being featured at the 1964 World's Fair in New York City. Arbour and Mallon called ''Ankylosaurus'' an "iconic" dinosaur in 2017, and noted that the World's Fair 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.
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. ''Ankylosaurus'' has been featured in the ''Jurassic Park
''Jurassic Park'', later referred to as ''Jurassic World'', is an American science fiction media franchise created by Michael Crichton, centered on a disastrous attempt to create a theme park of De-extinction#Cloning, cloned dinosaurs. It bega ...
'' franchise, where they are depicted as attacking with their tails and running, abilities that have been criticized as unlikely by paleontologists.
See also
* Timeline of ankylosaur research
This timeline of ankylosaur research is a chronological listing of events in the History of paleontology, history of paleontology focused on the ankylosaurs, quadrupedal herbivorous dinosaurs who were protected by a covering bony plates and spik ...
References
Notes
Citations
External links
{{authority control
Ankylosaurinae
Dinosaur genera
Maastrichtian dinosaurs
Hell Creek Formation
Lance Formation
Scollard Formation
Frenchman Formation
Dinosaurs of Canada
Fossil taxa described in 1908
Taxa named by Barnum Brown
Dinosaurs of the United States