Ankylosauridae () is a family of armored dinosaurs within

Ankylosauria Ankylosauria is a group of herbivorous dinosaurs of the order 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 ...

, and is the sister group to

Nodosauridae Nodosauridae is a family of ankylosaurian dinosaurs, from the Late Jurassic to the Late Cretaceous period in what is now North America, South America, Europe, and Asia. Description Nodosaurids, like their close relatives the ankylosaurids, w ...

. The oldest known Ankylosaurids date to around 122 million years ago and went extinct 66 million years ago during the

Cretaceous–Paleogene extinction event The Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary extinction) was a sudden extinction event, mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million y ...

. These animals were mainly herbivorous and were obligate quadrupeds, with leaf-shaped teeth and robust,

scute A scute or scutum (Latin: ''scutum''; plural: ''scuta'' "shield") is a bony external plate or scale overlaid with horn, as on the shell of a turtle, the skin of crocodilians, and the feet of birds. The term is also used to describe the anterior po ...

-covered bodies. Ankylosaurids possess a distinctly domed and short snout, wedge-shaped

osteoderm Osteoderms are bony deposits forming scales, plates, or other structures based in the dermis. Osteoderms are found in many groups of extant and extinct reptiles and amphibians, including lizards, crocodilians, frogs, temnospondyls (extinc ...

s on their skull, scutes along their torso, and a tail club. Ankylosauridae is exclusively known from the northern hemisphere, with specimens found in western North America, Europe, and East Asia. The first discoveries within this family were of the genus '' Ankylosaurus'', by Peter Kaiser and

Barnum Brown Barnum Brown (February 12, 1873 – February 5, 1963), commonly referred to as Mr. Bones, was an American paleontologist. Named after the circus showman P. T. Barnum, he discovered the first documented remains of ''Tyrannosaurus'' during a career ...

in Montana in 1906. Brown went on to name Ankylosauridae and the subfamily Ankylosaurinae in 1908.

Anatomy

Ankylosaurids are stout, solidly built, armoured dinosaurs. They possess accessory ossifications on cranial bones that cover some skull openings and form wedge-shaped, horn-like structures. Along the ankylosaurid torso are scute rows, which are filled in with smaller ossicles to create a fused shield of armour. Only two collars of armour plates can be found on the neck, as opposed to the sister group, nodosaurids, which have three. Nodosauridae and Ankylosauridae also share the unique attribute of abundant structural fibres in both primary and secondary bone. Ankylosaurids also have an S-shaped narial passage. The most distinguishing feature of ankylosaurids is the presence of a tail club. It is made out of modified interlocking distal caudal

vertebra The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characterist ...

e and enlarged, bulbous osteoderms. The “handle” of the tail club involves the vertebrae, and requires elongated prezygapophyses to overlap at least half of the preceding vertebral centrum length. These distal caudal vertebrae also lack transverse processes and neural spines, and therefore tend to be longer than they are wide; the reverse is true for proximal caudal vertebrae. Derived ankylosaurids possess a fusion of posterior dorsal, sacral, and sometimes anterior caudal vertebrae, which forms a singular structure called a “synsacrum complex”. There is a complete fusion between centra, neural arches, zygapophyses, and sometimes neural spines. In 2017, Victoria M. Arbour and David C. Evans described a new genus of ankylosaurine that preserved extensive soft tissues along the body. This animal, named '' Zuul'' after its resemblance to the ''

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'' monster, is also the first ankylosaur from the Judith River Formation.

History of study

Barnum Brown and Peter Kaisen discovered the first ankylosaurid genus, '' Ankylosaurus'', in 1906 in the Hell Creek Beds in Montana. The fossil material they found was a portion of the skull, two teeth, some vertebrae, a distorted scapula, ribs and more than thirty osteoderms. Reconstruction of the specimen was initially met with skepticism by those who believed it to be at least very close to, or completely a part of the genus ''Stegopelta'', and Brown himself placed it within the suborder Stegosauria. It has previously been interpreted that variation in ankylosaurid tail club shape is due to

sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most an ...

, which assumes that tail club morphology has a sex-linked

intraspecific Biological specificity is the tendency of a characteristic such as a behavior or a biochemical variation to occur in a particular species. Biochemist Linus Pauling stated that "Biological specificity is the set of characteristics of living organi ...

function. This is possible if the tail club was used for agonistic behaviour. However, a sexual dimorphism theory would predict roughly equal numbers of individuals with two distinct sizes of tail clubs. Obvious sexual dimorphism has not been documented, but if the clubs of one sex are much larger, then there would be a bias for preservation and discovery towards that sex.

Phylogeny

In 1978, W.P. Coombs, Jr. classified almost all valid species of Ankylosauria within either Nodosauridae or Ankylosauridae. This was a pivotal study and described many characters of ankylosaurs in the earliest phylogenetic analyses of the group. Later in 1998,

Paul Sereno Paul Callistus Sereno (born October 11, 1957) is a professor of paleontology at the University of Chicago and a National Geographic "explorer-in-residence" who has discovered several new dinosaur species on several continents, including at sites ...

formally defined Ankylosauridae as all ankylosaurs more closely related to '' Ankylosaurus'' than to '' Panaplosaurus''. Ankylosaurs not known to possess a tail club were included in Kenneth Carpenter's 2001 phylogeny of Ankylosauridae. In a study done in 2004 by Vickaryous et al., '' Gargoyleosaurus'', '' Gastonia,'' and ''

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'' were recorded as basal ankylosaurids, with the rest of the ankylosaurids filled out with '' Gobisaurus'', '' Shamosaurus'', and

ankylosaurines Ankylosaurinae is a subfamily of ankylosaurid dinosaurs, existing from the Early Cretaceous about 105 million years ago until the end of the Late Cretaceous, about 66 mya. Many genera are included in the clade, such as '' Ankylosaurus'', '' Pin ...

from China, Mongolia, and North America. In 2012, Thompson et al. undertook an analysis of almost all known valid ankylosaurs and outgroup taxa at the time. They based their resulting phylogeny on characters representing cranial, post-cranial, and osteodermal anatomy, and details of synapomorphies for each recovered clade. This study placed '' Gargoyleosaurus'' and '' Gastonia'' within basal Nodosauridae, and put ''

Cedarpelta ''Cedarpelta'' is a extinct genus of basal ankylosaurid dinosaur from Utah that lived during the Late Cretaceous period (Cenomanian to lower Turonian stage, 98.2 to 93 Ma) in what is now the Mussentuchit Member of the Cedar Mountain Formation. Th ...

'' and '' Liaoningosaurus'' as basal ankylosaurids. Gastonia_fossil

Most recently, Arbour and Currie have presented a phylogenetic analysis of Ankylosauridae consisting of '' Gastonia'', ''

'', '' Chuanqilong'', other basal ankylosaurids, and a number of derived ankylosaurids. Their phylogeny includes some uncertain phylogenetic relationships, between '' Ankylosaurus'', '' Anodontosaurus'', ''

Scolosaurus ''Scolosaurus'' is an extinct genus of ankylosaurid dinosaurs within the subfamily Ankylosaurinae. It is known from the lower levels of the Dinosaur Park Formation and upper levels of the Oldman Formation in the Late Cretaceous (latest middle Cam ...

'', and ''

Ziapelta ''Ziapelta'' is an extinct genus of ankylosaurid. Its fossils have been found in the Hunter Wash and De-na-zin members of the Kirtland Formation of Upper Cretaceous (Campanian) New Mexico. It was named in 2014, in a research paper led by anky ...

''.

Paleobiology

Posture and locomotion

Ankylosaurids were likely very slow-moving animals. In all Ankylosauria, the

fibula The fibula or calf bone is a human leg, leg bone on the Lateral (anatomy), lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long ...

is more slender than the

tibia The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it connects ...

, suggesting that the tibia carried most of the weight of the animal, while the fibula served as an area of muscular attachment. Hindlimb muscles of ''

Euoplocephalus ''Euoplocephalus'' ( ) is a genus of very large, herbivorous ankylosaurid dinosaurs, living during the Late Cretaceous of Canada. It has only one named species, ''Euoplocephalus tutus''. The first fossil of ''Euoplocephalus'' was found in 1897 ...

'' have been restored and the placement of several muscles inserting on the femur have very short

moment arm In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of t ...

s. Muscles inserting on the tibia and fibula have longer moment arms. This pattern of retractor muscles points to an elephantine locomotion, consistent with columnar posture. Restoration of ''

'' forelimbs demonstrate similarities to crocodilian forelimb musculature. The most well developed muscles in the pectoral region had more of a weight-bearing function than a rotational one. It has also been postulated that the carpals and

metacarpals In human anatomy, the metacarpal bones or metacarpus form the intermediate part of the skeletal hand located between the phalanges of the fingers and the carpal bones of the wrist, which forms the connection to the forearm. The metacarpal bones ar ...

bear resemblance to those of

tetrapod Tetrapods (; ) are four-limb (anatomy), limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant taxon, extant and extinct amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids (p ...

s with fossorial (burrowing) habits. Several muscles in the posterior of ankylosaurids (dorsalis caudae, ilio-caudalis, coccygeo-femoralis brevis, coccygeo-femoralis longus, ilio-tibialis, and ischio caudalis) were used for motion of the tail and tail club. Ankylosaurids tend to have horizontal rather than an obliquely vertical orientation of zygapophyseal articulations in the free caudal vertebrae of the tail. This arrangement is most effective for side-to-side rather than vertical mobility. The absence of musculature to elevate the tail, and this orientation of zygapophyses suggest that the tail and its club swept parallel to and slightly above the ground.

Biogeography

It is difficult to establish the geographical origin of Ankylosauridae at present. There is a mix of basal ankylosaurids from both North America and Asia, which carries on through accepted cladistic analyses. It appears that in the mid-

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

, Asian nodosaurids were replaced by ankylosaurine ankylosaurids. Some researchers postulate that Ankylosaurines migrated into North America from Asia between the

Albian The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous Epoch/ Series. Its approximate time range is 113.0 ± 1.0 Ma to 100.5 ± ...

and

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. Campani ...

, where they diversified into a clade of ankylosaurines characterized by arched snouts and flat cranial bone plates (caputegulae). There is no evidence for ankylosaurids in

Gondwana Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final st ...

Variation

Within Ankylosauridae there is much individual and interspecific variation in expression of armour. However, the most researched aspect of ankylosaurid armour is the tail club. There has been substantial

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 s ...

and individual variability found in the morphology of this feature. There have been at least 16 caudal vertebrae included in the handle of the tail club of ''

Pinacosaurus grangeri ''Pinacosaurus'' (meaning "Plank lizard") is a genus of ankylosaurid thyreophoran dinosaur that lived in Asia during the Late Cretaceous (Santonian-Campanian, roughly 75 million to 71 million years ago), mainly in Mongolia and China. The first ...

'', and ''

'' has an estimated 9 – 11 coossified caudals. Variations in tail knob shape, thickness, and length are attributed to individual variation, taxonomy, or representation of different growth phases. There are difficulties with this last aspect, however, in that known clubs do not conform to a single growth series, yet some differences must be ontogenetic and allometric.

Lifestyle

Most ankylosaurid teeth were leaf-shaped, implying a mainly herbivorous diet. Their teeth could be smooth or fluted, or may differ on labial and lingual surfaces. '' Euoplocephalus tutus'' possess ridges and grooves on their teeth that have no relation to their marginal cusps. With their downward-facing neck and head, it is plausible for ankylosaurids to feed in a grazing pattern. Non-herbivorous habits have been implicated for some species, however. ''

Pinacosaurus ''Pinacosaurus'' (meaning "Plank lizard") is a genus of ankylosaurid thyreophoran dinosaur that lived in Asia during the Late Cretaceous (Santonian-Campanian, roughly 75 million to 71 million years ago), mainly in Mongolia and China. The first r ...

'' has been speculated as being an ant-eater-like long tongued insectivore,Hill, R., D’Emic, M., Bever, G., Norell, M. 2015. A complex hyobranchial apparatus in a Cretaceous dinosaur and the antiquity of avian paraglossalia. Zoological Journal of the Linnean Society. doi: 10.1111/zoj.12293 while '' Liaoningosaurus'' has been proposed to be a

piscivore A piscivore () is a carnivorous animal that eats primarily fish. The name ''piscivore'' is derived . Piscivore is equivalent to the Greek-derived word ichthyophage, both of which mean "fish eater". Fish were the diet of early tetrapod evoluti ...

. Either would be exceptional evidence of carnivory among

ornithischia Ornithischia () is an extinct order 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 Greek ...

ns.

There are a few prevailing theories for ankylosaurid tail club function. The first is agonistic behaviour within a species. In most vertebrates, including dinosaurs, this behaviour is accompanied by structures for display or combat. Some researchers believe this phenomenon would have been implausible considering there is no modern tetrapod analogue that uses the tail for this purpose. These paleontologists instead propose that ankylosaurids made use of their broad, flat skull for head-butting between individuals. The second theory for tail club function is for defense against predators. It has been postulated that the club would be most effective against the metatarsals of an attacking theropod. The bones that form cranial ornamentation have physiological costs, and so would be inefficient to produce merely for protection against predation. The theory has therefore been posed that these wedge-shaped osteoderms could support a partly

sexually selected Sexual selection is a mode of natural selection in which members of one biological sex choose mates of the other sex to mate with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex (in ...

interpretation.

Timeline of genera

ImageSize = width:800px height:auto barincrement:15px PlotArea = left:10px bottom:50px top:10px right:10px Period = from:-201 till:-45 TimeAxis = orientation:horizontal ScaleMajor = unit:year increment:10 start:-200 ScaleMinor = unit:year increment:1 start:-201 TimeAxis = orientation:hor AlignBars = justify Colors = #legends id:CAR value:claret id:ANK value:rgb(0.4,0.3,0.196) id:HER value:teal id:HAD value:green id:OMN value:blue id:black value:black id:white value:white id:jurassic value:rgb(0.2,0.7,0.79) id:earlyjurassic value:rgb(0,0.69,0.89) id:middlejurassic value:rgb(0.52,0.81,0.91) id:latejurassic value:rgb(0.74,0.89,0.97) id:cretaceous value:rgb(0.5,0.78,0.31) id:earlycretaceous value:rgb(0.63,0.78,0.65) id:latecretaceous value:rgb(0.74,0.82,0.37) BarData= bar:eratop bar:space bar:periodtop bar:space bar:NAM1 bar:NAM2 bar:NAM3 bar:NAM4 bar:NAM5 bar:NAM6 bar:NAM7 bar:NAM8 bar:NAM9 bar:NAM10 bar:NAM11 bar:NAM12 bar:NAM13 bar:NAM14 bar:NAM15 bar:NAM16 bar:NAM17 bar:NAM18 bar:NAM19 bar:NAM20 bar:NAM21 bar:NAM22 bar:NAM23 bar:NAM24 bar:space bar:period bar:space bar:era PlotData= align:center textcolor:black fontsize:M mark:(line,black) width:25 shift:(7,-4) bar:periodtop from: -201 till: -174 color:earlyjurassic text: Early from: -174 till: -163 color:middlejurassic text:

Middle Middle or The Middle may refer to: * Centre (geometry), the point equally distant from the outer limits. Places * Middle (sheading), a subdivision of the Isle of Man * Middle Bay (disambiguation) * Middle Brook (disambiguation) * Middle Creek (d ...

from: -163 till: -145 color:latejurassic text: Late from: -145 till: -100 color:earlycretaceous text: Early from: -100 till: -66 color:latecretaceous text: Late bar:eratop from: -201 till: -145 color:jurassic text:

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...

from: -145 till: -66 color:cretaceous text:

PlotData= align:left fontsize:M mark:(line,white) width:5 anchor:till align:left color:cretaceous bar:NAM1 from:-122 till:-120 text: Liaoningosaurus color:cretaceous bar:NAM2 from:-119 till:-113 text:

color:cretaceous bar:NAM3 from:-113 till:-110 text: Shamosaurus color:cretaceous bar:NAM4 from:-108.5 till:-108 text:

color:cretaceous bar:NAM5 from:-105 till:-100 text: Zhongyuansaurus color:cretaceous bar:NAM6 from:-98 till:-83 text:

Talarurus ''Talarurus'' ( ; meaning "basket tail" or "wicker tail") is a genus of ankylosaurid dinosaur that lived in Asia during the Late Cretaceous period, about 96 million to 89 million years ago. The first remains of ''Talarurus'' were discovered in ...

color:cretaceous bar:NAM7 from:-98 till:-83 text:

Tsagantegia ''Tsagantegia'' (; meaning Tsagan Teg) is a genus of medium-sized ankylosaurid thyreophoran dinosaur that lived in Asia during the Late Cretaceous period. The genus is monotypic, including only the type species, ''T. longicranialis''. The specime ...

color:cretaceous bar:NAM8 from:-92 till:-90 text: Gobisaurus color:cretaceous bar:NAM9 from:-80 till:-79 text: Minotaurasaurus color:cretaceous bar:NAM10 from:-80 till:-75 text:

color:cretaceous bar:NAM11 from:-76.5 till:-76 text:

color:cretaceous bar:NAM12 from:-76.5 till:-76 text: Dyoplosaurus color:cretaceous bar:NAM13 from:-76.4 till:-75.5 text:

color:cretaceous bar:NAM14 from:-76.2 till:-75.2 text: Zuul color:cretaceous bar:NAM15 from:-76 till:-75 text:

Nodocephalosaurus ''Nodocephalosaurus'' (meaning "knob headed lizard") is a monospecific genus of ankylosaurid dinosaur from New Mexico that lived during the Late Cretaceous (late Campanian to early Maastrichtian stage, 73.49 to 73.04 Ma) in what is now the De-na- ...

color:cretaceous bar:NAM16 from:-76 till:-74 text:

Ahshislepelta ''Ahshislepelta'' (meaning "Ah-shi-sle-pah Wash shield") is a monospecific genus of ankylosaur dinosaur from New Mexico that lived during the Late Cretaceous (late Campanian stage, 74.5 Ma) in what is now the Hunter Wash Member of the Kirtland ...

color:cretaceous bar:NAM17 from:-75 till:-74 text:

Aletopelta ''Aletopelta'' (; meaning 'wanderer shield') is a monospecific genus of basal ankylosaurid dinosaur from Southern California that lived during the Late Cretaceous (upper Campanian stage, 75.5 Ma) in what is now the Point Loma Formation. The ty ...

color:cretaceous bar:NAM18 from:-75 till:-74 text: Tianzhenosaurus color:cretaceous bar:NAM19 from:-75 till:-74 text: Shanxia color:cretaceous bar:NAM20 from:-74 till:-73 text: Oohkotokia color:cretaceous bar:NAM21 from:-72.8 till:-67 text: Anodontosaurus color:cretaceous bar:NAM22 from:-70 till:-69 text: Saichania color:cretaceous bar:NAM23 from:-70 till:-69 text: Tarchia color:cretaceous bar:NAM24 from:-66.5 till:-66 text: Ankylosaurus PlotData= align:center textcolor:black fontsize:M mark:(line,black) width:25 bar:period from: -201 till: -174 color:earlyjurassic text: Early from: -174 till: -163 color:middlejurassic text:

from: -163 till: -145 color:latejurassic text: Late from: -145 till: -100 color:earlycretaceous text: Early from: -100 till: -66 color:latecretaceous text: Late bar:era from: -201 till: -145 color:jurassic text:

from: -145 till: -66 color:cretaceous text:

References

*''

Dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

s and other Prehistoric Creatures'', edited by Ingrid Cranfield (2000), Salamander books, pg. 250-257. * * Kirkland, J. I. (1996). Biogeography of western North America's mid-Cretaceous faunas - losing European ties and the first great Asian-North American interchange. J. Vertebr. Paleontol. 16 (Suppl. to 3): 45A

External links

Family TreeAnkylosauridae
Tree of Life web project
Pseudoplocephalus
Blog by an evolutionary biologist and vertebrate palaeontologist specializing in ankylosaurs {{Taxonbar, from=Q517099 Aptian first appearances Taxa named by Barnum Brown Prehistoric dinosaur families