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Ankylosauridae () is a family of armored dinosaurs within Ankylosauria, and is the sister group to Nodosauridae. The oldest known ankylosaurids date to around 122 million years ago and went extinct 66 million years ago during the Cretaceous–Paleogene extinction event. These animals were mainly herbivorous and were obligate quadrupeds, with leaf-shaped teeth and robust, scute-covered bodies. Ankylosaurids possess a distinctly domed and short snout, wedge-shaped on their skull, scutes along their torso, and a tail club.
Ankylosauridae is exclusively known from the Northern Hemisphere, with specimens found in North America, Europe, and Asia. The first discoveries within this family were of the genus Ankylosaurus, by Peter Kaiser and Barnum Brown in Montana in 1906. Brown went on to name Ankylosauridae and the subfamily Ankylosaurinae in 1908.
The most distinguishing feature of ankylosaurids is the presence of a tail club. It is made out of modified interlocking distal caudal 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 Ghostbusters monster, is also the first ankylosaur from the Judith River Formation.
It has previously been interpreted that variation in ankylosaurid tail club shape is due to sexual dimorphism, which assumes that tail club morphology has a sex-linked intraspecific 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.
Later in 1998, Paul Sereno formally defined Ankylosauridae as all ankylosaurs more closely related to Ankylosaurus than to Panoplosaurus. Ankylosaurs not known to possess a tail club were included in Kenneth Carpenter's 2001 phylogeny of Ankylosauridae. Daniel Madzia and colleagues in 2021 formally defined Ankylosauridae in the PhyloCode as "the largest clade containing Ankylosaurus magniventris, but not Nodosaurus textilis". The basal subfamily Shamosaurinae is formally defined as "the largest clade containing Gobisaurus domoculus and Shamosaurus scutatus, but not Ankylosaurus magniventris". This definition ensures that Shamosaurinae is only valid when both Gobisaurus and Shamosaurus form a clade to the exclusion of Ankylosaurus.
In a study done in 2004 by Vickaryous et al., Gargoyleosaurus, Gastonia, and Minmi were recorded as basal ankylosaurids, with the rest of the ankylosaurids filled out with Gobisaurus, Shamosaurus, and Ankylosaurinae from China, Mongolia, and North America. (2004). 9780520941434, University of California Press. ISBN 9780520941434
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 and Liaoningosaurus as basal ankylosaurids.
In 2016, Arbour and Currie have presented a phylogenetic analysis of Ankylosauridae consisting of Gastonia, Cedarpelta, Chuanqilong, other basal ankylosaurids, and a number of derived ankylosaurids. Their phylogeny includes some uncertain phylogenetic relationships, between Ankylosaurus, Anodontosaurus, Scolosaurus, and Ziapelta.
Restoration of Euoplocephalus 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 Carpal bones and Metacarpal bones bear resemblance to those of 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.
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 Allometry.
Non-herbivorous habits have been implicated for some species, however. Pinacosaurus 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. Either would be exceptional evidence of carnivory among . 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 Sexual selection interpretation.
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