Ankylosauridae
| Ankylosaurids | |
|---|---|
 | |
| Mounted skeleton of Scolosaurus thronus, Royal Tyrrell Museum of Palaeontology | |
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Thyreophora |
| Clade: | †Ankylosauria |
| Clade: | †Euankylosauria |
| Family: | †Ankylosauridae Brown, 1908 |
| Type species | |
| †Ankylosaurus magniventris Brown, 1908 | |
| Subgroups | |
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| Synonyms | |
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Ankylosauridae (/ˌæŋkɪloʊˈsɔːrɪdiː/) 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 the clade went extinct 66 million years ago during the Cretaceous–Paleogene extinction event.[1] 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 osteoderms on their skull, scutes along their torso, and a tail club.[3]
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.[4] Brown went on to name Ankylosauridae and the subfamily Ankylosaurinae in 1908.
Anatomy
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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.[3] Only two collars of armour plates can be found on the neck, as opposed to the sister group, nodosaurids, which have three.[1] Nodosauridae and Ankylosauridae also share the unique attribute of abundant structural fibres in both primary and secondary bone.[5] Ankylosaurids also have an S-shaped narial passage.[1]
The most distinguishing feature of ankylosaurids is the presence of a tail club. It is made out of modified interlocking distal caudal vertebrae and enlarged, bulbous osteoderms.[6] The "handle" of the tail club involves the vertebrae, and requires elongated prezygapophyses to overlap at least half of the preceding vertebral centrum length.[6] 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.[6] 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.[7]
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.[8]
History of study
[edit]
Barnum Brown and Peter Kaisen discovered the first ankylosaurid genus, Ankylosaurus, in 1906 in the Hell Creek Beds in Montana.[4] The fossil material they found was a portion of the skull, two teeth, some vertebrae, a distorted scapula, ribs and more than thirty osteoderms.[4] 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.[4]
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.[7] 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.[7][9]
Phylogeny
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In 1978, W.P. Coombs, Jr. classified almost all valid species of Ankylosauria within either Nodosauridae or Ankylosauridae.[10] This was a pivotal study and described many characters of ankylosaurs in the earliest phylogenetic analyses of the group.
Later in 1998, Paul Sereno formally defined Ankylosauridae as all ankylosaurs more closely related to Ankylosaurus than to Panoplosaurus.[11] Ankylosaurs not known to possess a tail club were included in Kenneth Carpenter's 2001 phylogeny of Ankylosauridae.[12] Daniel Madzia and colleagues in 2021 formally defined Ankylosauridae in the PhyloCode as "the largest clade containing Ankylosaurus magniventris, but not Nodosaurus textilis".[13] The basal subfamily Shamosaurinae is formally defined as "the largest clade containing Gobisaurus domoculus and Shamosaurus scutatus, but not Ankylosaurus magniventris".[13] This definition ensures that Shamosaurinae is only valid when both Gobisaurus and Shamosaurus form a clade to the exclusion of Ankylosaurus.[13]
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 ankylosaurines from China, Mongolia, and North America.[14]
In 2012, Thompson et al. undertook an analysis of almost all known valid ankylosaurs and outgroup taxa at the time.[15] 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.[15]
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