Ankylosauridae

Dyoplosaurus tail reconstruction, showing terms used for parts of ankylosaurid tails

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.[2] 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.[4] 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.[5] The “handle” of the tail club involves the vertebrae, and requires elongated prezygapophyses to overlap at least half of the preceding vertebral centrum length.[5] 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.[5] 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.[6]

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.[7]

Skull of first known ankylosaurid specimen, belonging to Ankylosaurus

Barnum Brown and Peter Kaisen discovered the first ankylosaurid genus, Ankylosaurus, in 1906 in the Hell Creek Beds in Montana.[3] The fossil material they found was a portion of the skull, two teeth, some vertebrae, a distorted scapula, ribs and more than thirty osteoderms.[3] 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.[3]

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.[6] 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.[6][8]

Club of indeterminate ankylosaurine, CMN 349

In 1978, W.P. Coombs, Jr. classified almost all valid species of Ankylosauria within either Nodosauridae or Ankylosauridae.[9] 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.[10] Ankylosaurs not known to possess a tail club were included in Kenneth Carpenter's 2001 phylogeny of Ankylosauridae.[11]

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.[12]

In 2012, Thompson et al. undertook an analysis of almost all known valid ankylosaurs and outgroup taxa at the time.[13] 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.[13]

Gastonia and other polacanthine dinosaurs have uncertain placement, possibly within Ankylosauridae

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.[14]

• Timeline of ankylosaur research

• Dinosaurs and other Prehistoric Creatures, edited by Ingrid Cranfield (2000), Salamander books, pg. 250–257.
• Carpenter K (2001). "Phylogenetic analysis of the Ankylosauria". In Carpenter, Kenneth (ed.). The Armored Dinosaurs. Indiana University Press. pp. 455–484. ISBN 978-0-253-33964-5.
• 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

• Family Tree
• Ankylosauridae Tree of Life web project
• Pseudoplocephalus, Blog by an evolutionary biologist and vertebrate palaeontologist specializing in ankylosaurs