Ankylosauridae

Ankylosaurids Temporal range: Early Cretaceous - Late Cretaceous, 122–66 Ma PreЄ Є O S D C P T J K Pg N
File:Euoplocephalus-tutus-1.jpg
Mounted skeleton of Euoplocephalus tutus, Senckenberg Museum
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Order:	†Ornithischia
Suborder:	†Ankylosauria
Family:	†Ankylosauridae Brown, 1908
Type species
†Ankylosaurus magniventris Brown, 1908
Subfamilies
†Ankylosaurinae Brown, 1908
Synonyms
Syrmosauridae Maleev, 1952

An ankylosaurid is a member of the armored dinosaur family Ankylosauridae that appeared 122 million years ago (along with another family of ankylosaurs, the Nodosauridae) and became extinct 66 million years ago during the Cretaceous–Paleogene extinction event. Ankylosaurids have been found in western North America, Europe and East Asia, though good specimens are rare; most are known only from bone fragments.

Features

File:Ankylosaurus tail terminology.png

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

The heavy armour, forming a veritable shell on the backs of ankylosaurids and their clubbed tails, makes them look superficially similar to the mammalian glyptodonts (and to a lesser degree to the giant meiolaniid turtles of Australia).

Their heavily armoured heads formed a toothless beak at the front (comparable to modern birds), though the sides of the mouth and the lower jaw did bear small teeth, deeply inset from the jaw. Among all the ornithischians, the endocranial anatomy of ankylosaurs is the most poorly known.

Armor

Ankylosaurids usually had a thick armour plating of fused bone, often interspersed with a variety of spikes and lumps. Ankylosaurids were so heavily armored that some advanced species even had armoured eyelids.

Tail

Many ankylosaurids also had an enlarged mass of bone forming a "club" on the end of their tails, made of two enlarged bone lumps. This tail club has traditionally been used to separate ankylosaurids from their close relatives the nodosaurids, although the many primitive ankylosaurids ("shamosaurines", and even basal ankylosaurines) also lacked bony tail clubs.

Earliest ankylosaurs and nodosaurids did not have tail clubs. Tail clubs evolved in the subgroup ankylosaurids with a handle and a knob. The handle was a sequence of interlocking vertebrae at the end of the tail, the knob was a bony end. The handle evolved first per studies with two dinosaurs: Liaoningosaurus paradoxus lived 122 million years ago, Zhongyuansaurus luoyangensis lived about 92 million years ago, each had versions of the handle but no tail knobs. Ankylosaurs with tail clubs evolved around 75 million years ago. The theory is the tail evolved first to be a stiff appendage then the tail end club evolved. Ankylosaurs had osteoderms on the sides of their tails since the earliest ancestors Scelidosaurus.

Relationships

File:Ankylosaurid skull anatomy.png

Diagram showing ankylosaurid skull anatomy

The polacanthids are sometimes included as a subfamily of ankylosaurids, as Polacanthinae. However, phylogenetic analyses since 2000 have shown the polacanthids to form either a natural group apart from the ankylosaurids, or to be an unnatural grouping of primitive ankylosaurs.

Ankylosauria and Stegosauria are now grouped together within the clade Thyreophora. This group first appeared in the Sinemurian age, and survived for 135 million years, until disappearing in the Maastrichtian. 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 Ankylosaurinae (members of which are called ankylosaurines) within Ankylosauridae. Ankylosaurus appears to be most closely related to Anodontosaurus and Euoplocephalus. The following cladogram is based on a 2015 phylogenetic analysis of the Ankylosaurinae conducted by Arbour and Currie:

Ankylosaurinae	Crichtonpelta Tsagantegia Zhejiangosaurus Pinacosaurus Saichania Tarchia Zaraapelta Ankylosaurini Dyoplosaurus Talarurus Nodocephalosaurus Ankylosaurus Anodontosaurus Euoplocephalus Scolosaurus Ziapelta

Since Ankylosaurus and other Late Cretaceous North American ankylosaurids grouped with Asian genera (in a tribe the authors named Ankylosaurini), Arbour and Currie suggested that earlier North American ankylosaurids had gone extinct by the late Albian or Cenomanian ages of the Middle Cretaceous. Ankylosaurids thereafter recolonised North America from Asia during the Campanian or Turonian ages of the Late Cretaceous, and diversified there again, leading to genera such as Ankylosaurus, Anodontosaurus, and Euoplocephalus. This explains a 30 million year gap in the fossil record of North American ankylosaurids between these ages.

Timeline of discoveries

Timeline of genera

See also

• Timeline of ankylosaur research

References

• Dinosaurs 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. Vert. Paleontol. 16 (Suppl. to 3): 45A

External links

• Ankylosauridae
• Family Tree
• Ankylosauridae Tree of Life web project