Ankylosauria

Unikonta

Ankylosaurs Temporal range: Middle Jurassic—Late Cretaceous, 167–65.5 Ma PreЄ Є O S D C P T J K Pg N
Mounted skeleton of Euoplocephalus tutus, Senckenberg Museum
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
clade:	Dinosauria
Order:	†Ornithischia
Node:	†Eurypoda
Suborder:	†Ankylosauria Osborn, 1923
Subgroups
†Ankylosauridae †Nodosauridae

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. Ankylosaurs were bulky quadrupeds, with short, powerful limbs. They are first known to have appeared in the early Jurassic Period of China, and persisted until the end of the Cretaceous Period. They have been found on every continent except Africa. The first dinosaur ever discovered in Antarctica was the ankylosaurian Antarctopelta, fossils of which were recovered from Ross Island in 1986.

Ankylosauria was first described by Henry Fairfield Osborn in 1923.^[1] In the Linnaean classification system, the group is usually considered a suborder or an infraorder. It is contained within the group Thyreophora, which also includes the stegosaurs, armored dinosaurs known for their combination of plates and spikes.

Classification

Ankylosauria is usually split into two families: Nodosauridae (the nodosaurids) and Ankylosauridae (the ankylosaurids). A third family, the Polacanthidae, is sometimes used,^[2] but is more often found to be a sub-group of one of the primary families.^[3]

The first formal definition of Ankylosauria as a clade was given in 1997 by Carpenter. He defined the group as all dinosaurs closer to Ankylosaurus than to Stegosaurus (a definition followed by most paleontologists today). This "stem-based" definition means that the primitive armored dinosaur Scelidosaurus, which is slightly closer to ankylosaurids than to stegosaurids, is technically a member of Ankylosauria. Upon the discovery of Bienosaurus, Dong Zhiming (2001) erected the family Scelidosauridae for both of these primitive ankylosaurs.

Major differences distinguishing the ankylosaurids from the nodosaurids is that the ankylosaurids had bony clubs at the end of their tails, domed snouts in front of the eyes, and large horns projecting from the top and bottom of each side of the skull, all of which nodosaurids lacked. The nodosaurids had narrow heads, and frequently had large spikes protruding from their bodies. This group traditionally includes Nodosaurus, Edmontonia, and Sauropelta.

The traditional ankylosaurids are from later in the Cretaceous. They had much wider bodies and have even been discovered with bony eyelids. The large clubs at the end of their tails may have been used in self-defense (swung at predators) or in sexual selection. This family included Ankylosaurus, Euoplocephalus, and Pinacosaurus.

Polacanthids

The family Polacanthidae was named by Wieland in 1911 to refer to a group of ankylosaurs which seemed to him intermediate between the ankylosaurids and nodosaurids. This grouping was ignored by most researchers until the late 1990s, when it was used as a subfamily (Polacanthinae) by Kirkland for a natural group recovered by his 1998 analysis suggesting that Polacanthus, Gastonia, and Mymoorapelta were closely related within the family Ankylosauridae. Kenneth Carpenter resurrected the name Polacanthidae for a similar group which he also found to be closer to ankylosaurids than to nodosaurids. Carpenter became the first to define Polacanthidae as all dinosaurs closer to Gastonia than to either Edmontonia or Euoplocephalus.^[4] Most subsequent researchers placed polacanthines as primitive ankylosaurids, though mostly without any rigorous study to demonstrate this idea. The first comprehensive study of 'polacanthid' relationships, published in 2012, found that they are ither an unnatural grouping of primitive nodosaurids, or a valid subfamily at the base of Nodosauridae.^[3]

Taxonomy

While ranked taxonomy has largely fallen out of favor among dinosaur paleontologists, a few 21st century publications have retained the use of ranks, though sources have differed on what its rank should be. Most have listed Thyreophora as an unranked taxon containing the traditional suborders Stegosauria and Ankylosauria, though Thyreophora is also sometimes classified as a suborder, with Ankylosauria and Stegosauria as infraorders. A simplified version of one possible classification follows:

• Order Ornithischia (the "bird hipped" dinosaurs)
  • Clade Thyreophora (armored herbivores)
    • Clade Eurypoda (advanced armored dinosaurs)
      • Clade Ankylosauria
        • Family Ankylosauridae
          • Subfamily Ankylosaurinae
        • Family Nodosauridae
          • Subfamily Polacanthinae?

Reproduction

Possible neonate sized ankylosaur fossils have been documented in the scientific literature.^[5]

Notable specimens

Oilsands ankylosaurian

A complete ankylosaurian skeleton was discovered by accident March 21, 2011 in approximately 110 million-year-old oil sands being excavated by Suncor near Fort McMurray, Alberta, Canada. It is the oldest known dinosaur specimen from the province.^[6]

References

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External links

• Ankylosauria at DinoData
• Ankylosaurus- The Bullet-Proof Dinosaur

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