Ankylosauria

Ankylosaurs Temporal range: Middle Jurassic—Late Cretaceous, Bajocian–Maastrichtian PreꞒ Ꞓ O S D C P T J K Pg N
Collection of ankylosaurs. From top left to right: Liaoningosaurus, Edmontonia, Tianzhenosaurus, Gargoyleosaurus, Scolosaurus, Denversaurus, Gastonia, Borealopelta and Akainacephalus.
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Order:	†Ornithischia
Clade:	†Eurypoda
Suborder:	†Ankylosauria Osborn, 1923
Subgroups
†Cryptosaurus? †Dracopelta †Kunbarrasaurus †Sarcolestes †Serendipaceratops? †Sinankylosaurus †Stegosaurides? †Tianchisaurus †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, similar to turtles. Ankylosaurs were bulky quadrupeds, with short, powerful limbs. They are known to have first appeared in the early Jurassic Period, and persisted until the end of the Cretaceous Period. The two main families of Ankylosaurs, Nodosauridae and Ankylosauridae are primarily known from the Northern Hemisphere, but more basal ankylosaurs are known from the Australia-Antarctica region during the Cretaceous.

Ankylosauria was first named by Henry Fairfield Osborn in 1923.^[1] In the Linnaean classification system, the group is usually considered either 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.

Etymology[]

The name of this group of dinosaurs is associated with a number of anatomical features in which small and large bony shields fused together, completely covering their back and sides. On the skull these shields fused with the underlying bones, and the dorsal ribs fit snugly to the vertebrae. The Latin name Ankylosauria is derived from the Greek ἀγκύλος [ankylos] — "curved", "bent" with the anatomical meaning "hard" or "fused" and σαῦρος [sauros] — "lizard".^[2]

In the 1908 description of the genus Ankylosaurus, Barnum Brown described the family Ankylosauridae as a group of representatives with a "rigid spine", but noted the wide, curved shape of the ribs, suggesting a "strongly curved" back^[3] (an error based on the alleged similarity to stegosaurs and glyptodonts, as ankylosaurs have flat backs). Therefore, "rigid lizard" and "curved lizard" could be additional meanings applied to the name of ankylosaurs.^[2]

Evolution[]

The origin of ankylosaurs is poorly understood, and only a few specimens from the Middle Jurassic are known.^[4] The ancestral ancestry of ankylosaurs has long been sought among stegosaurs, the closest group to ankylosaurs compared to other dinosaurs. Currently, ankylosaurs are a close group of stegosaurs within the Eurypoda clade.^[5] They are united by the presence of osteoderms in the skin, the narrow triangular skull of stegosaurs is similar to that of nodosaurids, and some similarity is found in the structure of the palate.^[6] Since stegosaurs are known from the Middle Jurassic,^[7] ankylosaurs are probably of the same age. They may have split up during the Aalen period, more than 170 million years ago. There are no well-preserved remains of ankylosaurs of that age. An incomplete radius and ulna from the Isle of Skye in Scotland are known, the exact affiliation of which to ankylosaurs or stegosaurs is not established.^[8] Most likely, ankylosaurs followed a different evolutionary path than stegosaurs. In the latter, the osteoderms become raised, and the lateral protection disappears.^[9] Ankylosaurs evolved towards the development of osteoderms on the surface of the skull, increased armor and further consolidation of the carapace,^[10] which suggests that the ancestor of the carapace consisted of separate non-fused osteoderms.^[11]

Paleobiology[]

Armor[]

All ankylosaurians had armor over much of their bodies, mostly scutes and nodules, with large spines in some cases. The scutes, or plates, are rectangular to oval objects organized in transverse (side to side) rows, often with keels on the upper surface. Smaller nodules and plates filled in the open spaces between large plates. In all three groups, the first two rows of plates tend to form a sort of half-ring around the neck; in nodosaurids, this comes from adjacent plates fusing with each other (and there is a third row as well), while ankylosaurids usually have the plates fused to the top of another band of bone. The skull has armor plastered on to it, including a distinctive piece on the outside-rear of the lower jaw.

Diet and feeding[]

Ankylosaurs were built low to the ground, typically one foot off the ground surface. They had small, triangular teeth that were loosely packed, similar to stegosaurs. The large hyoid bones left in skeletons indicates that they had long, flexible tongues. They also had a large, side secondary palate. This means that they could breathe while chewing, unlike crocodiles. Their expanded gut region suggests the use of fermentation to digest their food, using symbiotic bacteria and gut flora. Their diet likely consisted of ferns, cycads, and angiosperms. Mallon et al. (2013) examined herbivore coexistence on the island continent of Laramidia during the Late Cretaceous. It was concluded that ankylosaurs were generally restricted to feeding on vegetation at, or below, the height of 1 meter.^[12]

Reproduction[]

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Possible neonate-sized ankylosaur fossils have been documented in the scientific literature.^[13]

Brain[]

Members of Ankylosauria sported a very small brain size in proportion to their body, second only to the Saurischian sauropods.

Movement[]

Ankylosaurs were slow moving, largely because of the shortness of the limbs combined with being incapable of running. Their top speed was likely less than 10 km/h (2.8 m/s; 6.2 mph).

Classification[]

Ankylosauria is usually split into two families: Nodosauridae (the nodosaurids) and Ankylosauridae (the ankylosaurids). A third family, the Polacanthidae, is sometimes used,^[14] but is more often found to be a sub-group of one of the primary families.^[15]

The first formal definition of Ankylosauria as a clade, a group containing all species of a certain evolutionary branch, was given in 1997 by Carpenter.^[16] He defined the group as all dinosaurs (more precisely: all thyreophoran Ornithischia) closer to Ankylosaurus than to Stegosaurus. This definition is essentially 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.^[17] In 2001, Carpenter proposed a new group uniting Scelidosaurus, Ankylosauridae, Nodosauridae, and Polacanthidae, with Minmi (thought to be a primitive ankylosaurian), to the exclusion of Stegosaurus.^[18] However, many taxonomists find that Ankylosauromorpha is an invalid group.^[19]

Nodosauridae[]

This group traditionally includes Nodosaurus, Edmontonia, and Sauropelta. The nodosauridae had longer snouts than their ankylosaurid cousins. They did not sport the archetypal 'clubs' at the ends of their tails; but, rather, their most pronounced physical features were their spikes.

Nodosaurids had very muscular shoulders and a specialized knob of bone on each shoulder blade called the acromial process. It served as an attachment site for the muscles that held up their large parascapular spines. These spines would be used for self-defense against predators. They also had wide, flaring hips and thick limbs. They had smaller, narrow beaks than the ankylosaurids, which likely allowed them to be very selective over what plant matter they grazed on.

Most nodosaurid finds are from North America.

Ankylosauridae[]

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 squamosal plates projecting from the top and bottom of each side of the skull, all of which nodosaurids lacked. 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. The clubs were made of several plates of bone that were permeated by soft tissue, allowing them to absorb thousands of pounds of force. Their beaks were larger and broader than the nodosaurids, indicating that these ankylosaurs were generalists in their diet.

Polacanthidae[]

The family Polacanthidae was named by in 1911 to refer to a group of ankylosaurs that seemed to him to be intermediate between the ankylosaurids and nodosaurids.^[20] 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 that 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.^[21] 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 either an unnatural grouping of primitive nodosaurids, or a valid subfamily at the base of Nodosauridae.^[15]

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 dinosaurs)
    • Clade Eurypoda (advanced armored dinosaurs)
      • Clade Ankylosauria
        • Family Ankylosauridae
          • Subfamily Ankylosaurinae
        • Family Nodosauridae
          • Subfamily Polacanthinae?

See also[]

• Timeline of ankylosaur research

References[]