Ornithischia

Ornithischia is an extinct clade of mainly herbivorous dinosaurs characterized by a pelvic structure similar to that of birds.
Ornithischians with well known anatomical adaptations include the ceratopsians or "horn-faced" dinosaurs (e.g. Triceratops), armored dinosaurs (Thyreophora) such as stegosaurs and ankylosaurs, pachycephalosaurids and the ornithopods. Ornithischians shifted from bipedal to quadrupedal posture at least three times in their evolutionary history and it has been shown primitive members may have been capable of both forms of movement.

Thyreophora is a group of armored ornithischian dinosaurs that lived from the early Jurassic Period until the end of the Cretaceous.
Thyreophorans are characterized by the presence of body armor lined up in longitudinal rows along the body. Primitive forms had simple, low, keeled scutes or osteoderms, whereas more derived forms developed more elaborate structures including spikes and plates. Most thyreophorans were herbivorous and had relatively small brains for their body size.

Thyreophora includes various subgroups, including the suborders Ankylosauria and Stegosauria. In both the suborders, the forelimbs were much shorter than the hindlimbs, particularly in stegosaurs. The clade has been defined as the group consisting of all species more closely related to Ankylosaurus than to Triceratops. Thyreophora is the sister group of Cerapoda within Genasauria.


The largest thyreophoran was Ankylosaurus at 9 metres (30 ft) in length and 6 tonnes in weight.

Ankylosaurus was a large animal by modern standards. The skull of the largest known individual has been estimated at 64.5 cm (25.4 inches) long and 74.5 cm (29.3 inches) wide, the whole animal was estimated at 6.25 m (20.5 feet) long, 1.5 m (4.9 feet) wide and 1.7 m (5.6 feet) tall at the hip. The body shape was low-slung and quite wide. It was quadrupedal, with the hind limbs longer than the forelimbs. Although its feet are still unknown, comparisons with other ankylosaurs suggest Ankylosaurus probably had five toes on each foot. The skull was low and triangular in shape, wider than it was long. Like other ankylosaurs, Ankylosaurus had small, leaf-shaped teeth, which were the smallest relative to the body size, than in any other ankylosaurid species,and lacked the tooth batteries of the contemporaneous ceratopsid and hadrosaurid dinosaurs. Bones in the skull and other parts of the body were fused, increasing their strength.
The most obvious feature of Ankylosaurus is its armor, consisting of massive knobs and plates of bone, known as osteoderms or scutes, embedded in the skin. Osteoderms are also found in the skin of crocodiles, armadillos and some lizards. The bone was probably overlain by a tough, horny layer of keratin. These osteoderms ranged greatly in size, from wide, flat plates to small, round nodules. The plates were aligned in regular horizontal rows down the animal"s neck, back, and hips, with the many smaller nodules protecting the areas between the large plates. Smaller plates may have been arranged on the limbs and tail. Compared to the slightly more ancient ankylosaurid Euoplocephalus, the plates of Ankylosaurus were smooth in texture, without the high keels found on the armor of the contemporaneous nodosaurid Edmontonia. A row of flat, triangular spikes may have protruded laterally along each side of the tail. Tough, rounded scales protected the top of the skull, while four large pyramidal horns projected outwards from its rear corners.
The tail club of Ankylosaurus was also composed of several large osteoderms, which were fused to the last few tail vertebrae. It was heavy and supported by the last seven tail vertebrae, which interlocked to form a stiff rod at the base of the club. Thick tendons have been preserved, which attached to these vertebrae.
Ankylosaurus was named as the type genus of the family Ankylosauridae.Ankylosaurids are members of the larger taxon Ankylosauria, which also contains the nodosaurids. Ankylosaur phylogeny is a contentious topic, with several mutually exclusive analyses presented in recent years, so the exact position of Ankylosaurus within Ankylosauridae is unknown. Ankylosaurus and Euoplocephalus are often thought to be sister taxa. However, other analyses have found these genera in different positions. Further discoveries or research may clarify the situation.
Like other ankylosaurs, Ankylosaurus was herbivorous. It"s wide muzzle was adapted for non-selective low-browse cropping. The absence of tooth batteries indicates that it did very little chewing. It may have had a hindgut fermentation system like modern herbivorous lizard. This is based on the features of the ribcage.
The tendons of the tail were partially ossified (or bony) and were not very elastic, allowing great force to be transmitted to the end of the tail when it was swung. It seems to have been an active defensive weapon, capable of producing enough of a devastating impact to break the bones of an assailant. A 2009 study showed that "large tail knobs could generate sufficient force to break bone during impacts, but average and small knobs could not", and that "tail swinging behavior is feasible in ankylosaurids, but it remains unknown whether the tail was used for interspecific defense, intraspecific combat, or both". It has also been proposed that the tail club acted as a decoy for the head, although this idea is now largely discredited.
Ankylosaurus magniventris existed between 68 to 66 million years ago, in the final Maastrichtian stage of the Late Cretaceous Period, and was among the last dinosaur species that appeared before the Cretaceous–Paleogene extinction event. The type specimen is from the Hell Creek Formation of Montana, while other specimens have been found in the Lance Formation of Wyoming and the Scollard Formation in Alberta, Canada, all of which date to the end of the Cretaceous.
The Hell Creek, Lance and Scollard Formations represent different sections of the western shore of the shallow sea that divided western and eastern North America during the Cretaceous. They represent a broad coastal plain, extending westward from the seaway to the newly formed Rocky Mountains. These formations are composed largely of sandstone and mudstone, which have been attributed to floodplain environments. The Hell Creek is the best studied of these ancient environments. At the time, this region was subtropical, with a warm and humid climate. Many plant species were supported, primarily angiosperms, with less common conifers, ferns and cycads. An abundance of fossil leaves found at dozens of different sites indicates that the area was largely forested by small trees.Ankylosaurus shared its environment with dinosaurs including the ceratopsids Triceratops and Torosaurus, hypsilophodont Thescelosaurus, hadrosaurid Edmontosaurus, nodosaurid Edmontonia, pachycephalosaurian Pachycephalosaurus, and the theropods Struthiomimus, Ornithomimus, Troodon, and Tyrannosaurus.
Fossils of Ankylosaurus are considerably rare in these sediments, compared to Edmontosaurus and the super-abundant Triceratops, which make up most of the large herbivore fauna. Another ankylosaur, Edmontonia, is also found in the same formations. However, Ankylosaurus and Edmontonia seem to have been separated both geographically and ecologically. The non-selective low browsing of Ankylosaurus may have limited it to the upland regions, away from the coast, while Edmontonia, which had a narrower muzzle for a more selective diet, seems to have lived at lower elevations, closer to the coast.

Tarchia  gigantea

Size estimates of Tarchia have been largely based on Dyoplosaurus giganteus, the holotype of which is one of the largest ankylosaurian individuals known. This would make Tarchia the longest known Asian ankylosaur, with an estimated body length of 8 metres (26 ft).
Estimated at over eight meters long, Tarchia is one of the largest ankylosaurids currently known, rivalling even the more famous Ankylosaurus. In fact given that Ankylosaurus is still known only from partial remains, Tarchia may one day actually prove to be the biggest. Tarchia was named along with another large, but slightly smaller ankylosaurid called Saichania, and although quite similar to one another, there are a number of identifiable differences between the two, particularly differences associated with the skull proportions. Despite these differences however, both Tarchia and Saichania both share bulbous bone growths that are present across the tops of their skulls. A North American ankylosaurid called Nodocephalosaurus also has these bumps, strongly suggesting a possible relationship with Tarchia and Saichania.
Tarchia possessed a wide cropping beak across its mouth that allowed large amounts of vegetation to be indiscriminately pulled into the mouth. These plants would have likely been quite tough considering that Tarchia lived in an arid climate that was near desert in places, and would have required a large degree of processing in the mouth. Evidence for this comes from the teeth which show occlusion wear, basically meaning that the teeth of the upper and lower jaws regularly made contact. Like other ankylosaurids Tarchia had teeth more suited to chopping, and with every up and down movement of the jaw, the food in the mouth would be chopped into smaller and smaller pieces. This was not just to help swallowing but to increase the efficiency of digestion as the teeth chopping the food would provide a greater surface area to be exposed to the digestive acids of the stomach, greatly enhancing the nutritional gain.
Tarchia also had a hard palate and a network of air passages in the snout which would have helped to moisten the dry air of its ecosystem before it reached its lungs. This would greatly reduce the amount of water lost through the process respiration, a vital adaptation considered the climate that Tarchia lived in. Additionally the presence of the hard palate (unknown in most dinosaurs, but seemingly common in ankylosaurids) meant that Tarchia could still breathe while it processed food in its mouth.

Edmontonia

Edmontonia was one of the larger nodasaurids, dinosaurs that were similar to the ankylosaurids but lacked their tail clubs and had narrower mouths. Still, Edmontonia did possess armour plates called osteoderms along its back, and is particularly noted for having large spikes that point out from the sides of its body, the four largest of which are above the shoulder. It seems unlikely that these spines were just for defence as they only cover a relatively small part of the body, and they may not have provided all that much protection against large theropod dinosaurs such as tyrannosaurs like Daspletosaurus and Albertosaurus.
Edmontonia was bulky, broad and tank-like. Its length has been estimated at about 6.6 m (22 ft).In 2010, Gregory S. Paul considered both main Edmontonia species, E. longiceps and E. rugosidens, to be equally long at six metres and weigh three tonnes.
Edmontonia had small, oval ridged bony plates on its back and head and many sharp spikes along its sides. The four largest spikes jutted out from the shoulders on each side, the second of which was split into subspines in E. rugosidens specimens. Its skull had a pear-like shape when viewed from above.Its neck and shoulders were protected by three halfrings made of large keeled plates.
It’s plausible that these shoulder spikes may have been more like deer antlers, with the larger and more developed spikes belonging to the more mature individuals. Additionally it is possible that two Edmontonia may have walked up to one another and engaged in a pushing contest for dominance, the horns locking so that the Edmontonia could get a grip of one another. Again larger spikes would have been preferable in such a contest as they would have allowed the more mature individuals a reach advantage.
Study of other fossils found in association with Edmontonia, specifically petrified trees, has come to the conclusion that Edmontonia lived in an environment that saw extended wet and dry periods throughout the year. This would suggest that like other animals in similar climates, Edmontonia would lay eggs to hatch in time for the wet season so that the newly hatched young had a plentiful supply of fresh vegetation. Additionally the adults would have spent much of their time feeding upon as many plants as they could so that they could build up fat reserves to better survive the lean periods of the dry seasons. The oncoming of the wet season is also thought to be the reason why some Edmontonia remains have the spikes and armour in exactly the same position as they were in life. This is simply a case of flood water washing a large amount of sediment over an individual that had died during the dry season, burying and protecting it from carnivorous animals that may have scavenged and pulled the body to pieces had it not been buried from them.  

Panoplosaurus

Panoplosaurus is one of the more famous of the North American nodosaurs, and a genus that lived during the later stages of the Cretaceous. Like with other nodosaurs, Panoplosaurus would have looked similar to an ankylosaur, though with a few key differences. The most obvious difference at a glance would have been the lack of a tail club; instead the tail of Panoplosaurus would have terminated in a tapered point like so many other dinosaurs. Also like other nodosaurs, and unlike ankylosaurs, the snout and mouth of Panoplosaurus would have been relatively narrow, possibly indicating that Panoplosaurus was a selective browser of low growing plants.
Like its nodosaur relatives, Panoplosaurus would have had heavy bony scutes running down the back and sides of the body. These scutes, technically known as osteoderms, would have acted like mail armour against the teeth and claws of predatory dinosaurs. However, one of the more common armoured choices of nodosaurs, the large shoulder spikes seen in some other genera, were absent in Panoplosaurus. This might suggest that the presence and arrangement of shoulder spikes in nodosaurs may have also served a display as well as a defensive purpose. With the absence of spikes, Panoplosaurus seem to have favoured the development of very large osteoderms that grew very close together on the neck.

Euoplocephalus

Euoplocephalus  is one of the largest genera of herbivorous ankylosaurian dinosaurs, living during the Late Cretaceous of Canada. It has only one named species, Euoplocephalus tutus. Euoplocephalus was about five to six meters long and weighed over two tons. Its body was low-slung and very flat and wide, standing on four sturdy legs. Its head had a short drooping snout with a horny beak to bite off plants that were digested in the large gut. Like other ankylosaurids, Euoplocephalus was largely covered by bony armor plates, among them rows of large high-ridged oval scutes. The neck was protected by two bone rings. It could also actively defend itself against predators like Gorgosaurus using a heavy club-like tail end.
Among the ankylosaurids, Euoplocephalus was exceeded in size only by Ankylosaurus, and perhaps Tarchia and Cedarpelta. Euoplocephalus was 6 metres (20 ft) long and weighed about 2 tonnes (2.2 short tons). It was also 2.4 metres (7.9 ft) wide. Like other ankylosaurids, it had a very broad and flat low-slung torso, about four feet high, positioned on four short legs.
Whereas Ankylosaurus is the most famous ankylosaurid dinosaur, Euoplocephalus is one of the most important because of the huge number of remains that have been attributed to the genus. These include remains of over forty individual Euoplocephalus, comprising fifteen known skulls and some post cranial skeletons that are almost complete. This wealth of material has not only helped to increase our understanding of Euoplocephalus and the Ankylosauridae as a group, but has also revealed avenues of research that were not previously considered by palaeontologists.