Ornithischian dinosaurs
Although even the earliest ornithischians have a very different pelvic structure to the saurischians (see cladogram) they appear in the fossil record at the same time. Pisanosaurus, admittedly known only from teeth, is found in the Ishigualasto Formation along with Herrerasaurus and Eoraptor.
Right from the beginning ornithischian evolution proceeded along radically different lines to those of saurischians. For a start all ornithischians were herbivorous. As with saurischians, ornithischians were primitively bipedal but quadrupedalism developed independantly several times; each major group of ornithischians had quadrupedal members. Ornithischians all had a predentary bone (see diagram), not present in saurischians. This jaw bone did not bear any teeth but supported a horny beak which connected with a similar structure on the premaxilla above. Ornithischians also displayed a remarkable facility for novel head ornamentation (see diagram). Possible uses for this include vocalisation, defence, intraspecific combat and display.
Early Ornithischians
The earliest and most primitive ornithischians were very similar in superficial morphology to the most primitive theropods. However in contrast to the theropods, Triassic ornithischians are very poorly known; it is not until the Jurassic that relatively complete skeletons are found. Despite some notable early successes, e.g. stegosaurs, ornithischians did not really come into their own until the Cretaceous, when they became very numerous and diverse.
“Fabrosaurids”
Fabrosaurs is a kind of catch all name for early ornithischians. They are generally known only from fragmentary fossil remains and are therefore hard to classify to a higher taxonomic level. Lesothosaurus is the best known of all these early ornithischians and its skeleton (see diagram) illustrates the basic form of these dinosaurs. Although a primitive ornithischian Lesothosaurus is found in Early Jurassic rocks (contemporaneous with Scutellosaurus and Heterodontosaurus (see below)); none of the Triassic forms is well preserved (see diagram).
The earliest ornithischians have a retroverted pubis and characteristic leaf-shaped teeth with coarse denticles (see diagram). They were small (less than 1 or 2m adult length) and bipedal with a good turn of speed - they have a short femur and a long lower leg.
The rest of the ornithichians are separated from Lesothosaurus and other fabrosaurids by the presence of inset teeth, thought to indicate that all these forms had some kind of fleshy or muscular cheeks; thus all other Ornithischia are placed in a group called the Genasauria. The idea of cheeks in ornithischians has been challenged recently but research is still ongoing and the results are so far inconclusive.
Basal Thyreaphora
The earliest and most primitive of the armoured dinosaurs come from the Early Jurassic of Laurasia. The best known forms are Scutellosaurus from Arizona and Scelidosaurus from England.
If you compare the skeleton of Scutellosaurus with that of Lesothosaurus you can see that they look very similar. They were both small animals - Scutellosaurus was about 1.2m long and Lesothosaurus about 0.9m long. Scutellosaurus had relatively longer arms and may have been facultatively bipedal. But the main feature which places Scutellosaurus in the Thyreophora is the presence of armour plates or dermal bones which in life would have been embedded in the skin and would have provided some protection from predators.
Scelidosaurus was a larger and more derived form. It had larger dermal plates providing more complete coverage. It also probably had larger more robust front legs relative to its ize. I say probably because the lower arms of Scelidosaurus are not known but the shoulder girdle is and it is substantial. In fact, for many years Scelidosaurus was reconstructed as an obligate quadruped and very ankylosaurian in nature but recently research has indicated that it may well have been only facultatively quadrupedal.
Stegosaurs
Stegosaurs were the first group of successful and diverse ornithischians. They are a mainly Late Jurassic group and are distinguished by two rows of spikes or plate among their backs. Huayangosaurus from the Middle Jurassic of China is the most primitive true stegosaur. By the Late Jurassic stegosaurs had a worldwide distribution although their fossil remains are rare.
Stegosaurus is found in the Late Jurassic Morrison formation of the western United States and this genus is Colorado’s state fossil. Stegosaurus is the best known and largest of the stegosaurs, getting up to 9 or 10 metres in length. As a group stegosaurs were large quadrupedal herbivores; they seem to have been low browsers because the relative shortness of their hind legs forced their head and neck downwards. Stegosaurs had small narrow skulls and while the presence of cheeks would have meant chewing was possible most food processing was probably carried out in the large digestive system.
In most forms, with the notable exception of Stegosaurus itself, these spikes or plates are paired. The most complete skeleton of Stegosaurus found so far (1996) seems to indicate that these plates were arranged in a double alternating row. This new specimen also revealed that Stegosaurus had a throat pouch covered in small bones or ossicles. This is interesting because it would make sense to protect such a vulnerable area and also may indicate that the throat was a part of the body that predators tried to attack.
Debate has raged over the function of the dermal bone in these forms. Most stegosaurs had small plates or spikes along their backs for which a defensive function seems clear. They also had long spikes on their tails and it has been speculated that they swung these at any theropod that came too close. Indeed a pubic boot of Allosaurus with a hole the right size to have been made by a Stegosaurus tail spike has been found. The wound seems to have gone septic and would certainly have been very painful. However the large highly vascularised plates of Stegosaurus are more controversial. They don’t seem to have been designed as weapons, spikes, such as those sported by Kentrosaurus (see diagram) would be more effective, and the high degree of blood flow implied by the bone texture has lead some to suggest that the plates acted as radiators to warm the animal up in the morning (if dinosaurs were cold-blooded) or to cool it down on hot days.
Ankylosaurs
Ankylosaurs were basically living tanks; they had the most impregnable armour of all the dinosaurs. Ankylosaurs are a mainly Cretaceous group. As with the stegosaurs, ankylosaurs were never particularly numerous or diverse but they had a worldwide distribution and thrived throughtout the Cretaceous.
There were two main groups of ankylosaurs - the Nodosauridae and the Ankylosauridae. The nodosaurids had less armour plating. They had longer narrower skulls than the short high wide skulls of the ankylosaurids (see diagrams). Nodosaurids had relatively longer tails, and one noticeable feature of their armour is that they always had long spikes over their necks and shoulders, perhaps again indicating that this vulnerable area was a favourite with theropods.
Ankylosaurids took their dermal armour to an extreme; some forms even had bony eyelids! Ankylosaurids had bony clubs on the ends of their tails. The vertebrae in the distal half of the tail were bound into a stiff rod by ossified tendons, perhaps to increase the efficacy or accuracy of the swing.
Adult ankylosaurs were between 3 and 10 metres long when fully grown. Although they had cheeks and could have chewed their food they, like stegosaurs, probably let their bulky guts do most of the processing.