Australia: The Land Where Time Began

A biography of the Australian continent 

Ornithischia

In the order Ornithischia the pubis and ischium both point backwards and are close together. Other specialisations in the ornithischians are the presence of a predentary, a unique bone in the lower jaw situated where the 2 halves of the lower jaw meet. There are no teeth on the  predentary, but it is thought it may have had a horny beak covering it as in turtles.

According to the author² all ornithischian dinosaurs are believed to have been herbivorous and appear to have been much more numerous and diverse than their potential predators, as occurs among modern mammals.

Some ornithischians were bipedal and others were quadrupedal, and all were herbivorous. Some, like the hypsilophodonts, were small, running, agile dinosaurs that are found throughout the world. They filled the niches at present occupied by gazelles.

Iguanodontids were large, bipedal, herbivores with a spike on their thumbs.

Hadrosaurs were duck-billed, herbivores, some of which had head processes. The processes came in a variety of shapes and sizes, it is believed the structures were used to amplify the vocalisations of the animals, the different shapes and sizes producing different sounds. The duck bills of the hadrosaurs had up 700 teeth exposed at a time.

Ceratopsians were quadrupedal forms with an armoured frill around their necks, and often horns. It is believed these horns were used in mating fights as well as for defence. The best known ceratopsian is Triceratops.

Pachycephalosaurs were bipedal, had thick domed skulls. It is believed the thickened head bones were probably to protect the skull during mating fights that possibly involved head-butting, as many animals do today.

The thyreophorans are characterised by having bony plates in their body wall, clubs or spikes on their tails and for having been almost exclusively quadrupeds. Included in this group of heavily armoured animals are stegosaurs and ankylosaurs, heavily armoured dinosaurs such as Euplocephalus that was so heavily armour-plated that it even had bony shutters reinforcing its eyelids and a huge bony club on the end of its tail.

Cerapodans were typically bipeds that were not armoured, though a few had reverted to quadrupedal locomotion. One major group of cerapodans were the ornithopods, many of which were of medium size, about 2-5 m long, and are believed to have been quite abundant. It has been suggested that they probably filled the ecological niches presently occupied by sheep, goats, deer and antelopes. One of these was Hypsilophodon in which the body was balanced on the hips in a similar manner to the theropods, with slender legs for fast running, grasping hands, and their teeth, jaws and cheeks that were adapted for eating plants. Small to medium-sized ornithopods remained quite abundant throughout the time of the dinosaurs. Though some larger types evolved in the later parts of the Mesozoic, the iguanodontians were the hadrosaurs that the author² describes as ‘extraordinarily numerous’, that evolved in the Late Cretaceous of North America and Asia. Some of these dinosaurs did have snouts that were similar in shape to the beaks of ducks, and others had a wide range of hollow structures on their heads that have been suggested to have been associated with social signaling that may have been in the form of loud honking sounds.

The other major group of cerapodans were the Marginocephalians that arose in the Cretaceous. The pachycephalosaurs were included among this group, their bodies being very similar to those of the Ornithopods but they had a large bony dome on their heads that the author² suggests was vaguely similar to the structures on the heads of hadrosaurians, the main difference being that the dome on the heads of pachycephalosaurs were made of solid bone. It has been suggested that the bone thickening on the heads evolved because they were ‘headbangers’ in the manner of some of the modern cloven-hoofed animals.

Also included here were the ceratopsians, among which were Protoceratops and Triceratops. A singular narrow beak was present on the front part of the jaws of all these dinosaurs and there was a tendency to have at the back of the skull a collar of bone described as ruff-like. Some of this group, especially the earlier ones, were bipedal many greatly increased the size of their bodies, the enlarged head with huge frill-like bony collars and large eyebrow and nose horns.

It has been suggested that they became quadrupedal as a result of their great body mass, and they are not dissimilar to the rhinos of the present.

 Sources & Further reading

1. Long, John A, 1998, Dinosaurs of Australia and New Zealand, University of New South Wales Press.
2. Norman, David, 2005, Dinosaurs: A Very Short Introduction, Oxford University Press