Question

The order Ornithischia includes Ornithopoda, Pachycephalosauria, Stegosauria, Ankylosauria and Ceratopsia. List the similarities and differences among them.

Answer 1

ORDER - ORNITHISCHIA

The order Ornithischia includes Ornithopoda, Pachycephalosauria, Stegosauria, Ankylosauria and Ceratopsia.

The chief difference between two group of dinosaurs is the configuration of pelvis. Based on this the dinosaurian order can be divided in to two, Saurichia ( lizard hips) and ornithischia ( bird hips).

Like all other animals the pelvis of dinosaurs comprises three separate bones ilium, ischium and pubis. In the case of ornithischia the lateral profile of the pelvis is different from saurischians. They have long iliac bone and long and modified ischium and pubis structure below. The ischium bone is also long and thin and it is extended backward and slightly downward from hips socket and hence the name bird hips.

Ornithopoda

They are one of the most successful dinosaurs lineage. They have horny beak and specialised teeth. The tendency to reduce and loose front teeth and develop cheek pouches for processing food, the arrangement of teeth for crushing vegitation more easily etc..can be considered as evolutionary progression. Some ornithopods evolved blocky wrist bone, spike like thumb and divergent fifth finger.some have claws on middle finger.

Pachycephalosauria

They have basic ornithopod body plan.some evidences suggest that they are evolved from ornithopods. They have ornithopod like pelvis but a portion of ischium is different.

They are also known as dome heads. They have a thickning in the forehead of skull. Microscopic studies of this dome suggest that they are poorly designed to divert stress away from brain case.

Stegosauria

They have spike like tail and series of large triangular plates along it's back. The forelimbs are much shorter than hind limbs.This give an arches appearance to organism. It is believed that the plate act as thermoregulators. They store glycogen in it's sacral cavity, like present day animals.

Ankylosauria

They are called armored dinosaurs. They have extensive mosaic like large interlocking bony plates that completely encases it's back and flanks. Their Hind leg list longer than forelimbs but not much as that of stegosaurus. They have broader , more encrusted skulls.

Ceratopsia

They are plant eating dinosaurs.They are characterised by large bony frill on the back of skull and a unique upper beak bone called rostral.The skull of kosmoceratops of ceratopsia is considered to be the most ornate of any known dinosaurs. Ceratopsidae is of two lineage

1. Chasmosaurinae : large eye horns and small nose horns

2. Centrosaurinae : Small eye horns and large nose horns.