Question

what is the differences and similarities between the organisms found in order Ornithischia, order Saurischia and order Crocodilia ? Note that all are Archosauria subclass

Answer 1

SIMILARITIES BETWEEN ORNITHISCHIA, SAURISCHIA AND CROCODILIA:

1. They are all diapsids. A diapsid is characterized by the presence of two temporal openings in the skull.

2. They are all amniotes. An amniote is a clade of tetrapods in which the embryo develops inside the following protective extra-embryonic membranes- Amnion, Chorion and Allantois. They retain the fertilized egg within the mother or lay their eggs on land, as opposed to anamniotes, that usually lay their eggs in water.

3. They are all thecodonts. This means that their teeth are set in deep sockets.

4. They have antorbital and mandibular fenestrae. An antorbital fenestrae is an opening in the skull in front of the eye sockets. A mandibular fenestrae is an opening in the skull in front of the jaw. Their purpose is to reduce the weight of the skull and jaw.

5. They have a pronounced trochanter, which is a prominent ridge in the femur. It provides a large site for attachment of muscles. This results in strong muscles for an erect gait.

DIFFERENCES BETWEEEN ORNITHISCHIA, SAURISCHIA AND CROCODILIA:

ORNITHISCIA-

1. Includes herbivorous ancestors birds and all Archosaurians closely related to birds.

2. Belongs to clade Dinosauria.

3. Ornithischians have a reversed pubis (opisthopubic), pointing backwards and down, toward the tail. It lies alongside and parallel to the ischium. Some ornithischians have a projection at the frontal end of the pubis (four pronged pelvis structure)- "Bird Hip".

4. All species extinct.

SAURISCHIA-

1. Includes all carnivorous dinosaurs, Sauropodomorphs and birds.

2.Belongs to clade Dinosauria.

3. In saurischian dinosaurs, the pubis bone points toward the front of the animal, and flares into a keel at the forward end (three pronged pelvis structure)- "Lizzard Hip".

4. Living species present (birds).

CROCODILIA-

1. Includes true crocodiles, alligators, caimans, gharial and false gharial.

2. Belongs to clade Eusuchia.

3. They have ankles that flex in the middle so the feet can twist against the ankles as it walks sprawled out. However, this ankle joint can also stiffen so that the animal moves in a “high walk,” as seen in crocodiles.

4. Living species present.