Reptiles were the first fully terrestrial vertebrates. The group includes lizards, snakes, crocodiles, and turtles today. The Class 'Reptilia' is paraphyletic - that means it is a group that had a single ancestor - some time in the Carboniferous - but it excludes some ultimate descendants of that ancestor (namely the birds and mammals).

General Characteristics

Reptiles are amniotes - they lay eggs. This was one of the primary adaptations that allowed them to colonise land. The egg shell prevents the embryo from drying out before it is ready to hatch.

All modern reptiles are cold-blooded (ectotherms). This means that they cannot regulate their body temperature and rely on the temperature of their environment to maintain body heat. Some researchers have suggested that some, or all, of the dinosaurs were warm-blooded.

The skin of a reptile is covered in hard scales made of a protein called keratin. This is the same substance that forms bird feathers and mammal hairs.

Skull Morphology

Amniotes show four skull patterns:
  • Anapsid: Probably the primitive form of the amniote skill. It lacks any temporal fenestrae and is thought to be common to all early reptiles such as Hylonomus.
  • Synapsid: This form of skull has one temporal fenestra in a low position. It is seen in the 'mammal-like' reptiles, such as the cynodonts, and in mammals.
  • Diapsid: The skull type seen in the majority of reptiles both modern and fossil, including lizards, crocodiles, birds, dinosaurs and pterosaurs. There are two temporal fenestrae.
  • Euryapsid: Probably derived from the diapsid skull type, and perhaps more than once. It has one temporal fenestra, but its position is higher in the skull suggesting the loss on the lower fenestra of diapsids. Seen in extinct marine reptiles such as nothosaurs, plesiosaurs and ichthyosaurs.

Dinosaur Characteristics

Dinosaurs were the dominant group of fossil reptiles. Many of them were huge and adapted to life on land. Their key characters are in the hindlimb, and these reflect a major evolutionary change through the Triassic, when the archosaurs evolved from a sprawling posture (the primitive condition), through a semi-erect posture to the fully erect stance seen in all dinosaurs:
  • an erect (upright) posture,
  • primitively bipedal (standing up on their hind legs),
  • with a ball-and-socket hip joint,
  • a straight hinge-like knee and ankle,
  • roller-like main ankle bones (astragalus and calcaneum) that were closely fixed to the bottom of the main shin bone (the tibia).
  • most dinosaurs had more than the basic two sacral (hip) vertebrae seen on other reptiles - sometimes three or four, or even as many as six or seven.

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