REPTILES - CONQUERORS OF THE LAND

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Reptiles were the first truly terrestrial vertebrates, by virtue of the various modifications of the skull, axial skeleton, skin, excretory system, circulatory system and reproduction. The two main requirements for terrestrial life were greater skeletal support and mechanisms to prevent water loss through the skin and in the excretory products.

Mosasaurus sp., an extinct, fully aquatic reptile, related to modern varanids. (up to 9m)

Newly hatched Hypacrosaurus sp. (about 30 cm) and parents (up to 10m).

Evolution of the reptile skull

A: Lateral view and lower jaw of Sphenodon sp. (tuatara);
B - D: Lateral, dorsal and palatal views of an alligator skull.

Both species are diapsid reptiles. (Walker, 1987)

SKELETON

Tuatara skeleton

Crocodile skeleton

Two groups of chelonians are distinguished by the different mobility and articulation of the neck vertebrae.

Turtle skeleton

Cryptodira (top): The head can be retracted under the shell, with inversion of the neck skin. Lateral processes on the cervical vertberae are only slightly developed. Once the head is retracted, the front opening of the carapace can be closed almost competely by the forelimbs.

Pleurodira (bottom): The head can only be retracted sideways and the neck skin cannot be inverted. Lateral processes of the cervical verebrare are well-developed. The forelimbs cannot protect the head and neck.

SKIN

A: Vertical section through a horny scale and skin of a squamate reptile.
B: Enlargment of a section of the scale (Walker, 1987)

Epidermal changes of snake skin during a sloughing cycle.
  A: Resting stage.
  B: Basal cells have divided to form a new inner epidermal generation.
  C: A cleavage zone has formed betwen the two generations.
  D: Sloughing. The outer generation is shed.

EGGS

Schematic diagram showing the organisation of the eggs of amniote vertebrates: (a) Birds; (b) Chelonians and crocodiles; (c) Lepidosauria.

Note that the space shown between the inner and outer egg membranes is for clarity of illustration: in reality the egg mambranes adhere to each other, except at position of the air space in bird eggs.

PHYLOGENY

Cladogram depicting possible relationships between modern reptiles groups and some extinct groups.

Testudomorphs have anapsid skulls (no temporal fenestra).

Synapsida had a single temporal fenestra (and are thought to be ancestral to mammals).

The remaining groups are diapsid (two temporal fenestra, although the bony bars defining these are secondarily lost in some groups).

Archosauria includes the dinosaurs, pterosaurs, phytosaurs and modern crocodiles. (Pough et al. 1989)

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