Characters and classification of class: Reptilia

 General Characters

Reptiles are the class of vertebrates  fully adapted for life on land. The class name refers to the mode of locomotion (Latin: repere or reptum, to creep or crawl), and the study of reptiles is called Herpetology (Gr., herpeton, reptiles).

1. They are predominantly terrestrial, creeping or urrowing, mostly carnivorous, air-breathing, cold-blooded, oviparous and tetrapodal vertebrates.

2. Bilaterally symmetrical body and divisible into 4 regions—head, neck, trunk and tail.

3. Two pairs of pentadactyl limbs provided with digits having  horny claws. However, limbs absent in a

few lizards and all snakes.

4. Exoskeleton of horny epidermal scales, shields, plates and scutes.

5. Skin dry, cornified and devoid of glands.

6. Mouth terminal. laws bear simple conical teeth. In turtles teeth replaced by horny beaks.

7. Alimentary canal terminates into a cloacal aperture.

8. Endoskeleton bony. Skull with one occipital condyle (monocondylar). A characteristic T-shaped interclavicle present.

9. Heart usually 3-chambered, 4-chambered in crocodiles. Sinus venosus reduced. 2 systemic arches present. Red blood corpuscles

11. Kidneys metanephric. Excretion uricotelic.

12. Brain with better development of cerebrum than in Amphibia. Cranial nerves 12 pairs.

13. Lateral line system absent. Jacobson's organs present in the roof of mouth,

14. Sexes separate. Male usually with muscular copulatory organ.

15. Fertilization internal. Mostly oviparous. Large yolky meroblastic eggs, covered with leathery

shells, always laid on land. Embryonic membranes (amnion, chorion, yolk sac and allantois) appear during development. No metamorphosis. Young resemble adults.

16. Parental care usually absent.

Classification

 There are more than 7,000 living and several extinct species of reptiles.The class Reptilia is first

divided into 5 major groups or subclasses on the basis of presence or absence of certain openings

through the posterolateral or temporal region of the skull

Subclass I. Anapsida

Primitive reptiles with a solid skull roof. No temporal openings.

Order 1. Cotylosauria

1. Primitive reptiles.

2. Resemble Labyrinthodont amphibians.

Ex. Seymouria.

Order 2. Chelonia or Testudinata (Gr., chelone, turtle; L., testudo, turtle)

1. Body short, broad and oval.

2. Limbs clawed and/or webbed, paddle-like.

3. Body encased in a firm shell of dorsal carapace and ventral plastron, made of dermal bony plates. Thoracic  vertebrae and ribs usually fused to carapace,

Sub-class 2. Eurypsida

1. Skull with single dorsolateral temporal opening bounded

by post -orbital and squamosal.

2. Extinct.

Ex. Plesiosaurus.

Sub-class 3. Parapsida

1. Dorsolateral temporal opening in skull bounded by supratemporal

and post Frontal.

2. Extinct.

Ex. Ichthyosaurus.

Sub-class 4. Synapsida

1. Skull roof with lower opening behind eye bounded above

by post-orbital and squamosal.

2. Extinct mammal-like reptiles.

Ex. Dimetrodon, Varanosaurus.

Subclass 5. Diapsida

Skull with two temporal openings on either side separated by the bar of postorbital and squamosal bones.

Order -Rhyncocephalia

1.Small elongated lizard like body

2.Limbs pentadactyle, clawed and burrowing

3. Skin covered by granular scales and mid-dorsal row of spines.

4. Skull diapsid. Nasal openingsParietal foramen with vestigeal pineal eye

.present. Quadrate is fixed.

5. Vertebrae amphicoelous or biconcave.

6.Numerous abdominal ribs present.

7. Teeth acrodont. Cloacai aperture transverse.

8. Heart incompletely 4-chambered.

9. No copulatory organ in male.

Example : Represented by a single living

species, the "tuatara" or Sphenodon punctatum of

New Zealand.

Order 2. Squamata (L., squama, scale or squamatus, scaly)

Advanced, small to medium, elongated.

Limbs clawed, absent in snakes and in few lizards.

Exoskeleton of horny epidermal scales,shields and spines.

Skull diapsid. Quadrate movable.

Vertebrae procoelous. Ribs single—headed.

Teeth acrodont or pleurodont.

Heart incompletely 4-chambered.

Cloacai aperture is transverse.

Male with eversible double copulatory organs(hemipenes).

Order 3. Crocodilia

1. Body long, head large and long.

2. Jaws powerful with numerous bluntly conical teeth.

3. Short limbs ending into claws with web.

4. Tail long heavy and compressed.

5. Heart 4-chambered with separate ventricles.

Ex. Crocodile (Crocodilus), Gavials (Gavialis),

Alligators (Alligator).

Order 4. Pseudosauria

Extinct. Ex. Saltopseudois.

Order 5. Saurischia

1. Ischium and pubis diverge.

2. Bipedal or huge 4-footed.

Ex. (Extinct) Dinosaurs, Tyrannosaurus, Brontosaurus, Diplodiscus.

Order 6. Ornithischia

I. Pelvis bird-like.

2. Ischium and pubis together.

Extinct. Ex. Dinosaurs, Iguanodon, Triceratops, Stegosaurus.

Order 7. Pterosauria

1. Flying reptiles.

2. Forelimbs with wing membranes. Tail long.

Ex. (Extinct) Pteranodon, Rhamphorhynchus.