ORIGIN & EVOLUTION OF REPTILES

ORIGIN & EVOLUTION OF REPTILES

Reptiles evolved from amphibians of Carboniferous period[1], which depended on water bodies for laying eggs and development of larval stages and hence could not exploit arid/terristrial habitats far away from water bodies.

They developed a large yolk-laden [2]shelled egg that could be laid on land and in which an amniotic[3] sac contained fluid in which embryo could develop to an advanced stage, capable of fending for itself when hatched. The following anatomical changes transformed the ancestral amphibians into land adapted reptiles:

• Body developed a covering of epidermal scales to prevent loss of body moisture, and skin became cornified and devoid of glands.
• Skull became monocondylic[4] for better movement and flexibility. Atlas and axis vertebrae together permitted skull movement in all directions.
• Limb bones and girdles became stronger but limbs were attached on the sides of body, and belly touched the ground during creeping mode of locomotion.
• Sacral region involved two strong and fused vertebrae to support the body weight on hind legs.
• Pentadactyle limbs developed claws that helped in climbing on rocks and trees.
• Respiration through lungs became more efficient to use the oxygen available in the air.
• As a water conservation strategy, metanephric kidneys excreted uric acid which did not require water for excretion.
• Reptiles continued to be ectothermal since ventricle was not completely partitioned[5] by a septum and blood mixed in heart.
• Internal fertilization evolved as a large cleioid[6] shelled egg was laid on land.
• Embryonic membranes amnion, allantois and yolk sac evolved to enable embryonic development in arid conditions.

ANCESTORS OF REPTILES

1.       THE ANAPSIDS (THE COTYLOSAURS)

They were the most primitive stem reptiles that evolved from the labyrithodont amphibians (Embolomeri) in Carboniferous period.

Seymoria was a lizard-like animal, with pentadactyle limbs and a short tail. It had homodont labyrinthine teeth on the jaw bones as well as on vomer and palatine bones. Presence of lateral line indicates its amphibious habits. Skull was monocondylic for better movement of head. Seymoria indicates gradual transition from labyrinthodont amphibians to reptiles. Another 5 foot long cotylosaur fossil, Limnoscelis was found in Mexico that had large premaxillary teeth and long tail.

2.       THE PARAPSIDS

They possessed superior temporal vacuity in the skull and were adapted for aquatic mode of life.

Plesiosaurus was marine long-necked, fish-eating animal with 15 metre long fusiform body, short tail and paddle-like limbs modified for swimming. The skull was euryapsid type with a superior temporal vacuity. The fossils are from lower Jurassic (about 180 million years) and they are believed to have become extinct in end-Cretaceous mass extinction.

Ichthyosaurus  had fish-like body with fore limbs modified into paddle-like fins and hind limbs disappeared. There was a fleshy dorsal fin too. Caudal fin was large and bilobed. Jaws projected into an elongated snout and teeth were homodont, an adaptation for fish-catching. Skull was parapsid type with additional postfrontal and supratemporal bones behind the eye orbit. Vertebral column became secondarily simplified with amphicoelous vertebrae.

3.       THE SYNAPSIDS

Synapsids split off from the primitive reptilian stock very early in evolution, perhaps in the middle carboniferous period. Synapsids had started developing mammalian characteristics that enabled them to be fleet-footed and active predators. Their legs commenced to move under the body. Heterodont dentition and false palate started developing in pelycosaurs and had been completely formed in therapsids. Two types of synapsids occurred from carboniferous to Permian, namely, the primitive Pelycosaurs and advanced therapsids.

Pelycosaurs  are represented by Dimetrodon whose fossils were discovered from North America and Russia from the late Carboniferous to Permian periods. They were primitive reptile-like animals in which limbs had moved under the body but not completely and each limb had 5 digits with claws. Neural spines on the back were excessively long stretching highly vascularized skin between them that formed a fin-like or sail-like structure. They had heterodont dentition with incisors, canines and molars clearly defined but the false palate had not been completely formed.

Therapsids  were more advanced and active synapsids which were perhaps endothermic animals with high rate of metabolism. Heterodont dentition with false palate allowed these animals to chew and grind food for quick digestion in the gut so that high metabolic demand of the body could be fulfilled. Jaw muscles were attached to zygomatic arch to make chewing effective. Carnivore therapsids were called Cynodonts (ex. Cynognathus) and herbivores were Dicynodonts.

4.       THE THECODONTS

They evolved from the sauropsid Archosauria, a group of insignificant lizard-like reptiles that survived the Triassic mass extinction. They evolved into bipedal and highly agile predators.

Euperkeria and Ornithosuchus fossils were unearthed from South Africa and Europe. They were about 2 ft long bipedal lizard-like animals with small head but very long tail for balancing while they chased flying insects by rapid running. Endothermy must have evolved in thecodonts to meet the extraordinary energy demands of their predatory life style.

5.       THE SAURISCHIANS

They were dinosaurs with lizard-like pelvic girdle in which ischium and pubis bones radiated away from each other. They were both bipedal and quadrupedal and carnivores as well as herbivores.

6.       THE ORNITHISCHIANS

They were dinosaurs with bird-like pelvic girdle in which ischium and pubis bones were directed towards posterior as found in modern birds. These were also highly diversified carnivores as well as herbivores and both bipedal and quadruped.

7.       THE PTEROSAURIA

They were flying or gliding dinosaurs of Mesozoic that varied in size from sparrow-sized to some species, like Pteranodon, having a wing span of 8 meters. They had pneumatic bones. Last digit of the fore limb was extraordinarily long and served to attach the membranous patagium between fore limb, hind limb and the body. Hind limbs were used for clinging on to the rocks and cliffs and 3 digits of fore limbs also had curved claws, an adaptation for clinging. Their jaws were modified into beak that possessed homodont dentition but Pteranodon did not have teeth

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[1] The carboniferous period is famous for its vast swamp forests. The swamps produced coal from which the term carboniferous is derived. It lasted from about 359.2 to 299 million years ago.

[2] The eggs of reptiles are macrolecithal, they contain large amount of yolk for development of embryo into miniature adults which can feed and defend themselves.

[3] The amniotic egg of reptiles and birds is surrounded by a tough outer shell that protects the egg from predators, pathogens, damage and from drying. Oxygen passes through tiny pores in the shell, so embryo doesn’t suffocate. Inside the shell are four sac. The first sac inside the shell is chorion, which carries oxygen from the shell to the embryo and waste carbon dioxide from the embryo to the shell. Within the chorion is amnion, the membrane for which the amniotic egg is named. The amnion keeps the embryo from drying out, so it’s critical to living on land. A third sac, the allantois, stores wastes from the embryo and also fuses with the chorion to form the chorioallantoic membrane, ehich carries oxygen and carbon dioxide to and from the embryo, just like lungs. A fourth membrane, the yolk sac, holds and digests nutritious yolk for the developing embryo.

[4] Monocondylic skull has one occipital condyle in skull, it provides high degree of movement.

[5] Two atria and one ventricle. The two atria and one partially divided ventricle. There is a mixing of oxygenated and de-oxygenated blood because the ventricle is not split completely.

[6] Birds lay hard-shelled eggs, but most reptiles lay soft-shelled eggs. Bird’s eggs are incubated by body heat, but reptile eggs are incubated by natural heat. The reptiles eggs are hidden, thus are all white. The birds eggs are incubated in nests and are exposed thus show colouring and camouflage. * amniotic egg