Amphibians & Reptiles

Amphibians include frogs, newts and salamanders. They show their evolutionary origins as they develop from eggs through to adulthood. The midwife toad Alytes obstetricans exhibits an unusual version of this process in that the male collects the eggs on his back and cares for them until they hatch into tadpoles. Amphibians lay their eggs in water, just like their ancestors, the fish. Their descendants, the reptiles, evolved eggs with a waterproof coating that allowed them to be laid on land away from the numerous fish predators of the ancient rivers.

Reptiles occupy a wide variety of habitats, mostly in warmer areas of the world. The turtles and tortoises were around long before the dinosaurs and have shells as a very effective defence against predators. The giant tortoises of the Galapagos Islands off the coast of Ecuador sparked the imagination of Charles Darwin in 1835 when the local governor explained that animals on each island had a distinct shell shape. This example of the subspecies Geochelone ephippium elephantopus from Pinzon (Duncan Island) has a high front, allowing the tortoise to stretch its neck steeply upwards to reach the shrubs on which it feeds.

While some reptiles have ancient fossil histories, others are highly advanced. The chameleons, such as Bradypodion pumilum, can change colour to blend in with their backgrounds. They move slowly, swaying as if they were leaves in the breeze, to get within a tongue’s reach of their insect prey. Lizards show a great variety of behaviour; the armadillo lizards, such as Cordylus giganteus, roll up to expose their spiny skins. This puts off many a potential predator.

Snakes are highly evolved reptiles. The rattlesnake Crotalus viridus is a classic predator. Dried scales at the end of the tail produce the noise that gives this famous snake its common name. The tongue flicks out to taste the air for the scent of prey, while the head is held back in an S-shaped coil, ready to strike out and deliver a fatal bite. The fangs are thrust out as the snake opens its mouth, and the pressure on poison glands in the roof of the mouth delivers a fatal dose to small prey. The windpipe extends to the front of the mouth, allowing the rattler to swallow the drugged prey head first while still being able to breathe.

The crocodiles and their relatives have been successful predators for hundreds of millions of years. Among these, the gharial Gavialis gangeticus is specialised for stealth. The skull shows the lump of bone at the end of the snout, which supports the nostrils. Only these and the eyes are above water, allowing the gharial to breathe and search for prey without being spotted.

The evolution of vertebrates is a long story. Ancient species of crocodiles and turtles witnessed the rise and fall of dinosaurs. The only close dinosaur relatives to survive the extinction of 65 million years ago were the birds.



 
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