What were some of the earliest known reptiles?
Two of the earliest known reptiles, the Hylonomus and Paleothyris, both descended from amphibians during the Middle Carboniferous period of the Paleozoic era.
Just like this paddle tail newt, ancient amphibians survived on both land and sea; they were the first animals to survive for extended periods outside the water (iStock).
The best evidence of the change from amphibian to reptile was the early reptiles high skulls evidence of additional jaw muscles and thicker egg shells. The Hylonomus still claims the prize (so far) as the oldest-known reptile and lived about 315 million years ago. The Paleothyris evolved about 300 million years ago. The fossils of both these reptiles were found near Nova Scotia, Canada, in ancient tree stumps. Apparently, the animals fell into tree stumps in pursuit of insects or worms. There they were trapped and eventually died.
Why did the reptiles dominate during the Mesozoic but not the amphibians?
Besides the ability to not depend on water as much as amphibians, there are probably two main reasons why reptiles became dominant in the Mesozoic. First, reptiles developed adaptations in their skeletal structure, allowing them to move much quicker than amphibians. Second, during the Permian period the climate became hotter and drier, and many water sources disappeared. The reptiles new adaptations from the development of scales to retain water to eggs that could survive without staying in water allowed them to thrive where amphibians could not.
Did some reptiles return to the oceans?
Yes, as the reptiles spread out over the land, some of them returned to the water. Over a period of time, they evolved and adapted to the water again. Their legs gradually evolved back into fins and flippers; eyes adapted to seeing underwater; and bodies became streamlined for better speed in the water. In addition, they could no longer lay their eggs on land. Thus, they evolved a way of producing living young within their bodies, a process called ovoviviparous. The Ichthyosaurs, or fish lizards, were the most fishlike true reptiles.
Though it looks much like modem fish do today, the Ichthyosaur was still a reptile, and one of the first true reptiles to live in the water exclusively (iStock).
How are reptiles grouped?
During the 100 million years after the first reptiles appeared, various reptile lines continued to evolve. Today, it is difficult to find agreement about reptile classification. In most cases, they are divided into four living orders (the others have died out over time):
Crocodilia Crocodiles, alligators, gharials, and caimans, comprising 23 known species.
Squamata Lizards, snakes, and the worm lizards, or amphisbaenids, which make up about 7,900 species.
Testudines Turtles and tortoises, which includes about 300 species.
Sphenodontia The endangered tuatara, which can only be found in New Zealand and consists of two species.
There is also another older method of grouping reptiles: subclasses according to the positioning of the temporal fenestrae, or the openings in the sides of the skull behind the eyes: the anapsids, synapsids, diapsids, and euryapsids. The anapsids had no openings in the skull and eventually evolved into todays turtles and tortoises. The synapsids, or same hole, had a low skull opening, and were once thought to be the ancestors of modern mammals (and are now not considered to be true reptiles). It was the animals of the diapsid line, or two skull openings, that eventually gave rise to the dinosaurs. Another debatable line is the euryapsids, characterized by a single opening on the side of the skull, which are now usually included with the diapsids.