Age of the Dinosaurs I: Early Reptiles

The first reptiles started appearing in the Carboniferous period. They were small lizard-like creatures. It’s not exactly known how they evolved, but their advantage over amphibians is obvious, as they didn’t need to return to the water to procreate. (1) This caused them to be able to conquer land in a way not seen before.

One of the earliest species of reptiles was Hylonomus, the oldest known ancestor of crocodiles and dinosaurs, and eventually birds. During this period another early ancestor started to appear, but of an entirely different group. Synapsids, a group of egg-laying, vertebrate animals, eventually developed into mammals. (1)


1. Artist impression of Hylonomus lyelli, an early reptile from the Late Carboniferous of Nova Scotia, Canada

During the Permian period all land formed one great continent known as Pangaea. In many areas had a harsh and dry climate, giving the advantage to reptiles and synapsids over amphibians. Both groups of terrestrial animals flourished, but for unknown reasons synapsids were more successful and made up 75 % of vertebrate, four legged terrestrials. (1)

Then the climate began to change and synapsids started spreading from the north in southern direction. One group of these early synapsids was called pelycosaurs. They resembled large lizards with strong jaws. During the Permian period this group started to evolve into therapsids, who were an extremely varied group with many different shapes and sizes. This group contained the first predators. The Permian period also saw the evolution of the two main classes of reptiles: diapsids and anapsids. (1)


2. Artist impression of Pristerognathus vanderbyli, a member of the therapsids, from South Africa

The end of the Permian period was around 248 million years ago. One of the most massive extinctions ever took place then, and over 90 % of all species went extinct. The extinction hit the oceans the hardest, as they warmed up around 95 % of all species living in the oceans died out. Among them were large groups of coral species and trilobites. Land species fared a little better, ‘only’ 70 % died out. (1)

There was no clear cause for this extinction. What we know is that there were significant climate changes. The oceans withdrew and shallow inner seas dried up, which caused a lot of the oceanic creatures to go extinct. Also, due to volcanic activity there was a lot of dust and carbon dioxide in the atmosphere, causing warming of the Earth as well as a lowered oxygen content in sea water. (1) All of this together was too much for most species living at the time.

The time of the early reptiles ended soon after this mass extinction. A group of reptiles called archosaurs, members of the diapsids, became dominant during the Triassic Period and from this group the dinosaurs would evolve, giving rise to the Age of the Dinosaurs. (1)


References:

1. Ackroyd, Peter, The Beginning, 2003, Dorling Kinderley, London (page 49-60)

Picture credits:

All pictures from Wikipedia Commons