Evolution: Amniotic Egg Allowed Animals to Move Onto Land

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An artist's recreation of Hylonomus lyelli, a tiny reptile that lived more than 300 million years ago. ( Matteo De Stefano / MUSE via Wikicommons under CC BY-SA 3.0)

Tiny Animals Trapped in Fossil Trees Help Reveal How Fauna Moved Onto Land

by Hilary Maddin The Conversation Smithsonian.com 

Over 150 years ago, geologist Sir William Dawson made an astounding discovery in the Joggins Cliffs, along the shores of Nova Scotia’s Bay of Fundy. Within the lithified remains of a giant tree-like fern were the bones of a tiny, 310 million-year-old animal.

This animal was unlike any other seen thus far. It was able to venture where no vertebrate (back-boned) animal had ventured before, deep into the lycopsid forests, away from the water’s edge. This was all thanks to an evolutionary innovation: the amniotic egg.

Although animals had previously ventured onto land in the earlier Devonian Period, animals with an amniotic egg—such as modern reptiles, birds and yes, even mammals—do not need to return to the water to reproduce, as modern amphibians still do. The amniotic egg is a self-contained pond, where the embryo and all its food and waste are stored surrounded by a protective, desiccation-resistant shell.

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