Ordovician era (jan 1, 443800000 BC – jan 1, 48540000 BC)

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Introduction
The Ordovician Period, is the second period of the Paleozoic Era. It began 485.4 million years ago, after the Cambrian Period, and ended 443.8 million years ago, when the Silurian Period began. Ordovician rocks mainly occur at the highest elevation on Earth, many are at the top of Mount Everest. This period had a rich variety of marine life that flourished in the vast seas and the first primitive plants began to appear on land
In the late Ordovician period, a mass extinction eliminated around 85% of marine species. It was the second largest extinction in earth's history. Most research suggests that this was caused by a cooling climate or changes in oxygen lived in the ocean, one study suggests that it was triggered by a gamma-ray burst.

A example of an extinct animal from Ordovician era is Helicoprion. Helicoprion is an extinct genus of shark-like eugeneodont fish. Almost all fossil specimens are of spirally arranged clusters of the individuals' teeth, called "tooth whorls", which in life were embedded in the lower jaw. As with most extinct cartilaginous fish, the skeleton is mostly unknown. Alexander Karpinsky discovered the first Helicoprion in 1899 in Russia, he imagined the whorl as a fused-together coil of teeth that curled up over the shark's snout.

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