What Major Extinctions Took Place Before The Dinosaurs?

Question: What major extinctions took place before the dinosaurs?

Answer:

Ordovician-Silurian and Late Devonian.

To date, five major global extinctions have occurred on planet Earth. Mass extinction occurs when approximately 75% of all species die off over a period of fewer than 2.8 million years. The extinction of dinosaurs occurred about 253 million years ago, but two mass extinctions happened before these creatures roamed the planet: Ordovician-Silurian and Late Devonian.

What Major Extinctions Took Place Before The Dinosaurs?

Ordovician-Silurian Extinction

Occurring about 440 million years ago, this was the first mass extinction on Earth. About 85% of species expired. At the time, organisms weren’t land-dwelling and only resided in water. 

A change in the climate caused a drastic change in sea temperatures. This resulted in the death of most sea organisms, such as trilobites, shelled brachiopods, coral, and bivalves.

A mass glaciation caused a drastic drop in sea levels, causing habitats and food chains to be destroyed. The cause of this glaciation is unknown. 

Some scientists believe that the formation of the North American Appalachian mountains caused a cooling process that eventually resulted in mass glaciation. Erosion of the mountain range’s silicate rocks eliminated carbon dioxide from the atmosphere. 

A second theory involves a decrease in oxygen. This caused toxic metals to dissolve into the oceans, resulting in the extinction of sea life. 

Some scientists believe a supernova’s gamma-ray created a hole in the ozone layer. Ultraviolet radiation emitted through the hole resulted in the first mass extinction. 

Other scientists believe that the eruption of molten larvae onto the earth’s surface (volcanism) was the cause. 

Late Devonian Extinction

Also known as the ‘Age of Fish’, the Devonian period occurred about 370 million years ago and included a large variety of prehistoric marine life. The second mass global extinction resulted in the demise of 75% of all species. 

During this age, some species had evolved into land-dwelling animals, although most still lived in the ocean, e.g., the Dunkleosteus was a large, 33-foot-long, armored predator of the subtropical oceans.

It is thought by some scientists that trees, flowers, and other vascular plants were the main factors in the Late Devonian extinction. Initially characterized by rock, the land was transformed into soil as plants’ root systems evolved. As soil washed into the oceans, its high nutrient levels caused algae to grow in great numbers. 

The algae depleted the oxygen levels in the water. This destroyed food chains and resulted in the death of marine life that were unable to adapt to the lowered oxygen levels and food supply. 

Not all scientists agree with this theory. It is also believed that volcanic eruptions caused a dramatic decrease in oxygen levels in the water. This resulted in the death of much marine life.