Past Cobb-L-Stones Articles

One of the greatest climate shifts ever recorded on Earth occurred during the Ordovician Period, the second of six periods of the Palezoic Era (Age of invertebrates). The oceans had an average temperature of over 110 degrees F, as the Period began 500 million years ago, and nearly 40 million years later at the end of the period the Earth was experiencing a major ice age. The ice age had severe consequences that caused over 60% of the different (genera) marine organisms to go extinct. The Earth has experienced five events of mass extinction and the event at the end of the Ordovician is the second largest of the five.

What causes such a climate shift? It is blamed on a large drop in the amount of carbon dioxide gas in the atmosphere; a gas that traps heat on Earth. During the previous period (the Cambrian) widespread volcanic eruptions had released large volumes of carbon dioxide into the air. The concentration of carbon was nearly 20x greater than our current value. The dramatic drop in CO2 (resulted in a major loss of heat and the ice age) came about as the gas dissolved into the early ocean where it combined with calcium to form thick beds of limestone on the ocean floor of the Ordovician. Limestone is the most common rock made during the Ordovician.

A dramatic change in life (and fossils) came about because of the growing calcium carbonate content of the ocean. Those organisms that secrete shells made of calcite had an explosion in their populations. The top predator of the ocean became the nautilus. The Giant Orthocone grew to reach 35 feet in length, and was called the “great white shark of its time”. The arch enemy of the nautilus was the sea scorpian called Eurypterid, which grew to be over eight feet long.

The higher temperatures of the first half of the period melted nearly all ice from the poles, sea level rose, and the ocean covered much of the USA in a warm shallow sea which provided a vast area for these creatures to exist. But as time when by and the temperatures dropped, a large continent called Gondwana (consisting of Africa joined to South America, Australia and India) drifted directly over the south pole. A vast ice sheet several miles thick formed on the continent and trapped much of Earth’s water on land, causing a dramatic drop in sea level. The loss of the shallow sea over the USA was the major cause of the mass extinction of the carbonate loving organisms, which could not adapt to living in the cold, deeper waters off the continental shelf.

Early nautiloids had straight shells, and slabs of multiple Ordovician (and Devonian) nautiloids from the Atlas Mountains in southern Morocco are commonly sold at mineral shows. Unfortunately, nautiloids swim backwards by releasing jets of water from their large openings, and straight nautiloids had difficulty swimming in a straight line. They soon evolved curved shells for better swimming, and the most distinctive Ordovician nautiloids are those having an early formed curved shell and a straight living chamber.