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In the most recent ice age which lasted from two million to 10,000 years ago, vast seas of ice--glaciers--covered much of Indiana. As they advanced into southeastern Indiana, the glaciers scoured the land, exposing the sediments left by a much older, shallow, inland sea of the Ordovician Period (510 to 438 million years ago).
This sea covered most of North America, including Indiana, during the Ordovician. In southeastern Indiana, deposits from this sea are most easily viewed along road cuts and in stone quarries. In fact, if you drive one mile south of Richmond on U.S. 27 you can see a large road cut exposing a swath of the ancient Ordovician sea bottom. The road cut reveals what these seas were like over 400 million years ago.
Families of organisms were increasing, making the seas richer in invertebrate life. Among those that flourished in diversity and sophistication were the marine predators, including one of the most ferocious--nautiloids, shelled marauders of the shallow seas.
Nautiloids belong to a group of invertebrate animals called cephalopods ("head-foot"), relatives of the familiar octopuses and squid. Cephalopods originated in the Cambrian Period (570 to 500 million years ago), and nautiloids originated in the Ordovician.
Nautiloids were very successful and grew rapidly in number of species in the Ordovician and Silurian Periods (the Silurian ended 410 million years ago). Then, in the Devonian (410 to 360 million years ago), their diversity began to decline.
They suffered large extinctions at the end of the Triassic Period (205 million years ago) and again at the end of the Miocene Epoch (5 million years ago). Today, only six species of nautiloids remain--the chambered or pearly nautiluses.
Like nautiloids of today, Ordovician nautiloids were animals that lived in a shell created by the creature's bodily secretions. As the nautiloids grew they "remodeled," secreting a new, larger living chamber which was added on to the smaller, old living chamber.
The evolutionary process resulted in a changing nautiloid form. In the Ordovician Period nautiloids had straight shells. Because the size of the chambers increased as the animal grew--sometimes to 15 feet in length--the straight shell took on a cone-like shape. Nautiloid shells of the Silurian were curved, while those in the Devonian became coiled. In present-day nautiloids, the shell twists around itself in beautiful whorls as the animal adds larger and larger chambers.
Can you think of a factor that could have contributed to the evolution of nautiloid shells from straight to curved to coiled? (Hint: Would you rather move around, feed and reproduce with a barrel tied to your back or a fifteen-foot length of stove pipe?)
The animal would extend its many tentacles from the large end of the nautiloid "cone" and swim along above the ocean bottom, searching for prey such as trilobites. Present-day octupi have eight tentacles, but nautiloids in the Ordovician had many more--perhaps as many as one hundred. Imagine being a trilobite--an ancient arthropod that looked like a pill bug--and seeing a 15-foot-long, coned nautiloid with 100 tentacles reach out to grasp, subdue and devour you. With this image in mind you can appreciate that these nautiloids were fearsome creatures, indeed!
At the end of the Ordovician, there was a dramatic extinction in which 100 families of marine invertebrate animals disappeared. This extinction was second in scope only to that at the end of the Permian Period (225 million years ago) in which 90 to 95 percent of all marine species vanished. The Permian extinction is thought to have resulted from greenhouse, gas-induced global warming.
But why was there a sudden decrease in species diversity at the end of the Ordovician? One contributing factor may have been that glaciation, resulting from the Ordovician cooling trend, caused sea levels to drop as more and more water was deposited in the ice sheets. As sea levels dropped, the size of the seas decreased, leaving less space in which marine species could live.
This marine day trip is brought to you by the Indiana Geological Survey:
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