Polished nautiloid fossils make unusual gemstones.
During the Ordovician Period 510 to 438 million years ago, a vast, shallow, inland sea covered most of North America, including Indiana, according to the Indiana Geological Survey. At the bottom of this sea, sedimentary deposits led to the formation of a wide variety of fossils that represent many different types of life present in that ancient sea.
Nautiloids
Nautiloids are a group of invertebrate aquatic animals that, along with modern-day octopuses and squid, are cephalopods. Cephalopods first appeared in the Cambrian Period 570 to 500 million years ago; the nautiluses were first seen in the Ordovician Period. Nautilus populations increased rapidly in diversity until the Devonian Period 410 to 360 million years ago, when they began to decline, notes a writer on the Indiana Geological Survey website. The Ordovician nautiloid fossils found in Indiana are the shells in which the animal lived. As nautiloids grew, the animal added new chambers to its shell, resulting in a cone-shaped shell with many compartments. In subsequent time periods, the nautiloids began to develop curved, and then coiled, shells which are similar to the nautiloids living today.
Conostichus
Conostichus is a cone-shaped fossil that was first described in 1876 by paleobotanist Leo Lesquereaux. This fossil is radially symmetrical and was first thought to be a marine plant, but when no internal structures were detected, it was instead determined that the fossil was not a plant, but a mold of the burrow of a sea anemone, according to the Indiana Geological Survey. It is thought that sand collected in these burrows and then became cemented in place by iron compounds. The mold was later released when the shale in which it formed eroded faster than the mold itself.
Foraminifera
The most abundant fossils found in the limestone of the Salem Formation of Indiana are the Foraminifera. These are single-celled, shelled protists that appear as small, smooth grains, only revealing the complex, chambered structure of their shells upon microscopic examination.
Bryozoans
Bryozoans are aquatic organisms, generally smaller than a millimeter in size, that live in colonies. While they encrust hard surfaces and also form branching colonies, the most common formation in the limestone of Indiana are the fan-like fenestrate bryozoans that left lacy, net-like fossils.
Crinoids
Crinoids are animals in the Echinoderm class, which also includes sea stars. Crinoids consist of a cup-shaped body with five or more feathery arms and a stem to hold the organism in one place. The disc-shaped ossicles that formed the stems of the crinoids would often disarticulate and fossilize, making them another common Indiana fossil.
Tags: Geological Survey, Indiana Geological, Indiana Geological Survey, million years, Period million, Period million years
