Fossil Representative of Indiana
Nautiloid (common name)
Nautiloids are the most primitive of the diverse taxonomic class of marine mollusk called cephalopods. Cephalopods are defined by their bilateral symmetry, prominent head, and arms or tentacles. Extinct cephalopods include ammonoids and belemnites. Modern cephalopods, all descendants of a species of nautiloid from the Cambrian period, include squid and octopus. In addition to the existing species, the subclass includes thousands of shelled organisms known only by fossils, and possibly some that never fossilized.
A STATE SYMBOL
Nautiloid fossils are commonly found in Paleozoic rock. The Cincinnati Arch region of Ohio, Kentucky, and Indiana contains abundant fossilized nautiloids from the middle and late Ordovician period of the Paleozoic era, which makes them approximately 450 to 520 million years old. Fossils were deposited in this area by a shallow, inland sea that once covered most of North America. Indiana has not designated an official state fossil; however, the nautiloid, found in the state’s Ordovician sea deposits, represents the state’s geology and paleontology well.
The word nautiloid is derived from the Greek word ναυτικός (nautilus), which means "sailor." The word "cephalopod" comes from the Greek words κεφάλι (kefali), meaning "head," and πόδι (podi), meaning "foot." The tentacles or arms protruding from the heads of cephalopods are what gave them this name.
Characteristics that distinguish nautiloids are their external shells, their simple chamber walls called "septa," and a tube of tissue called a "siphuncle." As it grows a new chamber, the organism secretes a new septum and its body resides in the last chamber, the living section. The siphuncle connects the living section with the previous ones. These chambers are called "camerae."
Fossil shells are straight, curved, or coiled. Normally about one foot (30.5 cm) in diameter, straight-shelled nautiloids are estimated to have reached lengths of up to 20 or 30 feet (6.1 to 9.1 m). Nautiloids are thought to have been fierce predators. They sometimes employed jet propulsion for movement, but probably mainly used their many tentacles, up to 100 of them, to swim along the bottom of the ocean and search for prey. Their diet consisted of fish, shellfish, and other marine invertebrates.
Nautiloids first appeared approximately 500 million years ago, in the Cambrian period of the Paleozoic era. They were most abundant in the Ordovician, Silurian, and Devonian periods of about 370 to 520 million years ago. These cephalopods evolved into a wide variety of organisms with different shell types, shapes, and structures. Nautiloids began to decline in numbers in the Devonian period of the Paleozoic era, approximately 400 million years ago. Evolving and diversifying, some species of nautiloids survived into the Mesozoic era. Species alive today include nautilus, cuttlefish, squid, and octopi.
The earliest nautiloid fossils known were found in northeastern China and date from the late Cambrian period. In addition to this location and the north-central United States, fossilized nautiloids have been found in the United Kingdom, Morocco, and off the Baltic coast.
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