Goniatite

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Goniatite
Drawing of a goniatite fossil displaying characteristic 'zigzag' sutures
Conservation status
Extinct (fossil)
Scientific classification
Kingdom: Animalia Phylum: Mollusca Class: Cephalopoda Subclass: Ammonoidea Order: Goniatitida Hyatt, 1884
Families
all extinct

Goniatites are an extinct group of ammonite, which are related to the nautiloids. They lived early in the Devonian period, some 400 million years ago, and became extinct during the Permian period.

The goniatites are classified as ammonoids, an extinct group that includes the ammonites. The ammonoids are part of a larger group - cephalopods, which includes belemnites and nautiloids. The cephalopods are part of a larger group of animals called the molluscs (phylum mollusca), which also includes gastropods and bivalves.

The goniatites all possessed an external shell, which is divided internally into chambers. The animal lived in the largest of the external chambers, and the internal chambers would have been filled with gas, making the animal buoyant in the water. The general structure of the goniatites would have been similar to that of their relatives the ammonites, being a free swimming animal possessing a head with two well developed eyes and arms (or tentacles).

Goniatites are small to medium in size, often being less than 15 centimetres in diameter (6 inches). The shell is always coiled, unlike that of the later ammonites of the Mesozoic era, some of which evolved into partially coiled or completely uncoiled forms (called heteromorphs). The shape of most goniatite shells suggests that they were poor swimmers.

The thin walls between the internal chambers of the shell are called the septa, and as the goniatite grew it would move its body forward in the shell secreting septa behind it, thereby adding new chambers to the shell. The sutures (or suture lines) are visible as a series of narrow, wavy lines on the surface of the shell. The sutures appear where each septa contacts the wall of the outer shell.

A distinctive feature of the goniatites is the "zigzag" pattern of their sutures. The sutures of nautiloids are by comparison somewhat simpler, being either straight or slightly curved.

Goniatites are found in North America, Europe, North Africa and Australasia. Certain limestones in western part of the Republic of Ireland are packed with beautifully preserved goniatite fossils. They are also found in marine bands of the Carboniferous coal measures in Europe, and in marine rocks of the Pennsylvanian period in Arkansas. Large numbers of goniatites occur in rocks from the Devonian period of Morocco, and they are important zone, or index fossils used in dating the rocks of that period.

[edit] See also

• Baculite
• Orthoceras
• Lituites