Goniatite

Taxobox
name = Goniatite
fossil_range = fossil range|400|251.4Early Early Devonian - Late Permian


image_width = 155px
image_caption = Drawing of a goniatite fossil displaying
characteristic 'zigzag' sutures
regnum = Animalia
phylum = Mollusca
classis = Cephalopoda
subclassis = Ammonoidea
ordo = Goniatitida
ordo_authority = Hyatt, 1884
| subdivision_ranks = Suborder
subdivision =
* Goniatitina
* Tornoceratina

Goniatites are an extinct group of ammonoid, which are shelled cephalopods related to squids, belemnites, octopuses, and cuttlefish, and more distantly to the nautiloids. Early in the Devonian period, some 400 million years ago, goniatites originated from within the more primitive anarcestine ammonoids. Surviving the Late Devonian biotic crises, goniatites flourished during the Carboniferous and Permian periods and finally became extinct in the Permian–Triassic extinction event at the close of the Paleozoic Era some 251.4 million years ago. They were survived by their cousins the ceratite ammonoids, also descendants of the anarcestine ammonoids.

Morphology

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, almost always being less than 15 centimetres (6 inches) in diameter and often smaller than 5 centimetres (2 inches) in diameter. The shell is always planispirally 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, whereas later ammonoids showed suture patterns of increasing complexity. One explanation for this increasing extravagancy in suture pattern is that it leads to a higher strength of the shell.

Ecology

Ecologically, goniatites were limited to environments of normal-marine salinity -- as appears to be the case for all cephalopods throughout their history. Goniatites are much more abundant and speciose in rocks understood to represent cratonic (also called epicontinental or inland) sea sediments than they are in rocks understood to represent open ocean sediments. Within these inland seas, goniatites' greatest abundance and diversity appears to have been achieved in offshore deep ramp and basinal environments rather than in nearshore environments. Known nearshore (e.g., lagoonal) occurrences have generally been ascribed to wash-in of shells from offshore waters.

Due to lack of strong evidence for any particular life mode (e.g., nektonic, planktonic, demersal, planktivorous, piscivorous), it remains unclear what resources goniatites were capitalizing on in these offshore environments. Only a few goniatites' full trophic apparatuses have ever been described, and reports of stomach contents in these creatures' fossils remain questionable at best. However, goniatites clearly lacked the calcified jaw apparatuses developed later in ammonoid history by the ammonites; this has been cited as evidence against a durophagous (shell-crushing) diet for goniatites.

Distribution

Goniatites are found in North America, Europe, North Africa and Australasia. However, they seem to occur mostly in areas which at the time would have been tropical to subtropical. Most any fossil-bearing limestone or shale from inland seas of the late Paleozoic tropics or subtropics is likely to yield some goniatites. In the USA, such rocks are found from Maine, New York, and Virginia and in every state west to Nebraska and south to Texas and Alabama; as well as in parts of almost every western state (with North Dakota, Oregon, Washington, and Hawai'i as the exceptions).

Notable goniatite occurrences include the following: 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.

ee also

* Baculite
* Orthoceras
* "Lituites"