Taxonomy of the Treatise on Invertebrate Paleontology In some classifications, these are left as suborders, included in only three orders: Goniatitida, Ceratitida and Ammonitida. Ammonitida, Lower Jurassic – Lower Paleocene.Ceratitida, Upper Permian – Upper Triassic.Prolecanitida, Upper Devonian – Upper Triassic.Goniatitida, Middle Devonian – Upper Permian.Agoniatitida, Lower Devonian – Middle Devonian.The Ammonoidea can be divided into six orders, listed here starting with the most primitive and going to the more derived: Iridescent ancient ammonite fossil on display at the American Museum of Natural History, New York City, around 2.5 feet in diameter This suture type is characteristic of Jurassic and Cretaceous ammonoids, but extends back all the way to the Permian. Ammonoids of this type are the most important species from a biostratigraphical point of view. Ammonitic – lobes and saddles are much subdivided (fluted) subdivisions are usually rounded instead of saw-toothed.This suture pattern is characteristic of Triassic ammonoids and appears again in the Cretaceous "pseudoceratites". Ceratitic – lobes have subdivided tips, giving them a saw-toothed appearance and rounded, undivided saddles.This pattern is characteristic of the Paleozoic ammonoids. Goniatitic – numerous undivided lobes and saddles typically 8 lobes around the conch.Three major types of suture patterns are found in the Ammonoidea: Lobes and saddles which are so far towards the center of the whorl that they are covered up by succeeding whorls are labelled internal lobes and saddles. Additional lobes developing towards the inner edge of a whorl are labelled umbilical lobes, which increase in number through ammonoid evolution as well as an individual ammonoid's development.
The lateral region involves the first saddle and lobe pair past the external region as the suture line extends up the side of the shell. On suture diagrams the external saddle is supplied with an arrow which typically points towards the aperture. The external saddle lies directly on the lower midline of the shell and is edged by external lobes. The external or ventral region refers to sutures along the lower (outer) edge of the shell, where the left and right suture lines meet. While nearly all nautiloids show gently curving sutures, the ammonoid suture line (the intersection of the septum with the outer shell) is variably folded, forming saddles ("peaks" that point towards the aperture) and lobes ("valleys" which point away from the aperture). Regions of the suture line and variants in suture patterns The topology of the septa, especially around the rim, results in the various suture patterns found. Often, the name of an ammonite genus ends in - ceras, which is from κέρας ( kéras) meaning "horn".Īmmonites (subclass Ammonoidea) can be distinguished by their septa, the dividing walls that separate the chambers in the phragmocone, by the nature of their sutures where the septa join the outer shell wall, and in general by their siphuncles.Īmmonoid septa characteristically have bulges and indentations and are to varying degrees convex when seen from the front, distinguishing them from nautiloid septa, which are typically simple concave, dish-shaped structures. 79 AD near Pompeii) called fossils of these animals ammonis cornua (" horns of Ammon") because the Egyptian god Ammon ( Amun) was typically depicted wearing rams' horns. The name "ammonite", from which the scientific term is derived, was inspired by the spiral shape of their fossilized shells, which somewhat resemble tightly coiled rams' horns. Their fossil shells usually take the form of planispirals, although some helically spiraled and nonspiraled forms (known as heteromorphs) have been found. The earliest ammonites appeared during the Devonian, with the last species vanishing during the Cretaceous–Paleogene extinction event.Īmmonites are excellent index fossils, and linking the rock layer in which a particular species or genus is found to specific geologic time periods is often possible. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) than they are to shelled nautiloids such as the living Nautilus species. Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda.
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