Ammonoids are extinct, typically coiled-shelled cephalopods composing the subclass Ammonoidea. They are more closely related to living octopuses, squid, and cuttlefish (which compose the clade Coleoidea) than they are to nautiluses (family Nautilidae), which their shells resemble. The earliest ammonoids appeared during the Emsian stage of the Early Devonian, around 410-408 million years ago, with the last species vanishing during or soon after the Cretaceous–Paleogene extinction event approximately 66 million years ago. They are often called ammonites, which is most frequently used for members of the order Ammonitida, the only remaining group of ammonoids from the Jurassic up until their extinction.
Ammonoids exhibited considerable diversity over their evolutionary history, with over 10,000 species having been described. Ammonoids are excellent index fossils, and they have been frequently used to link rock layers in which a particular species or genus is found to specific geologic time periods. Their fossil shells usually take the form of planispirals, although some helically spiraled and nonspiraled forms (known as heteromorphs) have been found, primarily during the Cretaceous period. Because ammonites and their close relatives are extinct, little is known about their way of life. Their soft body parts are very rarely preserved in any detail. Nonetheless, much has been worked out by examining ammonoid shells and by using models of these shells in water tanks.
Etymology 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. Pliny the Elder ( 79 AD near Pompeii) called fossils of these animals ("horns of Ammon") because the Egyptian god Ammon (Amun) was typically depicted wearing rams' horns.NH 37.40.167 Often, the name of an ammonite genus ends in -ceras, which is from ancient Greek () meaning "horn".
Classification
Orders and suborders The Ammonoidea can be divided into six orders, listed here starting with the most primitive and going to the more derived: Agoniatitida, Lower Devonian – Middle Devonian Clymeniida, Upper Devonian Goniatitida, Middle Devonian – Upper Permian Prolecanitida, Upper Devonian – Upper Triassic Ceratitida, Upper Permian – Upper Triassic Ammonitida, Lower Jurassic – Lower Paleocene
In some classifications, these are left as suborders, included in only three orders: Goniatitida, Ceratitida and Ammonitida. The classification of ammonoids is based in part on the ornamentation and structure of the septa comprising their shells' gas chambers.
Taxonomy of the Treatise on Invertebrate Paleontology The Treatise on Invertebrate Paleontology (Part L, 1957) divides the Ammonoidea, regarded simply as an order, into eight suborders, the Anarcestina, Clymeniina, Goniatitina and Prolecanitina from the Paleozoic; the Ceratitina from the Triassic; and the Ammonitina, Lytoceratina and Phylloceratina from the Jurassic and Cretaceous. In subsequent taxonomies, these are sometimes regarded as orders within the subclass Ammonoidea.
Evolutionary history
Ammonoids are widely thought to have originated from straight-shelled (orthocone) "nautiloids" belong to Bactritida during the early Devonian (Emsian) around 409-408 million years ago, with transitional fossils showing the transition from a straight shell, to a curved (cyrtoconic) shell to a relaxed (gyroconic) spiral and finally to a tight spiral.De Baets K. 2012. EARLY EMSIAN AMMONOIDEA: STRATIGRAPHY, INTRASPECIFIC VARIABILITY AND MACROEVOLUTION Ph.D. thesis, Universität ZürichKlug, C. and Korn, D. 2004. The origin of ammonoid locomotion. Acta Palaeontologica Polonica 49 (2): 235–242 The Late Devonian mass extinction (Kellwasser event) at the end of the Frasnian led to a dramatic decline in ammonoid diversity, with only a handful of lineages belong to Tornoceratina (a subgroup of Goniatites) surviving, becoming ancestral to all later ammonoids. Ammonoids rediversified during the following Famennian, which also saw the radical shift of the siphuncle from a lower (ventral) to upper (dorsal) position. Ammonites were nearly completely exterminated by the Hangenberg Event (end-Devonian mass extinction) at the end of the Devonian, with only a handful of lineages surviving, with one of the surviving goniatite lineages becoming ancestral to all post-early Carboniferous and later ammonoids. Ammonoids again rediversified during the Early Carboniferous. During the Carboniferous ammonoids underwent alternating periods of diversification and decline, and during the late Carboniferous ammonioid diversity became concentrated in a few geographical regions.
During the Permian, the Capitanian mass extinction event severely reduced the diversity of Goniatitida and Prolecanitida, while the Ceratitida, which originated during the Middle Permian, likely from the Daraelitidae, was largely unaffected and radiated in the Late Permian, becoming the dominant group of ammonoids in this period represented by two groups, the araxoceratids and xenodiscids. The end-Permian mass extinction again reduced ammonoids to the verge of extinction, though both main ceratitd lineages survived, though the xenodiscids were more successful and ancestral to all later ammonoids.
Ammonites were devastated by the end-Triassic extinction, with only a handful of genera belonging to the family Psiloceratidae of the suborder Phylloceratina surviving and becoming ancestral to all later Jurassic and Cretaceous ammonites. Ammonites explosively diversified during the Early Jurassic, with the orders Psiloceratina, Ammonitina, Lytoceratina, Haploceratina, Perisphinctina and Ancyloceratina all appearing during the Jurassic. Heteromorph ammonites (ammonites with open or non-spiral coiling) of the order Ancyloceratina became common during the Cretaceous period.
At least 57 species of ammonites, which were widespread and belonged to six superfamilies, were extant during the last 500,000 years of the Cretaceous, indicating that ammonites remained highly diverse until the very end of their existence. All ammonites were wiped out during or shortly after the K-Pg extinction event, caused by the Chicxulub impact. It has been suggested that ocean acidification generated by the impact played a key role in their extinction, as the larvae of ammonites were likely small and planktonic, and would have been heavily affected. Nautiloids, exemplified by modern nautiluses, are conversely thought to have had a reproductive strategy in which eggs were laid in smaller batches many times during the lifespan, and on the sea floor well away from any direct effects of such a bolide strike, and thus survived. Many ammonite species were filter feeders, so they might have been particularly susceptible to marine faunal turnovers and climatic change. Some reports suggest that a few ammonite species, including those belonging to the genera Hoploscaphites, Baculites and Fresvillia, may have persisted into the very early Danian stage of the Paleocene, with those found in the Cerithium Limestone of Denmark suggested to have survived at least 68,000 and up to a maximum of 200,000 years after the K-Pg extinction event (marked by Fiskeler), before going extinct.
Description
Size
The smallest ammonoid was Maximites from the Upper Carboniferous. Adult specimens reached only in shell diameter. Few of the ammonites occurring in the lower and middle part of the Jurassic period reached a size exceeding in diameter. Much larger forms are found in the later rocks of the upper part of the Jurassic and the lower part of the Cretaceous, such as Titanites from the Portland Stone of Jurassic of southern England, which is often in diameter, and Parapuzosia seppenradensis of the Cretaceous period of Germany, which is one of the largest-known ammonites, sometimes reaching in diameter. The largest-documented North American ammonite is Parapuzosia bradyi from the Cretaceous, with specimens measuring in diameter.
Basic shell anatomy
The chambered part of the ammonite shell is called a phragmocone. It contains a series of progressively larger chambers, called camerae (sing. camera) that are divided by thin walls called septa (sing. septum). Only the last and largest chamber, the body chamber, was occupied by the living animal at any given moment. As it grew, it added newer and larger chambers to the open end of the coil. Where the outer whorl of an ammonite shell largely covers the preceding whorls, the specimen is said to be involute (e.g., Anahoplites). Where it does not cover those preceding, the specimen is said to be evolute (e.g., Dactylioceras). A thin living tube called a siphuncle passed through the septa, extending from the ammonite's body into the empty shell chambers. Through a hyperosmotic active transport process, the ammonite emptied water out of these shell chambers. This enabled it to control the buoyancy of the shell and thereby rise or descend in the water column. A primary difference between ammonites and nautiloids is the siphuncle of ammonites (excepting Clymeniina) runs along the ventral periphery of the septa and camerae (i.e., the inner surface of the outer axis of the shell), while the siphuncle of nautiloids runs more or less through the center of the septa and camerae.
Siphuncle The siphuncle in most ammonoids is a narrow tubular structure that runs along the shell's outer rim, known as the venter, connecting the chambers of the phragmocone to the body or living chamber. This distinguishes them from living nautiloides (Nautilus and Allonautilus) and typical Nautilida, in which the siphuncle runs through the center of each chamber. However the very earliest nautiloids from the Late Cambrian and Ordovician typically had ventral siphuncles like ammonites, although often proportionally larger and more internally structured. The word "siphuncle" comes from the Neo-Latin siphunculus, meaning "little siphon".
Septa and suture patterns
Ammonites (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.
Ammonoid 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. The topology of the septa, especially around the rim, results in the various suture patterns found. The septal curvature in nautiloids and ammonoids also differ in that the septa curves towards the opening in nautiloids, and away from the opening in ammоnoids.Phylum Mollusca Class Cephalopoda 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). The suture line has four main regions.
The external or ventral region refers to sutures along the lower (outer) edge of the shell, where the left and right suture lines meet. The external (or ventral) saddle, when present, lies directly on the lower midline of the shell. As a result, it is often called the median saddle. On suture diagrams the median saddle is supplied with an arrow which points towards the aperture. The median saddle is edged by fairly small external (or ventral) lobes. The earliest ammonoids lacked a median saddle and instead had a single midline ventral lobe, which in later forms is split into two or more components.