Ammonoidea
Ammonoids are extinct, typically coiled-shelled cephalopods composing the subclass Ammonoidea. They are more closely related to living octopuses, squid, and cuttlefish than they are to nautiluses, which they 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 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.
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 called fossils of these animals ammonis cornua because the Egyptian god Ammon was typically depicted wearing rams' horns. Often, the name of an ammonite genus ends in -ceras, which is from ancient Greek κέρας 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
Taxonomy of the ''Treatise on Invertebrate Paleontology''
The Treatise on Invertebrate Paleontology 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 "nautiloids" belong to Bactritida during the early Devonian around 409-408 million years ago, with transitional fossils showing the transition from a straight shell, to a curved shell to a relaxed spiral and finally to a tight spiral. The Late Devonian mass extinction at the end of the Frasnian led to a dramatic decline in ammonoid diversity, with only a handful of lineages belong to Tornoceratina 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 to upper position. Ammonites were nearly completely exterminated by the Hangenberg Event 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 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, 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 that are divided by thin walls called septa. 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. Where it does not cover those preceding, the specimen is said to be evolute. 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 runs along the ventral periphery of the septa and camerae, 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 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 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. While nearly all nautiloids show gently curving sutures, the ammonoid suture line is variably folded, forming saddles and lobes. The suture line has four main regions.
The external or ventral region refers to sutures along the lower edge of the shell, where the left and right suture lines meet. The external 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 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.
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. The lateral saddle and lobe are usually larger than the ventral saddle and lobe. 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. In many cases the distinction between the lateral and umbilical regions are unclear; new umbilical features can develop from subdivisions of other umbilical features, or from subdivisions of lateral features. 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.
Three major types of suture patterns are found in the Ammonoidea:
- Goniatitic – numerous undivided lobes and saddles. This pattern is characteristic of the Paleozoic ammonoids.
- Ceratitic – lobes have subdivided tips, giving them a saw-toothed appearance. The saddles are rounded and undivided. This suture pattern is characteristic of Triassic ammonoids in the order Ceratitida. This pattern convergently re-evolved in the Cretaceous engonoceratid ammonites, commonly referred to as "pseudoceratites".
- Ammonitic – lobes and saddles are much subdivided ; subdivisions are usually rounded instead of saw-toothed. Ammonoids of this type are the most important species from a biostratigraphical point of view. This suture type is characteristic of Jurassic and Cretaceous ammonoids, but extends back all the way to the Permian.