Old Earth Ministries Online Earth History Curriculum
Presented by Old Earth Ministries (We Believe in an Old Earth...and God!)
This curriculum is presented free of charge for use by homeschooling families.
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Chapter 6 - The Devonian Period
Lesson 35: Species In-Depth: Ammonites
Ammonites, as they pertain specifically to the order Ammonitida, are an extinct group of marine animals belonging to the cephalopod subclass Ammonoidea. They are excellent index fossils, and it is often possible to link the rock layer in which they are found to specific geological time periods.
Although most commonly known ammonites date to time periods after the Devonian, the first ammonites appeared during the Devonian, therefore they will be covered in this period.
Chapter 6 - The Devonian Period
Drawing of a Goniatite (Picture Source)
Known as Goniatites, these extinct Devonion ammonoids, shelled cephalopods are related to squid, octopus, and belemnites, and they form the order Goniatitida. The Gonatitida originated from within the more primitive anarcestine ammonoides in the Middle Devonian some 390 million years ago. Surviving the Late Devonian extinction, goniatitids flourished during the Carboniferous and Permian only to become extinct at the end of the Permian some 251.4 million years ago. They were survived by their cousins the ceratite ammonoids, indirect descendants of the Anarcestida.
Their fossil shells usually take the form of planispirals, although there were some
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.
Many ammonoids probably lived in the open water of ancient seas, rather than at the sea bottom. This is suggested by the fact that their fossils are often found in rocks that were laid down under conditions where no bottom-dwelling life is found. Many of them (such as Oxynoticeras) are thought to have been good swimmers with flattened, discus-shaped, streamlined shells, although some ammonoids were less effective swimmers and were likely to have been slow-swimming bottom-dwellers. Ammonites and their kin probably preyed on fish, crustaceans and other small creatures, while they themselves were preyed upon by such marine reptiles as mosasaurs. Fossilized ammonoids have been found showing teeth marks from such attacks. They may have avoided predation by squirting ink, much like modern cephalopods; ink is occasionally preserved in fossil specimens.
The soft body of the creature occupied the largest segments of the shell at the end of the coil. The smaller earlier segments were walled off and the animal could maintain its buoyancy by filling them with gas. Thus the smaller sections of the coil would have floated above the larger sections.
Shell Anatomy and Diversity
Basic Shell Anatomy
The chambered part of the ammonite shell is called a phragmocone. The phragmocone 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. 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 that 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.
One feature found in shells of the modern Nautilus is the variation in the shape and size of the shell according to the sex of the animal, the shell of the male being slightly smaller and wider than that of the female. This sexual dimorphism is thought to be an explanation for the variation in size of certain ammonite shells of the same species, the larger shell (called a macroconch) being female, and the smaller shell (called a microconch) being male. This is thought to be because the female required a larger body size for egg production. A good example of this sexual variation is found in Bifericeras from the early part of the Jurassic period of Europe.
It is only in relatively recent years that the sexual variation in the shells of ammonites has been recognized. The macroconch and microconch of one species were often previously mistaken for two closely related but different species occurring in the same rocks. However, these "pairs" were so consistently found together that it became apparent that they were in fact sexual forms of the same species.
Variations in Shape
The majority of ammonite species feature a shell that is a planispiral flat coil, but other species feature a shell that is nearly strght (as in baculites). Still other species' shells are coiled helically, superficially like that of a large gastropod (as in Turrilites and Bostrychoceras). Some species' shells are even initially uncoiled, then partially coiled, and finally straight at maturity (as in Australiceras). These partially uncoiled and totally uncoiled forms began to diversify mainly during the early part of the Cretaceous and are known as heteromorphs.
Perhaps the most extreme and bizarre looking example of a heteromorph is Nipponites,
Ammonites vary greatly in the ornamentation (surface relief) of their shells. Some may be smooth and relatively featureless, except for growth lines, and resemble that of the modern Nautilus. In others various patterns of spiral ridges and ribs or even spines are shown. This type of ornamentation of the shell is especially evident in the later ammonites of the Cretaceous.
Although ammonites do occur in exceptional lagerstatten such as the Solnhoffen, their soft part record is surprisingly bleak - beyond a tentative ink sac and possible digestive organs, no soft parts are known at all. It can be tentatively assumed that they had numerous tentacles, each quite weak, and engulfed prey almost whole.
Few of the ammonites occurring in the lower and middle part of the Jurassic period
Due to their free-swimming and/or free-floating habits, ammonites often happened to live directly above seafloor waters so poor in oxygen as to prevent the establishment of animal life on the seafloor. When upon death the ammonites fell to this seafloor and were gradually buried in accumulating sediment, bacterial decomposition of these corpses often tipped the delicate balance of local redox conditions sufficiently to lower the local solubility of minerals dissolved in the seawater, notably phosphates and carbonates. The resulting spontaneous concentric precipitation of minerals around a fossil is called a concretion and is responsible for the outstanding preservation of many ammonite fossils.
When ammonites are found in clays their original mother-of-pearl coating is often preserved. This type of preservation is found in ammonites such as Hoplites from the Cretaceous Gault clay of Folkestone in Kent, England.
The Cretaceous Pierre Shale formation of the United States and Canada is well known for the abundant ammonite fauna it yields, including Baculites, Placenticeras, Scaphites, Hoploscaphites, and Jeletzkytes, as well as many uncoiled forms. Many of these also have much or all of the original shell, as well as the complete body chamber, still intact. Many Pierre Shale ammonites, and indeed many ammonites throughout earth history, are found inside concretions.
Other fossils, such as many found in Madagascar and Alberta (Canada), display iridescence. These iridescent ammonites are often of gem quality (ammolite) when polished. In no case would this iridescence have been visible during the animal's life; additional shell layers covered it.
Ammonite fossils became less abundant during the latter part of the Mesozoic, with none surviving into the Cenozoic era. The last surviving lines disappeared along with the dinosaurs 65 million years ago in the Cretaceous-Tertiary extinction event. That no ammonites survived the extinction event at the end of the Cretaceous, while some nautiloid cousins survived, might be due to differences in ontogeny. If their extinction was due to a bolide strike, plankton around the globe could have been severely diminished, thereby dooming ammonite reproduction during its planktonic stage.
In medieval Europe, fossilised ammonites were thought to be petrified snakes, and were called "snakestones" or, more commonly in medieval England, "serpentstones". They were taken to be evidence for the actions of saints such as Saint Hilda and Saint Patrick. Traders would occasionally carve the face of a snake into the empty, wide end of the ammonite fossil and sell them to the public. Ammonites from the Gandaki river in Nepal are known as saligrams, and are believed by Hindus to be a concrete manifestation of God or Vishnu.
The words "ammonite" and "ammonoid" are both used quite loosely in common parlance to refer to any member of subclass Ammonoidea. However, in stricter usage the term "ammonite" is reserved for members of suborder Ammonitina (or sometimes even order Ammonitida).
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