Fossils & Minerals:  Mollusks:  Ammonoids

AmmoniteClass: Cephalopoda
Sub Class: Ammonoidea

Ammonites are the common name given to the Ammonoidea, an extinct order of cephalopods. These relatives of today's squid, octopi, and Nautilus were abundant, intelligent, active, and predatory molluscs that lived in shallow (100 meters or less) marine environments throughout the world. Competing with fish for food, they fed on fishes, molluscs (including other cephalopods) and any other prey they could catch. Most ammonites had large jaws capable of crushing shell, as well as grasping arms and excellent vision. Ammonites first appeared in the Late Silurian seas about 415 million years ago, and then abruptly disappeared from the world's oceans 65 million years ago, at the end of the Cretaceous Period.

The Shell

  • Ammonite Shell Features Ammonites developed a variety of shell adornments. Ribs added strength and streamlined their shell while spines and tubercles probably protected them from predators and may have made them more attractive to the opposite sex. Microconchs (males), which are nearly always considerably smaller than macroconchs (females), have proportionally larger spines than macroconchs (females). Perhaps they not only needed this added protection from their predators but also from their prospective mates.

  • An ammonite shell consists of two parts: the Phragmocone, composed of progressively larger chambers that filled with gas to create near neutral buoyancy; and the Body Chamber, the large, terminal section which houses and protects the body.

  • IridescenceThe ammonite's mother-of-pearl like (nacreous) shell is comprised of alternating layers of aragonite and conchiolin. These layers were deposited by the ammonite's mantle in the same manner that modern molluscs create their nacreous shells. The magnificent array of colors in ammonites comes from light refracting and reflecting through the nacreous shell. Ammonite shells have been discovered in virtually every naturally-occurring color. The wide variety of colors exhibited by fossil ammonites is probably the result of the minerals, iron or manganese, combining with the original mother of pearl. Probably most ammonites possessed some color banding that provided camouflage both for protection and for stalking.

  • The suture pattern is seen beneath the external shell wall of the phragmocone. This pattern is formed where the chamber walls (the septa) contact the ammonite's outside shell.Sutures Each ammonite species developed its own distinctive pattern; and each ammonite family, its own unique pattern style. Ammonoids from the Devonian Period exhibit the simple goniatitic suture pattern. The later Jurassic and Cretaceous Period ammonites possess the highly complex ammonitic (dentritic) suture patterns.

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Other Physical Characteristics

  • Ammonites were quite small when they hatched (less than 1 mm) but they grew rapidly. It is thought that, like squids and octopi, most ammonites lived only one to two years before reaching maturity, breeding, and dying.

  • As a result of adaptation to differing marine environments, ammonites evolved a wide variety of shapes and sizes. Like modern cephalopods, they occupied many positions in the water column. Although most ammonite species grew in perfectly symmetrical spirals, others constructed shells shaped like hairpins, corkscrews, coils, cones, and even long, straight forms.

Ammonite Forms

Varied Forms

  • It appears that ammonites exhibited similar gender size differences as are found in living squids and octopi with the macroconch (female) generally being 25% to 400% larger than than the microconch (male).

  • Ammonite jaws were much larger in comparison to their body size than any living cephalopod. They used their jaws for crushing their prey which consisted of pretty much anything they could catch.

  • Some ammonites had parasites (epibionts) that lived on their shell. Often ammonites are discovered with bryozoans, limpets, oysters, and even worm tubes attached. At times, these animals and other molluscs have actually bored holes into the ammonite's shell. The discovery of these holes led some paleontologists to mistakenly identify them as the bite marks of larger predators.

Ammonite Cross Section

Ammonites through Time

Ammonites are found in Paleozoic and Mesozoic marine rocks worldwide. Each type of rock has its own mode of preservation. Most common in shales, limestones and sandstones, is the preservation of only the shell. Other parts of the ammonite are rarely found. In clay beds, the shells are often replaced by pyrite. Lithographic limestones and chalk beds preserve only a faint outline of the shell, but jaws and other parts are sometimes also found. No location has been discovered where all parts of the ammonite are preserved. Compiling data about various parts of different ammonites from many locations gives us only a glimpse of how these creatures may actually have looked.

Ammonites first appeared as a small, straight shelled cephalopod, more than 400 million years ago in the seas that covered what is now Europe. Called Bactrites, these animals were an offshoot of the nautiloids. By the middle of the Devonian (380 million years ago), ammonites had developed diverse shapes and sizes and colonized nearly all the world's seas. They flourished until the great Permian extinction event (250 MYA) that exterminated 90% of all living species including nearly all ammonite species.

During the Triassic Period (250-206 MYA), ammonites once again diversified and inhabited seas worldwide. Another major extinction occurred at the end of the Triassic which annihilated all but one family of ammonites. This one surviving family again evolved, diversified, and flourished in the seas of the world throughout the Jurassic and Cretaceous Periods (205-65 MYA). Ammonites again developed numerous strange and aberrant shapes. The extinction event at the end of the Cretaceous Period (65 MYA) which ended the reign of the dinosaurs on land, also extinguished the ammonites, one of the most successful animals that ever lived.

During the Late Cretaceous Period (65 to 99 MYA) much of the central portion of North America was covered by a large, shallow sea that extended from New Mexico and Texas north into Canada. Ammonites were the most numerous predators inhabiting that sea. Evidence of their massive numbers is apparent by the abundance of their remains in the marine rocks of today's Western Interior region. These ammonites competed with fishes of comparable size for food while also providing an ample supply of food for the larger fishes, reptiles, and cephalopods who preyed on them and shared this sea with them.

The Institute has several excellent books on Ammonites. Two of which were written by the Institute's Vice President Neal Larson, the "Ammonite Guru."

Books and other Literature section of our Catalog...


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