aka: Ammonoids

Ammonites are an extinct group of marine invertebrates in the Phylum Mollusca, Class Cephalopoda, and Subclass Ammonoidea. The subclass can be divided into six orders as follows: Agoniatitida (present in the Lower Devonian–Middle Devonian), Clymeniida (Upper Devonian), Goniatitida (Middle Devonian–Upper Permian), Prolecanitida (Upper Devonian–Upper Triassic), Ceratitida (Upper Permian–Upper Triassic), and Ammonitida (Lower Jurassic–Upper Cretaceous). The name “ammonite,” from which the common name is derived, was inspired by the spiral shape of their fossilized shells, which somewhat resembles tightly coiled ram’s horns. These molluscs are more closely related to living coleoids (e.g., cuttlefish, octopuses, and squid) than they are to shelled nautiloids, such as extant chambered Nautilus species.

The earliest ammonites originated from within the bactritoid nautiloids, appearing during the Devonian (about 409 million years ago), and the last surviving lineages went extinct at the close of the Cretaceous–Paleogene (K-Pg) extinction event (66 million years ago). As a group, the ammonoids continued through several major extinction events, although often only a few species survived. After each instance, however, this minority diversified into a multitude of forms. Ammonite fossils became less abundant during the latter part of the Mesozoic, with none surviving into the Cenozoic era.

Ammonites lived in schools and were explosive breeders—making them among the most abundant fossils found today. However, their soft body parts are very rarely preserved in any detail. They are excellent index fossils, and it is often possible to relate the rock layer in which a particular species or genus is found to specific geologic time periods.

There have been several hypotheses given for the demise of ammonites. One reason that has been provided is that the ammonites of the Cretaceous, being closely related to coleoids (shell-less cephalopods), had a similar reproductive strategy in which vast numbers of eggs were laid in a single spawn at the end of their lifespan. These, along with younger ammonites, are thought to have been part of the zooplankton at the surface of the ocean, where they were destroyed by the effects of a meteor impact. In addition, many ammonites were filter-feeders, so they might have been particularly susceptible to climate change and marine faunal turnovers, meaning that lack of food resources may have been the dominant cause of their extinction.

Their fossil shells usually take the form of planispirals (flat-coiled shells), but other species possess nearly straight shells (as in baculites), and some were even helically spiraled (e.g., Bostrychoceras and Turrilites) or partially uncoiled and non-spiraled forms (heteromorphs). Some species’ shells were even initially uncoiled, then partially coiled, and finally straight at maturity (as in Australiceras).

Perhaps the most extreme and unusual-looking example of a heteromorph ammonite is Nipponites, which appears to be a tangle of irregular whorls lacking any obvious symmetric coiling. Upon closer examination, though, the meandering shell is a three-dimensional network of connected ox-bow bend shapes. Nipponites can be found in strata of the upper part of the Cretaceous in Japan and the United States.

Ammonites can be morphologically distinguished by three structures as follows: (1) their septa (dividing walls) that separate the chambers in the phragmocone (chambered part of the ammonite shell), (2) the construction of their sutures where the septa joints the outer shell wall, and (3) their siphuncles. Ammonoid septa regularly have bulges and indentations and are to varying degrees convex from the cephalic end, distinguishing them from living nautiloids (Nautilus and Allonautilus) and typical Nautilida septa, which are typically simple concave dish-shaped structures and from which the siphuncle (a thin living tube) runs through the center of each chamber. This siphuncle passes through the septa, extending from the ammonite’s body into the empty shell chambers. The topology of the septa, especially around the rim, results in the various three major suture patterns found: ammonitic, ceratitic, and goniatitic.

The siphuncle in most ammonoids is a narrow cylindrical structure that runs along the shell’s outer rim (venter), connecting the chambers of the phragmocone to the body or living chamber. Viewed in cross section, an ammonite shell reveals a series of internally progressively larger chambers (called camerae) divided by thin walls called septa. Only the last and largest chamber, the body chamber, was occupied by the living specimen at any given moment in time. As the ammonite grew, it added newer and larger chambers to the open end of the coil. Through a hyperosmotic active transport process requiring cellular energy, the ammonite emptied water out of its shell chambers. This enabled it to control the buoyancy of the shell and thereby rise or descend in the water column.

When the outer whorl of an ammonite shell chiefly covers the former whorls, the specimen is said to be “involute,” such as those in the genus Anahoplites. When it does not cover those preceding, the specimen is said to be “evolute,” such as those in the genus Dactylioceras. The soft body of the organism 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 and swim by taking in and expelling water. Thus, the smaller sections of the coil would have floated above the larger sections.

The taxonomic classification of ammonoids is based partially on the structure and ornamentation of the septa composing their shells’ gas chambers. The ammonoid suture line (the intersection of the septum with the outer shell) is variably folded, forming saddles (or peaks) and lobes (or valleys), and this structure holds important taxonomic utility. Ammonites vary greatly in the ornamentation (surface relief) of their shells. Some, except for their growth lines, may be smooth and relatively dull and resemble 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 latter Cretaceous ammonites.

Because ammonites and their close relatives are extinct, little is known about their natural history and ecology. Nonetheless, many ammonoids probably lived in the open waters of primal seas, rather than on the sea bottom, because their fossils are often found in rocks laid down under conditions where no bottom-dwelling life has been found. Many of them (such as those in the genus Oxynoticeras) are thought to have been good swimmers, with flattened, cylindrical, sleek shells, although some ammonoids were thought to be less effective swimmers and were likely to have been slow-swimming bottom-dwellers. Due to their free-swimming and/or free-floating habits, ammonites often happened to dwell directly above seafloor waters.

In terms of food, synchrotron X-ray microtomography radiation (light-source) analysis of an aptychophoran (the largest group) ammonite revealed remains in their buccal cavity of larval isopods and molluscs, indicating that at least this particular kind of ammonite fed on tiny plankton. Others were assumed to be filter feeders. Much like modern cephalopods, they may have avoided predation by squirting ink, since ink is occasionally found to be preserved in fossil specimens.

Many ammonite shells have been found with round holes once thought to be a result of attached limpets (aquatic snails). However, the triangular formation of the holes, their size and shape, and their presence on both sides of the shells, corresponding to the upper and lower jaws, are more likely evidence of the bites from a giant mosasaur (large marine extinct reptile Tylosaurus) preying upon them.

Only recently has sexual variation in the shells of ammonites been recognized. 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, such as 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 macroconch (larger shell) being female and the microconch (smaller shell) being male. This is thought to be the result of the female requiring a larger body size for reproduction (egg production). An excellent example of this dimorphism is found in the genus Bifericeras from the early part of the Jurassic period of Europe.

In terms of relative sizes, few of the ammonites occurring in the lower and middle part of the Jurassic period reached a size beyond 23 cm (9 in.) in diameter. However, grander forms are found in the latter rocks of the upper part of the Jurassic and the lower part of the Cretaceous, such as Titanites from the Portland Stone of the Jurassic of southern Britain, which is often 53 cm (2 ft.) in diameter, and Parapuzosia seppenradensis of Germany (Cretaceous period), which is one of the largest known ammonites, sometimes attaining a diameter of 2 m (6.5 ft.). The largest reported North American ammonite is P. bradyi from the Cretaceous, with specimens reaching 137 cm (4.5 ft.) in diameter.

In North America, the Cretaceous Pierre Shale formation, which occurs east of the Rocky Mountains in the Great Plains, from the Pembina Valley in Canada to New Mexico, is well known for the abundant ammonite fauna it produces (often inside concretions), including Baculites, Hoploscaphites, Jeletzkytes, Placenticeras, and Scaphites, as well as many uncoiled forms. Other ammonite fossils, such as many found in Alberta, Canada, and in Madagascar, exhibit iridescence. When polished, these iridescent ammonites are often valued for their gemstone (ammolite) as well as scientific value.

During the Early Carboniferous, limy sediments accumulated in a shallow sea that covered what now is northern Arkansas. At the same time, deeper waters covered the central and southern portions of the state. As the streams and rivers washed sand and mud into the deeper water and eroded the continent, further reducing the water depth, clastic sediments began to accumulate on top of the shallow water deposits. Fossils of rare ammonites and of plants that washed in from the terrestrial environment, as well as trace fossils (such as the footprints, tracks, burrows, borings, and feces left behind by animals), can be found in these rocks. Middle Carboniferous cephalopod assemblages across northern Arkansas are dominated by a small number of ammonoid species that occur in unusual abundances. Individuals of a particular species exhibit strikingly similar size, yet both small and large species are represented. One such rich area is the Saratoga Chalk, which has yielded a rich ammonite fauna of at least seventeen species, referred to as the Nostoceras (N.) hyatti zone. Elements of the N. (N.) hyatti zone fauna occur in the Western Interior of the United States. Others have been found in the Fayetteville Shale in northwestern Arkansas; there are some black shale deposits off the creek beds in the southern side of Fayetteville (Washington County), and some flat concretions up to 30.5 cm (12 in.) and ammonites of various small sizes have been found here, some with pyrite. There are also large gray septarian concretions with ammonites. Calcareous concretions in the Nacatoch Sand in Washington (Hempstead County) yield abundant fossils of the lower Maastrichtian. Arkanites is a goniatitid ammonite that lived during the Early Pennsylvanian that has been found in Arkansas and Oklahoma.

For additional information:
Bottjer, D. J. “Campanian–Maastrichtian Chalks of Southwestern Arkansas: Petrology, Paleoenvironments, and Comparison with Other North American and European Chalks.” Cretaceous Research 7 (1986): 161–196.

———. “Structure of Upper Cretaceous Chalk Benthic Communities, Southwestern Arkansas.” Palaeogeography, Palaeoclimatology, Palaeoecology 34 (1981): 225–256.

———. “Trace Fossils and Palaeoenvironments of Two Arkansas Upper Cretaceous Discontinuity Surfaces.” Journal of Paleontology 59 (1985): 282–298.

Cobban, William A., and W. James Kennedy. “Some Upper Cretaceous Ammonites from the Nacatoch Sand of Hempstead County, Arkansas.” In Shorter Contributions to Paleontology and Stratigraphy, edited by William J. Sando. U.S. Geological Survey Bulletin, 1985.

———. “Upper Cretaceous (Maastrichtian) Ammonites from the Nostoceras alternatum Zone in Southwestern Arkansas.” In Shorter Contributions to Paleontology and Stratigraphy, edited by William J. Sando. U.S. Geological Survey Bulletin, 1985.

Dane, C. H. “Upper Cretaceous Formations of Southwestern Arkansas.” Bulletin of the Arkansas Geological Survey 1 (1929): 1–215.

Dolloff, J. H., R. A. Rozendal, E. N. Siratovich, F. M. Swain, and J. Woncik. “Subsurface Upper Cretaceous Stratigraphy of Southwestern Arkansas.” Transactions of the Gulf Coast Association of Geological Societies 17 (1967): 76–104.

Freeman, T. “Fossils of Arkansas.” Arkansas Geological Commission Bulletin 22 (1966): 1–53.

Kennedy, W. J., and W. A. Cobban. “Ammonites from the Saratoga Chalk (Upper Cretaceous), Arkansas.” Journal of Paleontology 67 (1993): 404–434.

———. “Campanian Ammonites from the Annona Chalk near Yancy, Arkansas.” Journal of Paleontology 67 (1993): 83–97.

Kruta, Isabelle, Neil Landman, Isabelle Rouget, Fabrizio Cecca, and Paul Tafforeau. “The Role of Ammonites in the Mesozoic Marine Food Web Revealed by Jaw Preservation.” Science 331 (2011): 70–72.

Manger, W. L., D. A. Stephen, and L. K. Meeks. “Possible Cephalopod Reproductive Mass Mortality Reflected by Middle Carboniferous Assemblages, Arkansas, Southern United States.” In Advancing Research on Living and Fossil Cephalopods, edited by F. Olóriz and F. J. Rodríguez-Tovar. Boston: Springer, 1999.

Staaf, Danna. Squid Empire: The Rise and Fall of the Cephalopods. Lebanon, NH: University Press of New England, 2017.

Ward, Peter. “Ammonoid Extinction.” In Ammonoid Paleobiology, edited by Neil H. Landman, Kazushige Tanabe, and Richard Arnold Davis. New York: Springer Science, 1996.

Wright, C. W. “A Classification of the Cretaceous Ammonites.” Journal of Paleontology 26 (1952): 213–222.

Chris T. McAllister
Eastern Oklahoma State College

Henry W. Robison
Sherwood, Arkansas

Last Updated: 09/22/2021