In Our Region
Ammonites were free-swimming mollusks that had external shells that were either straight or coiled. Although we lack fossilized soft parts for ammonites, we assume their bodies were similar to those of the chambered Nautilus. The Nautilus has a fleshy head equipped with a beak and very well-developed eyes, clustered in the middle of tentacles used for catching prey.
The shells of ammonites were divided into chambers by intricately folded walls or septae. These walls left distinct suture lines where they joined with the outer shell, making patterns that are important in ammonite classification.
The animal lived in the outermost chamber of the shell. As the animal grew, the old chambers could be filled with gas or liquid, in order to regulate the animal's buoyancy in the water, much like buoyancy tanks in a submarine. As a counterbalance, some of the chambers had calcareous deposits acting as ballast.
The smallest ammonite species had shells less than an inch in size, but the larger, coiled species were huge—some reached more than 9 feet (3 meters) in diameter!
There was an amazing diversity in the variety of shapes and ornamentation of ammonites. In addition to the typical coiled shape, ammonites in the heteromorph group could be straight like a tusk, or helically coiled, or even shaped like a compressed paper clip. Many ammonite shells have knobs, spines, tubercles, or ribs, which may have helped in defense from predators.
The genus Baculites included many species of straight-shelled ammonites, which only displayed a coiled form as juveniles. The shells could reach up to 6.5 feet (2 meters) in length. Males were believed to be only about half the size of females. Most scientists think they occupied a vertical orientation in the water column.
Nautiloids are another related group of cephalopods, distinguished from the ammonoids by the fact that the septa separating each chamber had a simple form, rather than being convoluted as in the ammonites. Nautiloids also had external shells, and although their species once numbered over 100, there are only five or six living species left, all within the Nautilus and Allonautilus genera.
We know that ammonites were carnivorous marine animals, but we have little data on what they actually ate. The rare ammonite fossils that have been recovered with evidence of food in the living chamber reveal tiny benthic animals known as ostracods and foraminiferans, or larger animals such as echinoderms, the group including sea urchins, sea stars, and sand dollars.
Some ammonites were sexually dimorphic, with the male (called the microconch) being far smaller than the female (the macroconch). The two sexes look so different that for a long time they were believed to be separate species.
Tooth marks on some ammonite shells indicate they were preyed upon by marine reptiles.
With more than 10,000 species of ammonites known from the fossil record, this group is one of the most characteristic marine fossils of Cretaceous-age rocks around the world. Because they were so diverse, ammonites are considered an index fossil.
Of Historical Interest
Text: Margaret Dykens and Lynett Gillette