Where have all the ammonites gone?

Where have all the ammonites gone?

By Robert A. Hedeen, Naturalist

The dinosaurs are undoubtedly the most famous of extinct animals, and their fossil remains create great interest among paleontologists, zoologists, students of evolution, and the general public. It amazes me that evidence in recent decades clearly indicates that certain dinosaurs were warm-blooded, had feathers, and evolved into birds. This means the cardinal munching on seeds in my feeder at this moment has a direct phylogenetic link to these ancient reptiles. Why the dinosaurs became extinct as a group some 135 million years ago is a question that has not been answered satisfactorily.

Few individuals realize, however, that at about the same time in geological history that the dinosaurs were dying off, another group of animals, albeit not so well known as the celebrated reptiles, was being extirpated from the earth. They were the ammonites, members of the highly specialized group of mollusks called the cephalopods (headfoot).

The sea-dwelling ammonites first appeared during the late Silurian and early Devonian periods (about 400 million years ago), flourished while evolving into some 5,000 different species, and abruptly became extinct during the Cretaceous period (about 135 million years ago). To be certain we would not overlook their existence, they left their fossilized shells behind for us to find and to speculate on their existence.

The only members of this group of mollusks living today are the squids, octopuses (octopi, take your choice), cuttlefish, and the chambered nautilus. Fortunate indeed is he whose praises are sung by a famous poet. The chambered nautilus would be “just another mollusk” had not Oliver Wendell Holmes immortalized him in his incomparable poem. The chambered nautilus undoubtedly evolved from an ammonite ancestor, but today it has so many different characteristics it no longer qualifies to be included in its ancestral group.

Like the nautilus, ammonites secreted protective, calcareous shells from the minerals in the seawater in which they lived. As they grew in size and the shell became too cramped for them, they simply added another room to their house by manufacturing a new and larger living chamber. The previously occupied chamber was sealed off by a partition. Room after room was added to the shell, as the animal grew larger, resulting in a heavy mass of unused real estate to be carried around on the ammonite’s back. But, each abandoned chamber became filled with a buoyant gas that served to lighten the homeowner’s load. Ammonites did not rise to the surface of the sea but moved easily along the bottom, scouring for food.

The head of the creature would be thrust out from its shell, and, if you saw one today, you would immediately recognize it as closely resembling the front end of a squid. The head was supplied with a series of tentacles that were used in the gathering of food and also as a means of locomotion, hence the scientific name of cephalopoda, or head-foot.

Invertebrate paleontologists use the zigzagging sutures on the surface of an ammonite shell as one means of identifying a specific species. These lines represent the internal areas where the outgrown and abandoned chambers were partitioned off. The shells were tightly coiled and many were ornamented with spines, bumps, and ribs.

As in the case of the dinosaurs, no one knows why this once successful group of animals died out. The Devonian period in which they flourished is also know as the Age of Fishes, and it could be that fish with strong jaws ate them out of existence. More probable, however, is that the environment gradually changed during that long period, and the ammonites were unable to adapt. “Adapt or perish” is a fundamental law of nature and one of the cornerstones of organic evolution.

Fossil hunters in the Rock River Valley are not apt to find many ammonites in the bedrock around Rockford. Our foundation of limestone dates primarily from the Ordovician period that was laid down earlier than the Silurian limestone (dolomite) where ammonites first appear. However, a short trip a bit south and east to the Chicago area or west to the mouth of the Rock River in Rock Island County will prove rewarding for ammonite collectors. The bedrock in these areas is dated from the Silurian age and is apt to contain fossils of these interesting animals that lived so long ago.

Dr. Robert Hedeen is a former resident of Maryland’s eastern shore and resided in the Chicago area from 1960-1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.

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