StoryImage( ‘/Images/Story//Auto-img-113761320722335.jpg’, ‘Photo by Dr. Robert A. Hedeen’, ‘The origin of shells in certain animal groups is still a mystery.’);
Though we in the Rock River Valley live far away from the oceans of the world, we are familiar with shells. The limestone that forms the bedrock of our area is riddled with various types of fossil shells, which once housed a host of animals that lived in the tropical sea that covered northern Illinois some 400 million years ago. You dont have to be a fossil hunter to verify this. Look downward the next time you stroll along a concrete sidewalk, and you will probably see small fossil shells embedded in the concrete. These relics of the past were in the limestone that was used in the making of the concrete.
The second largest group in the animal kingdom is the phylum Mollusca, which includes oysters, clams, snails, chitons, squids and octopuses (or is spelled it octopi?). Most of the members of this great and diverse group are characterized by having their bodies enclosed in a calcareous shell. A few show no trace of a shell, and the squid, though it appears naked in this regard, does have a rudimentary shell within its body. Many other groups of animals without backbones also have shells, including the brachiopods, barnacles, and even some one-celled animals.
A large group of one-celled animals known as the Foraminifera synthesize a shell, and the fossilized remains of these animas are of help to geologists when they explore for petroleum. Millions of years ago thick layers of these tiny shells covered the beds of some oceans. The famous White Cliffs of Dover along the eastern coast of England are made up almost entirely of untold zillions of these tiny shells.
Many will be content merely to say it is pretty when they encounter the large shell of a conch on the beach or in a souvenir store before they put it to their ear to hear the sound of the sea. But, others with a more artistic frame of mind will see inherent beauty in any sea shell and marvel at the process that produced such a feat of biological engineering.
If we look carefully at an ordinary clam we can see the outside of each of its shells bears a concentric series of evenly curved lines. Each of these lines records a stage in the growth of the shell, and the most recent stage is on the outer edge, while the baby shell is at the top. Thus the clam carries on its back a record of growth stages from infancy to old age. We can see how this is useful to the clam as its suit of armor is gradually increased to accommodate its growth.
True shells consist mainly of calcium carbonate, which is absorbed from the water and processed by numerous shell glands within the body of the animal. The shell- making material oozes out equally from these glands, and a shell will grow equally in all dimensions. A very old clam will be similar to a young clam, only larger. The process by which this occurs is called polyisomerization, which in Greek means many equally functioning parts. The mechanism that directs this equal distribution of the shell-forming material is embedded deep in the genetic material of the clam.
Biologists have wondered for years how shells first arose. Most assume it was an original, advantageous variation that appeared in some primitive animal and was picked up by natural selection and modified as it was passed on for countless generations. Obviously, the development of a shell afforded great protective value to the animal living inside.
Recently, however, a new idea about how shells first arose has been advanced. Scientists of the U.S. Geological Survey have evidence of a tremendous increase in the amount of calcium in oceans during the Cambrian Period of geological history (515 million years ago) that may have resulted in the formation of shells in many creatures of that time. These researchers have shown that residues of evaporated sea water trapped in rock salts from that period show three times as much calcium as the period before when the concentration of this element was similar to what we have in todays oceans.
It is theorized that this great increase in calcium was lethal to many animals, but some were able to cope with the changed environment by using their primitive kidneys to excrete the excess calcium as shell forming material. Thus, the evolution of the shell was due to a physiological adaptation rather than a physical one, advantageous for protection. Needless to say, this new hypothesis is at present far from being accepted by the majority of evolutionary biologists.
Whatever caused the evolution of shells did us a great favor by creating innumerable objects of natural art and providing an interesting hobby for those who collect them.
Dr. Robert Hedeen is a former resident of Marylands eastern shore and resided in the Chicago area from 1960 to 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.
From the Jan. 18-24, 3006, issue