A local daily newspaper* recently published an account the discovery of giant fossil clams. These measured four feet across, twice the size of any living species, and found in Utah for the first time. Similar fossils are said to have been found in Colorado and Kansas, but this is the first time such prehistoric invertebrates have been found this far west. L. Logan and K. Carpenter of the Utah State University-Eastern’s Prehistoric Museum* found a number of fossils in the Book Cliffs, near the Green River. A number of clam remnants encountered were eroded so badly they could not be collected, but finally one was found with little erosion damage. This specimen was collected and taken to the Museum. The giant clam fossils were found in mancos shale, named for the Mancos Sea that is thought to have cut through North America from the Arctic Ocean to the Gulf of Mexico in the far distant past. The foto above shows typical extant East coast clams.
A little bit about fossils. Fossils are the remains, impressions or traces of the bodies of organisms which have been preserved in rocks from previous geological ages.** Fossils that retain perfect or only slightly modified forms of plant and animals are rare. Over many years, plants and animals have had their outer skeletal parts dissolved by chemical processes, then replaced by minerals such as silica, calcite, etc. There is almost always the loss or dissolution of the original structure. Again, various minerals replace dissolved skeletal parts so that none of that original part remains. However, sometimes fossils have retained the original patterns perfectly both externally and internally (exoskeleton, endoskeleton); these are the most desired and sought after, but—also the most rare and difficult to find.
Indeed, there are some few examples of actual fossil skeletal materials remaining after thousands of years. These include skeletal parts found frozen within ice in Siberia and the skeletons of higher animals preserved in asphalt in Rancho la Brea in California.
Of course, the likelihood of fossilization for any given plant or animals is very small. After death, as these are most often eaten or destroyed or simply decay away on the surface. Obviously hard body components such as teeth, bones, shells, and wood are the most likely candidates for fossilization. Vertebrate paleontologists have created new species of vertebrate animals based on fossil teeth (dentition) found, often in caves. Certain conditions must be met, however, for fossils to form. First, the body or plant must be covered fairly rapidly after death—most commonly with sediment. Sediments offer protection from mechanical damage, normal biological decomposition, or the actions of water or atmospheric events. Thus marine or aquatic organisms are much more likely to become fossils than land dwelling (terrestrial) animals or plants. Additional important requirements for fossilization are extreme pressures generated by multiple layers of sediment and detritus above and…. time. Extreme pressure over many thousands, even hundreds of thousands of years, often produce recognizable animal and plant species.
Now some more about clams in general. Clams or “mussels” are often called “bivalves.” The “bi” refers to two, and the “Valve” refers to the shell; or in other words, two shells. The innermost layer of the shell is called the “mother of pearl.” It consists of alternating layers of limey and horny material. This produces an attractive iridescence which diffracts light and which in the past, has made clam shells much sought after to make buttons and jewelry.
Clams are, of course, marine or aquatic organisms. They lived in the sandy bottom of bodies of water and can be dug up from sandy seaside beaches. Clams and their kin may be found throughout the earth from the deepest marine seas to shallow fresh water rivers. Clams have no head. They have a mass of flesh in the ventral (lower) part of their bodies called a foot, which, when the shells open can be extended and provides slow movement as it contracts and expands. The two “mantles” are fleshy lobes inside the shell that secrete the bivalve shell and the “hinge.” The hinge opens to allow food-bearing water to circulate within the mantle. Food is captured and enters a crude digestive system. When enemies threaten, the clam simply closes up tight to protect its inner self. A single large muscle is responsible for governing opening or closing of the bivalves. This muscle is the sought after morsel that humans find so delectable in making clam chowder.
My own experience with clams came as a result of an invertebrate biology class I took while a student at the University of Oklahoma’s Lake Texoma Biological Station with four classmates. Dr Barry Valentine (Ohio State University) taught the class and we traveled throughout southern Oklahoma waterways in pursuit of fresh water clams of which there are a number of species with varying architecture. Most of the specimens we collected were from animals already expired. However, at the end of the class, besides the master collection which yielded new distribution information of several species, we each had a collection of attractive and varied fresh water clams. Mine eventually went to Weber State University. Overall, It was a most memorable biological collecting experience!
*Liesk, J. 2013 (4 Feb). Giant Clam Fossils Unearthed. Deseret News, SLC, UT.
*Prokop, R. and V. Krb. 1995. Fossils. Magna Books, Leicester, England.
***Utah State University – College of Eastern Utah. 2013. News, Big Find-Giant Clams. website: eastern.usu.edu