FOSSIL FINDS
MERCENARIA permagna: Nature’s Blend of Fossils and Crystals
Several years ago, I obtained several rare and quite prized fossilized Mercenaria clams from Florida. Both fossil and mineral collectors prized them for their beauty and natural preservation. These natural history wonders came from the once closed “Rucks Pit.” As of this writing, collecting is once again possible at Rucks Pit Crystal Mine located in Fort Drum, Florida. They still have a pay-to-dig-site where people can pick through the spoils piles; however, the most prized complete specimens are hardly ever found. Geologically, they generally were found within specific zones just above the Tamiami Formation within the Lower Nashua Unit.
The Mercenaria is a marine bivalve, and a bivalve is an aquatic mollusk with two similar halves hinged together and containing the soft parts of the creature within the calcium carbonate shell. Various Mercenaria species, also called Venus clams, are still alive in the oceans today and are edible, such are the quahog, cherrystone and littleneck clams. The valves usually have bilateral symmetry, which means only that the two haves are nearly equal in size and shape.
During the Pleistocene Epoch, 78,000 to 1.8 million years ago, these animals thrived in the warm saltwater tropical seas that periodically transgressed Florida. A transgression is the inundation of a landmass with water, be it salt or fresh, that remains for an extended period. The area of discussion, near Fort Drum, once was part of a warm saltwater tropical sea present during the Pleistocene period.
Throughout history, sea levels would occasionally rise and fall. Sometimes this was due to the sea rising, and sometimes it was due to the land sinking because of geological forces such as plate tectonics.
Regardless, the clams lived and died in this space because they were trapped beneath the waves and sand or tossed up onto the beach. Being a hard substance, the calcium carbonate shells would generally survive decay while the soft tissues decayed, creating a void. Over time, calcium-rich water would permeate the shells and deposit calcite within the void, creating beautiful amber-colored rhombohedral crystals.
These amber-colored crystals are sometimes called honey amber. The impurities determine the color of the crystals during formation.
Calcite is one of the most common and abundant minerals on earth and has a Moh’s hardness of 3, and a soft Coquina matrix generally surrounds the clams.
During the era in question, when the sea level dropped significantly, acidic rainwater dissolved some of the shells and beach sand that had accumulated and “cemented” them together to form Coquina. This Coquina is comprised of many shell fragments and other dissolved or partially dissolved calcium carbonate structures. As sea levels again rose, successive sediment layers covered the area until recently, when the quarry was opened for commercial use.
The spectacular specimens once found in this area are rarely, if ever, found today. However, people are known to find pieces of clams, whelks, and other shell material along with small and large calcite crystals.
Joseph J. “PaleoJoe” Kchodl is an award-winning paleontologist, author, storyteller, collector, a revered expert on the topic of trilobites, highly-sought-after presenter of school programs about fossils, keynote speaker and lecturer, and a global ambassador promoting digging and the appreciation of fossils. More can be learned by visiting his website: www.paleojoe.com.