Fossil Gastropod in Chazy Formation

May 01, 2009


Photographer: Rob Sheridan
Summary Author: Rob Sheridan, Stu Witmer

Humans are not the only great builders on Earth. Marine reefs, assembled by generations of calcareous animals, are one of the few earthly constructions large enough to be seen with the unaided eye from Earth orbit. Before the evolution of corals, marine reefs were formed by branching bryozoans which trapped sediment in webs of their calcareous skeletons, creating complex warrens of habitats in warm shallow seas. One particularly spectacular example is the Chazy Formation of the Eastern United States.

In Ordovician times, 450-480 million years ago, atmospheric CO2 levels were higher, O2 levels were lower, global temperatures were warmer, and sea levels were higher. This was before the evolution of corals. During this time, one of the earliest marine reefs was constructed by generations of branching calcareous bryozoans on the continental shelf of ancient Laurentia. The massive Chazy Reef trapped dark organic-rich silt and became home for evolving mobile animals, such as early univalve predators. With closure of the Iapetus Sea and formation of the ancient continent of Pangea, the Chazy Reef was buried in mid-continent and exposed to pressures and temperatures that transformed its organic-rich silt and calcium carbonate into a dark limestone. About 200 million years later Pangea rifted apart forming the Atlantic Ocean. Smaller terrestrial rifts also formed, exposing portions of the massive Chazy Reef, which runs in bedrock from Tennessee to Newfoundland. Perhaps the best exposure is on Isle La Motte, Vermont. The fossil gastropod photo shown above was taken on Isle La Motte, on September 8, 2008.

The dark limestone takes a polish well, and was once prized as "black marble". It was mined from Isle La Motte's Fisk Quarry in the early 1800s. The characteristic white markings, more notable when the limestone is polished, were only attributed to fossil animals many years later. The gastropod pictured here may have died while munching on algae and bryozoans, in the warm shallows of the Iapetus Sea, 450 million years ago.