Gryphaea

Gryphaea (commonly known as devil's toenails) is a genus of marine gryphaeid oysters known from the Mesozoic Era. Its fossils are almost worldwide-known but they are mostly common in Jurassic strata from Europe, North America, and South America. Gryphaea fossils are particularly important in the tectonically deformed Andean basins of Argentina and Chile and serve as index fossils. Gryphaea species appeared during the Late Triassic period and flourished during the Early Jurassic, eventually becoming extinct around the Late Jurassic. The name "devil's toenail" originated because of the curved shells of Gryphaea resembling a toenail. There used to be a common folk belief that carrying one of these fossils could prevent rheumatism. A complete Gryphaea shell consists of two articulated valves: a larger gnarly-shaped valve and a smaller, flattened valve. The soft parts of the animal occupied the cavity between the two shells, just like modern oysters. The larger, curved valve sat within the soft sediment on the sea floor. In seabeds dominated by soft sediments, Gryphaea shells provided stable surfaces for colonization by benthic organisms. The shells also feature prominent growth bands and some of them can provide sclerochronological measurements. Some species of Gryphaea reacted greatly to the paleoenvironments in which they lived, such as G. arcuata, where cold marine environments favored the formation of thicker shells and warmer environments produced thinner shells. Gryphaea lived on the sea bed in shallow waters, likely in large colonies. It has been suggested that Gryphaea and similar bivalves became extinct near the Cretaceous due to the evolution of durophagous predators and the increasing reworking of sediments which negatively affected their reclining mode of life on the sea floor. The material from Cretaceous units referred to Gryphaea (G. accipiter, G. newberryi, G. convexa, among others) has been progressively separated from this genus and reassigned...