GRYPHAEA – a bivalve (shell with 2 parts)
The extinct, Jurassic oyster Gryphaea has a thick shell made of calcite. It has two unequal valves
which are attached together by a hinge. The concave right valve fits like a lid within the larger
left valve. Gryphaea lived on muddy sea beds.
two unequal shells
the concave right valve fits like a lid
within the larger left valve
GEOLOGICAL TIME RANGE: Lower Triassic – lower Jurassic
Gryphaea, the fossil oyster, is commonly found in Jurassic rocks. There are different species
which show a change of shape through time. Burrowing and boring bivalves (=infaunal) have
valves that are mirror images, giving the shell a streamlined shape. Bivalves that live on the
sediment are not so constrained and can show different patterns of asymmetry. Cooler
conditions were optimal for Gryphaea arcuata which encouraged large shell growth, thick and
narrow.
Geological range of Gryphaea arcuata = Blue Lias- Hettangian,
Jurassic
EVOLUTIONARY TRENDS
Gradually through time, the shell of Gryphaea became bigger, thinner and flatter. The
interpretation is that the wider shell provided better stability in energetic waters. The
periods of evolution were driven by the need to adapt the shell for reclining on muddy,
early Jurassic sea-floors. Adaptation to a stable food supply is thought to have driven the
evolutionary increase in shell size; this trend can also be seen other early Jurassic bivalves.
Increase in size affects physical stability which probably explains the shell becoming bigger,
broader, thinner and flatter.
Gryphaea - Fossil folklore
Gryphaea arcuata was known in English folklore as “devil’s toenail” due to the curved
shape.
http://www.nhm.ac.uk/nature-online/species-of-the-day/collections/ourcollections/gryphaea-obliquata/index.html
Gryphaea - at Lyme Regis