You are looking at the interior side of an Ordovician
bivalve in which the aragonitic shell has dissolved. Borings
(known as Palaeosabella)
had penetrated that shell before it dissolved, making thin, clavate
excavations. These holes filled with sediment which was cemented
before the shell dissolved away, leaving casts of the holes.
A trepostome bryozoan at the top of the image can be seen encrusting
the casts, suggesting that this entire process of cementation
and dissolution took place on the seafloor under "Calcite
Sea" conditions. The scale bar is in millimeters; the specimen
is from the Whitewater Formation, Upper Ordovician, near Richmond,
Indiana.
|