Index fossils are the forms of life which existed during limited periods of geologic time and thus are used as guides to the age of the rocks in which they are preserved. John McPhee's analogy from Basin and Range (1981) describes the concept well:
These marine bivalves were one of the main components of the widespread Tethys Sea between Laurasia and Gondwana about 200 million years ago as Pangaea was breaking up:
Rudists were one of the main components of the reefs that formed then:
Rudists were widespread and had very different shapes making them excellent index fossils for fairly narrow time periods:
The earlier forms were elongate, with both valves being similarly shaped, often pipe-shaped, while the later, reef-building Cretaceous forms had one valve that become a flat lid, with the other valve becoming an inverted spike-like cone. The size of these conical forms ranged widely from just a few centimeters to over a meter in length.
Rudists' morphology consisted of a lower, roughly conical valve that was attached to the seafloor or to neighboring rudists, and a smaller upper valve that served as a kind of lid for the animal. The small upper valve could take a variety of different forms, including: a simple flat lid, a low cone, a spiral, and a star-shape.
The earlier forms tended to be more solitary but the Cretaceous forms were generally more colonial. Rudist colony: they started it millions of years ago. The first naturists died off at the major Cretaceous-Paleogene extinction about 65 million years ago.
Looking forward to your tales of nudist, er, rudist colonies. ;-)
Holiday Hummer in the Colorado Mountains 7/3/14 (photo by C. Fiss)
First clue (these are my photos) to location in the CO mts. See if you can win the geography quiz, at least a bit of a challenge this Sunday morning ;-):
More to come (if needed). Clue number three. Hint: It's very, very clear.