April 13. Waldron Shale

 

Eucalyptocrinus calyx.
The lower portion is
commonly preserved.

One of the most famous assemblages of Silurian fossils in the United States is the Waldron Shale, which crops out in east-central Indiana and western Ohio. Shale is a very fine grained rock that solidified from mud, often including clay-sized particles that are just a fraction of a millimeter across. Some parts of the Waldron include limestones and dolomites, and that’s where most of the richest fossils are found. In some places the animal fossils amount to small reefs. 

The life included the usual suspects for a warm, shallow Silurian sea – trilobites, crinoids, brachiopods, corals, and gastropods – those are snails. My collection includes a specimen of Eucalyptocrinus – a type of crinoid that had a relatively rigid calyx, or cup, where the organs were centered. It was given to me by an 8th grader who found it while on a field trip led by the Indiana University geology club. On the top of the cup-like calyx there’s a snail, a gastropod, common in the Waldron. The gastropod’s name is Platystoma, and it’s found commonly in close association with crinoid calyxes. Such close association, in fact, that it’s thought that they lived in a symbiotic relationship with the crinoids. Crinoids’ mouths and their anuses were both in the calyx, next to each other, and the interpretation is that the snail Platystoma lived on top of the crinoid, eating the crinoid’s waste products. That gives it the technical name coprophagus – which means poop eating. An alternative explanation is that there was no symbiosis at all – the snail was there because it was eating, and killing, the crinoid. 

—Richard I. Gibson 

Eucalyptocrinus drawn by James Hall (1881)

Links:
Waldron shale 
Probable coprophagus snail
Coprophagus snail 
Waldron fossils