File:Lichenocrinus & Sphenothallus encrusting a paleocobble (Kope Formation, Upper Ordovician; Aurora, Indiana, USA) 1.jpg

English: Encrusted limestone paleocobble from the Ordovician of Indiana, USA.

This rock is from the famous Cincinnatian Series of the tristate area of Ohio-Kentucky-Indiana. Rocks in the Cincinnatian were deposited in relatively shallow marine facies during the Late Ordovician. The Cincinnatian succession is mostly interbedded limestones and shales. Most of the limestones are event beds (= tempestites), deposited during ancient storms.

The specimen seen here is a paleocobble composed of lime mudstone - it was originally a loose clast on an ancient Cincinnatian seafloor. Such rocks are also called mobile hardgrounds. Organisms that occupy cobble substrates include various encrusters and borers.

The mottled grayish buttons attached to the cobble are encrusting Lichenocrinus crinoid holdfasts. The two black spots near the bottom of the picture are Sphenothallus holdfasts. Sphenothallus is an enigmatic organism that is essentially a jet-black "worm tube" attached to a small, circular holdfast. The worm tube itself is rarely preserved.

Stratigraphy: float from the lower Kope Formation, lower Edenian Stage, lower Cincinnatian Series, Upper Ordovician

Locality: Aurora cobble cut - construction site and cut wall immediately west of & adjacent to the intersection of Florence Drive & Route 50, just northeast of the town of Aurora & southwest of Lawrenceburg, southeastern Dearborn County, southeastern Indiana, USA (39° 04' 47.87" North latitude, 84° 53' 12.03" West longitude)