The Mt. Orab Bed: Fauna and Evidence
The Sunset Member butter shale, repeated storm burial, and the difference between a Mt. Orab record and a regional possibility.
One interval, repeated burial events
The Mt. Orab butter shale is an informal bed in the Sunset Member of the Arnheim Formation, Late Ordovician (Richmondian). The studied interval is approximately 0.46 m thick and contains at least seven recognizable clay-silt couplets interpreted as stacked storm-generated event beds. It is not one single burial event.
Clay and silt preserve different records
The basal silt layers contain more transported skeletal debris and disarticulated trilobite remains. The clay-rich layers preserve more articulated organisms and are interpreted as largely in-place communities rapidly buried at or near where they lived. The distinction is between the layers within the studied interval, not simply limestone versus shale across the formation.
Enrolled, prone and partially enrolled trilobites occur alongside carcasses and molts. Enrolment alone does not establish how an individual died. The study found no strong evidence for one reproductive or molting aggregation.
Confirmed Mt. Orab fauna
Flexicalymene retrorsa, a calymenid (Calymenidae), is the characteristic named trilobite. Isotelus is documented at genus level. Crinoids, Modiolopsis bivalves, bryozoans and the trace fossil Chondrites are also documented. Lingulids are reported but brachiopods are outside this guide’s main focus.
Not every Arnheim fossil is a Mt. Orab record
Pycnocrinus dyeri and associated platyceratid gastropods are reported at Arnheim Formation level in my working reference, not securely from the butter-shale horizon. Named gastropods, cephalopods, additional crinoids and rare trilobites require locality-specific evidence before inclusion in a Flat Run fauna list.
Identification is specimen-specific
Use the narrowest identification supported by the specimen. Formation-wide records do not establish a Mt. Orab or Flat Run occurrence. A locality study does not authenticate an individual lot.
Use Isotelus sp. unless diagnostic morphology supports a species. Do not automatically replace an existing Flexicalymene identification with F. retrorsa merely because that species is documented in the bed.
Source and scope
Hunda, B. R., Hughes, N. C., & Flessa, K. W. (2006). Trilobite taphonomy and temporal resolution in the Mt. Orab Shale Bed (Upper Ordovician, Ohio, U.S.A.). PALAIOS, 21, 26–45.
This summary follows the supplied Mt. Orab working fauna research draft. Its formation-wide and regional comparisons are provisional and need stronger locality-specific documentation.