Chesapecten fossils

Among the many shells that can be found on the beaches of southern Maryland and some other mid-Atlantic states are scallop shells. In many instances, the shells are actually fossil shells rather than of recent vintage. This post is primarily meant to alert other naturalists in localities such as Calvert County, Maryland that the majority of shells found on the beach are actually fossil shells and might be misidentified as belonging to a recent species. The secondary intent is to provide some help to those that might be interested in learning more about the fossils. In Calvert County, the best resource would be the Calvert Marine Museum which houses an extensive collection of local fossils.

Fossil Genus Chesapecten

Class Bivalvia
Family Pectinidae
Genus Chesapecten

The genus Chesapecten existed from the mid-Miocene Epoch, approximately 20 million years ago, until its extinction during the late Pliocene Epoch, between 3 to 4 million years ago. Of historical significance, Chesapecten jeffersonius was the first described fossil from the New World. In 1687, a pecten specimen was the first figured and described fossil from America in Martin Lister’s publication Historiae Conchyliorum Liber III. Lister did not give the specimen a scientific name. Thomas Say, a naturalist at the Philadelphia Academy of Science, recognized Lister’s plate #167 as his named species “Pecten jeffersonius” in 1824. In 1975, the species was reassigned to the newly created genus Chesapecten by Ward and Blackwelder.

The following is a guide to the various Chesapecten species arranged by formation:

Calvert Formation:
Plum Point Marl Member (Beds 4-10), middle Miocene Epoch, Burdigalian/Langhian Stage
Calvert Beach member (Beds 14-16), middle Miocene Epoch, Langhian Stage
Chesapecten coccymelus (Dall, 1898) ​​​​bed 10​
Chesapecten skiptonensis (Mansfield, 1936) beds 14-15​ rare
Chesapecten nefrens Ward & Blackwelder, 1975​​ beds 14-16​
Chesapecten marylandica (Wagner, 1839)​​​ beds 15-16​ uncommon
​
Choptank Formation:
Drumcliff Member (Bed 17), middle Miocene Epoch, Serravalian Stage
St. Leonard Member (Bed 18), middle Miocene Epoch, Serravalian Stage
Boston Cliffs Member (Bed 19), middle Miocene Epoch, Serravalian Stage
Chesapecten marylandica (Wagner, 1939) ​​bed 17​ uncommon
Chesapecten nefrens Ward & Blackwelder, 1975​​ beds 17-19​

St. Marys Formation:
Little Cove Point Member (Beds 21-23), upper Miocene Epoch, Tortonian Stage
Windmill Point Member (Bed 24), upper Miocene Epoch, Tortonian Stage
Chesapecten covepointensis Ward, 1992 ​​​beds 21-23
Chesapecten santamaria (Tucker, 1934) ​​​bed 24

Eastover Formation:
Claremont Member, upper Miocene Epoch, Tortonian Stage
Chesapecten middlesexensis (Mansfield, 1936)
C. middlesexensis middlesexensis (Mansfield, 1936)
C. middlesexensis ceccae Ward, 1992
Cobham Bay Member, upper Miocene Epoch, Tortonian/Messinian Stage
Chesapecten middlesexensis middlesexensis (Mansfield, 1936)

Yorktown Formation:
Sunken Meadow Member, lower Pliocene Epoch, Zanclean Stage
Chesapecten jeffersonius (Say, 1824)
Rushmere, Morgarts Beach, and Moore House Members, upper Pliocene Epoch, Piacenzian Stage
Chesapecten madisonius (Say, 1824)
Chesapecten septenarius (Say, 1824)

Unless noted, the relative occurrence of the various species is common to abundant. Biostratigraphic assignments based on Ward, 1992 (see below).

References:
LW Ward, Molluscan Biostratigraphy of the Miocene, Middle Atlantic Coastal Plain of North America, Virginia Museum of Natural History, Martinsville, VA (1992).

LW Ward & BW Blackwelder, Chesapecten, a New Genus of Pectinidae (Mollusca: Bivalvia) from the Miocene and Pliocene of Eastern North America, United States Government Printing Office, Washington, DC (1975).

LW Ward & BW Blackwelder, Stratigraphic Revision of Upper Miocene and Lower Pliocene Beds of the Chesapeake Group, Middle Atlantic Coastal Plain, United States Government Printing Office, Washington, DC (1980).

Posted on February 3, 2019 09:23 PM by rosalie-rick