This is strongly shell-related, as the upper Claibornian units in the U.S. Gulf Coast have several hundred species of mollusks, and the Lutetian of the Paris Basin has over 1000.

The most recent microfossil correlations actually move the boundaries a bit further.  The top of the Claibornian (Claibornian-Jacksonian boundary) is within the Bartonian.  The base of the Claibornian is within the Ypresian (lower Eocene).  None of the European or global stage boundaries correlate exactly with the major changes in the southeastern U.S., although the Eocene-Oligocene boundary is almost equal to the next stage boundary (Jacksonian-Vicksburgian boundary).

U.S. Jacksonian=global upper Bartonian, all of Priabonian, and very base of Rupelian
U.S. Claibornian=global upper Ypresian, all of Lutetian, lower Bartonian

Thus, the diverse faunas of the upper Claibornian are lower Bartonian (upper Middle Eocene).  Note also that some older literature refers only to the Gosport Sand as Claibornian and the rest of the modern Claibornian as St. Maurice.  The Gosport Sand is the top of the Claibornian as presently defined.

    Dr. David Campbell
    "Old Seashells"
    Biology Department
    Saint Mary's College of Maryland
    18952 E. Fisher Road
    St. Mary's City, MD  20686-3001 USA
    [log in to unmask], 301 862-0372 Fax: 301 862-0996
"Mollusks murmured 'Morning!'.  And salmon chanted 'Evening!'."-Frank Muir, Oh My Word!