The oldest sinistral cones and Busycon from eastern North America
appear in the lower Pliocene (Tamiami/lower Goose Creek and
equivalents), well before 3.8 million years ago (a minimum age of
younger layers).  The Pliocene began about 5 million years ago, and
there's one major layer below the appearance of the sinistral forms,
so a reasonable guess would put them around 4.2 million or so.  The
youngest of the cones are in the Caloosahatchee and Waccamaw
formations, right around the standard Pliocene/Pleistocene boundary at
1.8 million.  However, there are long-standing efforts to put the
Pliocene/Pleistocene boundary earlier, and a recent decision put the
Quaternary-Tertiary boundary (traditionally equated with the
Pliocene-Pleistocene boundary) back in what's currently the upper
Pliocene.

--
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"

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