THE EVOLUTION OF LAURENTIAN (NORTH AMERICAN) SPECIES OF TRETASPIS (ORDOVICIAN TRILOBITA)

Discovery of a new species of Tretaspis (trinucleid trilobite) in the Ordovician Myerstown Formation near Lebanon, Pennsylvania sheds new light on the evolution of Laurentian (North American) Tretaspis species. All five known North American species of Tretaspis lived in deep ramp environments of the Taconic foreland basin, on the southern margin of Laurentia. Tretaspis sp. nov. occurs in southeast Pennsylvania, is of early Sandbian age, and is the oldest known North American species of Tretaspis. The fringe of its cephalon has just four anterior (sag.) rows of pits. T. sp. nov. is likely a common ancestor to T. sagenosus of Virginia (middle Sandbian age) and T. reticulatus of Virginia and New York (middle Sandbian to early Katian age), which have six and seven anterior rows of fringe pits, respectively. T. canadensis of Quebec is also of early Katian age, has seven anterior fringe pits, and may be ancestral to T. reticulatus. T. clarkei of Quebec is of middle to late Katian age and does not appear to be ancestral to any of the older North American species. It has just four anterior rows of fringe pits and is nearly identical to European species of the T. seticornisgroup. It likely represents a migratory species from Baltica.