NEW MARINE MACROALGAE FROM THE ORDOVICIAN OF WISCONSIN AND MICHIGAN

Ordovician material from the Platteville Formation (Sandbian) of southern Wisconsin and Big Hill Formation (Katian) of northern Michigan is reported that provides novel information about the phylogenetic affinity, taxonomic diversity, and stratigraphic range of the nonbiomineralized taxa Buthograptus, Callithamnopsis, and Chaetocladus. Two new species of Buthograptus, a previously monotypic genus, are recognized on the basis of the Platteville Formation material, and new occurrences of the type species, B. laxus, are documented from several localities and two distinct stratigraphic levels within this unit, these collectively comprising new examples of algal-Lagerstätten. Results of scanning electron microscopic investigation of the Buthograptus material, as well as the nature of morphological variation between the various Buthograptus species, indicate a likely phylogenetic affinity with the green algal order Bryopsidales, the plumose thallus morphology resembling in some key respects that of extant Caulerpa. Additional material from the Platteville Formation provides new morphological details for the Callithamnopsis type species, C. fruticosa, aspects of which indicate that the genus belongs to the family Triploporellaceae rather than Seletonellaceae within the green algal order Dasycladales, and Chaetocladus material from the Big Hill Formation, which occurs in association with eurypterids and medusae, includes specimens referable to Chaetocladus dubius, a species of dasycladalean alga known previously only from the mid-Silurian of Ontario. All of these algal taxa show monopodial branching, a mode of growth common in the “Ordovician Flora” but generally lacking in the preceding “Cambrian Flora.” Collectively, the Wisconsin and Michigan material provides important new insights about macroalgae in the Ordovician biosphere and the early evolution of the Ulvophyceae (Chlorophyta).