Codium-like taxa from the Silurian of North America: morphology, taxonomy, paleoecology, and phylogenetic affinity

Abstract

AbstractA 1901 report by the Smithsonian Custodian of Paleozoic Plants noted that the nonbiomineralized taxaButhotrephis divaricataWhite, 1901,B. newliniWhite, 1901, andB. lesquereuxiGrote and Pitt, 1876, from the upper Silurian of the Great Lakes area, shared key characteristics in common with the extant green macroalgaCodium. A detailed reexamination of theseCodium-like taxa and similar forms from the lower Silurian of Ontario, New York, and Michigan, including newly collected material ofThalassocystis striataTaggart and Parker, 1976, aided by scanning electron microscopy and stable carbon isotope analysis, provides new data in support of an algal affinity. Crucially, as withCodium, the originally cylindrical axes of all of these taxa consist of a complex internal array of tubes divided into distinct medullary and cortical regions, the medullary tubes being arranged in a manner similar to those of livingPseudocodium. In view of these findings, the three study taxa originally assigned toButhotrephis, together withChondrites verusRuedemann, 1925, are transferred to the new algal taxonInocladusnew genus, thereby establishingInocladus lesquereuxinew combination,Inocladus newlininew comb.,Inocladus divaricatanew comb., andInocladus verusnew comb. Morphological and paleoecological data point to a phylogenetic affinity forInocladusn. gen. andThalassocystiswithin theCodium-bearing green algal order Bryopsidales, but perhaps nested within an extinct lineage. Collectively, this material fits within a large-scale pattern of major macroalgal morphological diversification initiated in concert with the Great Ordovician Biodiversification Event and apparently driven by a marked escalation in grazing pressure.UUID:http://zoobank.org/97c5c737-b291-41a2-aceb-f398cac9537a