Noncalcified macroalgal communities in Ordovician Midwestern seas; an overview

Document Type


Publication Date



Geography and Geology


Among fossils currently regarded as noncalcified macroalgae, four species from a single occurrence in the Ordovician of Wisconsin, described in an 1894 report by R. P. Whitfield, were the first to be recognized as such from the North American fossil record (two of these species were initially described by James Hall in 1861 and 1865 as graptolite taxa). In the century since Whitfield's report, nine additional occurrences of noncalcified macroalgal taxa, comprising some 22 species, have been identified in the Ordovician of North America, including a new occurrence from Wisconsin reported here for the first time. Comparison of these occurrences from taphonomic and sedimentologic perspectives reveals that all comprise essentially autochthonous biotas that inhabited poorly circulated, shallow, muddy-bottom settings. Nearly all of the biotas are alga-dominated and all include taxa assignable to the extant green algal order Dasycladales. For seven (including the new occurrence), the flora comprises only dasycladalean algae, and in two of the remaining three, dasyclads are numerically dominant. All of the algal taxa in these occurrences are haptophytes and all lack a cortex. In contrast, macroalgal communities in analogous settings of modern tropical seas tend to be dominated by heavily corticated rhizophytic taxa. Forms similar to those observed in the Ordovician persist in these settings as opportunists, rapidly colonizing bare zones when they become available. This study is part of a larger-scale effort to construct a comprehensive morphological and ecological history for early Paleozoic macroalgae. This in turn will allow for (1) identification and characterization of macroalgal morphological diversification events during this timeframe, (2) development of broader ecological contexts for the Cambrian Explosion and Ordovician Radiation biodiversification events, and (3) detailed comparisons of macroalgal floras across the Proterozoic/Phanerozoic boundary. From a sedimentological perspective, documentation of "algal-Lagerstatten" contributes important information about depositional and diagenetic environments and sequence stratigraphic architectures of sedimentary basins, and the nature and origin of organic material within Lower Paleozoic source rocks.