GEOGRAPHIC AND ENVIRONMENTAL DISTRIBUTION OF TWO LATE ORDOVICIAN REMOPLEURIDID TRILOBITES AND THEIR IMPLICATIONS FOR LIFESTYLE

Hypodicranotus is a Middle to Late Ordovician remopleuridid with a distinctively long, forked hypostome that would have prohibited enrollment. Its widespread distribution across Laurentia, with reported occurrences from the Ural Mountains and far northeast Asia, have led to speculation concerning mode of life. Numerous authors have suggested a nektobenthic or epipelagic lifestyle. Distribution from high-energy, shallow ramp (e.g. Kimmswick Formation, Missouri; Trenton Group, New York) to slope (e.g. Mifflin Formation, Platteville Group, Wisconsin and Minnesota; Hanson Creek Formation, Nevada) environments suggests a lack of substrate control as is expected for benthic organisms. Wide geographic distribution should be accompanied by low species-level diversity, which appears to be the case with Hypodicranotus, although a modern systematic assessment has not been attempted. With the exception of the Laurentian species H. striatulus and H. virgulatulus from the Ural Mountains, most have been described only as Hypodicranotus sp. The closely related Remopleurides has a pandemic, equatorial distribution, occurring from Gondwana through China to Baltica, and across Laurentia and is another candidate for an at least limited pelagic mode of life. Like Hypodicranotus, it is preserved in a wide range of depths and has low species-level diversity.

This study assesses the likelihood of a nektobenthic or epipelagic mode of life for Hypodicranotus and Remopleurides. Stratigraphic data and environmental interpretations from the Late Ordovician Viola Group, Oklahoma and Whittaker Formation, Mackenzie Mountains, allow us to document facies distribution in Laurentia, as well as data from all other localities for which environmental interpretations can be made from the literature. Additional insight is gained by comparison of the global distribution of Hypodicranotus and Remopleurides to that of Carolinites, which is a taxon that is widely accepted as epipelagic.