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Background  Chasmataspidids are a rare group of chelicerate arthropods known from 12 species assigned to ten genera, with a geologic range extending from the Ordovician to the Devonian. The Late Ordovician (Richmondian) fauna of the Big Hill Lagerstätte includes a new species of chasmataspidid represented by 55 specimens. This taxon is only the second chasmataspidid described from the Ordovician and preserves morphological details unknown from any of the previously described species.    Results  The new chasmataspidid species is described as  Hoplitaspis hiawathai  gen. et sp. nov.. Comparison with all other known chasmataspidids indicates that  Hoplitaspis  occupies an intermediate morphological position between the Ordovician  Chasmataspis  and the Silurian-Devonian diploaspidids. While the modification of appendage VI into a broad swimming paddle allies  Hoplitaspis  to the Diploaspididae, the paddle lacks the anterior ‘podomere 7a’ found in other diploaspidids and shows evidence of having been derived from a  Chasmataspis -like chelate appendage. Other details, such as the large body size and degree of expression of the first tergite, show clear affinities with  Chasmataspis , providing strong support for chasmataspidid monophyly.    Conclusions  The large body size and well-developed appendage armature of  Hoplitaspis  reveals that chasmataspidids occupied a greater breadth of ecological roles than previously thought, with the abundance of available specimens indicating that  Hoplitaspis  was an important component of the local community. The miniaturization and ecological limiting of diploaspidids potentially coincides with the major radiation of eurypterids and may suggest some degree of competition between the two groups. The geographic distribution of chasmataspidid species suggests the group may have originated in Laurentia and migrated to the paleocontinents of Baltica and Siberia as tectonic processes drew the paleocontinents into close proximity.

A common arthropod from the Late Ordovician Big Hill Lagerstätte (Michigan) reveals an unexpected ecological diversity within Chasmataspidida