First middle ordovician ostracods from Western Avalonia: Paleogeographical and paleoenvironmental significance

摘要

Two new species of ostracods, Conchoprimitia cassidula n. sp. and Sorornanopsis avalonensis n. gen. n. sp., represent the first described Middle Ordovician ostracods from western Avalonia. They were recovered as phosphatized carapaces dissolved out of a late early Darriwilian (ca. 467 Ma) limestone boulder from the Triassic Lepreau Formation of New Brunswick, Canada. The ostracods form a low-diversity component of a higher energy, near-shore, shelf marine fauna dominated by the trilobites Neseuretus and Stapleyella and by the conodonts Drepanoistodus and Baltoniodus. The low diversity of this Avalonian ostracod fauna contrasts with more diverse (tens of species), coeval ostracod faunas from Laurentia and Baltica. The association of Darriwilian ostracods and trilobites from New Brunswick demonstrates continuing exchange of open marine, cool water biota between Avalonia, Baltica, and West and North Gondwana that began in the late early Cambrian.