NEW OBSERVATIONS ON THE BASAL STRUCTURE OF THE GRAPTOLITE GENUS APPENDISPINOGRAPTUS, AND IMPLICATIONS FOR BIODIVERSITY AND BIOGEOGRAPHY

Graptolites of the genus Appendispinograptus are major components of the Late Ordovician oceanic ecosystem, and feature prominently in biostratigraphy and studies of biodiversity. Secondary structures extending along the basal spines occur in mature specimens of three of the four common late Katian species of Appendispinograptus: A. supernus, A. longispinus, and A. venustus. Using SEM photography of an isolated, 3-dimentional specimen of A. supernus?, we confirm the suggestion of previous authors that these structures are composed of hollow tubes extending from the sicular and thecal apertures. We also describe a collection of Canadian (northern Yukon) appendispinogratids possessing a remarkable heart-shaped basal structure. Detailed comparison (width, thecal length and inclination) with immature specimens from our collection, as well as collections from China and Nevada, identify these specimens as A. leptothecalis, a species not previously known to exhibit secondary spine modification. This observation allows enigmatic specimens from China, Siberia, and Nevada to be assigned to A. lepthecalis, clarifying our understanding of appendispinograptid biogeography and diversity. In addition to tubes (parasiculae), our Yukon specimens also commonly possess a large membrane-like structure that completely encloses the proximal end of the rhabdosome, a feature unique amongst appendispinograptids. This feature is also present in Siberian specimens, but is absent in Scotland, Nevada, and China, suggesting ecological or biogeographic affinity between northwestern Laurentia and the Kolyma region of Siberia.