Abstract

An investigation of the morphology and astogenetic pattern of isolated specimens of two species of Paraclimacograptus permits an assessment of the phylogenetic relationships of this genus. Forty-two specimens previously assigned to Paraclimacograptus innotatus ssp. isolated from turbiditic limestone from the Southern Urals of Russia (Cystograptus vesiculous Zone) and limestone concretions from the Canadian Arctic (Coronograptus cyphus Zone) were examined using the method of infrared video microscopy. Paraclimacograptus has a Pattern H astogeny and is therefore a member of the family Normalograptidae. At least three species can be distinguished using biometric criteria, P. innotatus, P. exquisitus, and P. obesus. Rhabdosomal characteristics of Paraclimacograptus indicate that it is phylogenetically related to early species of the genus Neodiplograptus. The point of divergence of the proximal thecae is defined by the interfingering of fuselli of markedly different widths. This suggests that fuselli were not necessarily secreted at a constant rate. In addition, this pattern of fusellar interfingering is here regarded as more consistent with a pterobranch mode of secretion than with a model of growth under an enveloping extrathecal mantle.

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