Abstract

The genus Archiclimacograptus represents one of the earliest taxa of axonophoran graptolites with geniculate thecae, but its current status and evolutionary relationships are difficult to access even after a number of cladistic analyses have been executed. Archiclimacograptus? pungens (Ruedemann), a poorly known little species from the lower Darriwilian of eastern North America is interpreted as a possible member of the genus. The species belongs to a group of axonophorans with highly asymmetrical proximal end and pattern C astogeny, characterized by the lack of an apertural spine on th12, ranging from the Levisograptus dentatus Biozone (Darriwilian) to the Nemagraptus gracilis Biozone (Sandbian). Specimens are common in North America and the Argentinian Precordillera, but have not been reported from any Atlantic Faunal Realm localities. The distribution of the group indicates a restriction to the Pacific Faunal Realm, providing the earliest indication of a developing biogeographic differentiation of axonophoran graptolites after their origin in the oceanic biofacies during the Upper Dapingian to lower Darriwilian time interval. Archiclimacograptus? ambiguus is described as a new species.

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