The graptolite Amplexograptus praetypicalis n. sp. and the origin of the typicalis group

Abstract

The graptolite Amplexograptus praetypicalis n. sp. is described and assigned to a more precisely defined and amended genus Amplexograptus Elles and Wood. This graptolite ranges through the lower half of the Climacograptus spiniferus Zone of northeastern North America and evolves, through a distinct process of phyletic gradualism, into the better known Climacograptus typicalis Hall. Climacograptus typicalis ranges through the upper half of the C. spiniferus Zone and the succeeding Climacograptus pygmaeus Zone, giving rise to three related species: the dwarfish Climacograptus pygmaeus Ruedemann, the gigantic Climacograptus magnificus Twenhofel, and the atavistic Climacograptus posterus Ruedemann. Together, these four species form a distinct group, termed the typicalis group, and a distinct lineage that had a brief existence from the late Middle to the early Late Ordovician. The genus Uticagraptus n. g. is here proposed for the graptolites of the typicalis group. This group is characterized by an advanced prosoblastic type of proximal-end development and an aseptate rhabdosome and, thus, differs fundamentally from Climacograptus bicornis (Hall), the type species of Climacograptus, with a more primitive diplograptid type of proximal-end development and a septate rhabdosome.