Acta Palaeontologica Polonica

The affinities of Isograptus, Glossograptus, Cryptograptus, Corynoides, and allied graptolites

Stanley C. Finney

Acta Palaeontologica Polonica 23 (4), 1978: 481-495

Numerous weif:preserved specimens referred to Glossograptus ciliatus Emmons and Cryptograptus marcidus (Hall), (= C. schaejeri Lapworth), have been isolated from limestones of the Middle Ordovician Athens Shale of Alabama. Early growth stages of both these species show an isograptid type of development. This, fact, together with new data from studies of the detailed thecal morphology, and re-interpretations of previously described species suggest that: 1) Glossograptus arose from an isograptid ancestor, possibly Isograptus itself; 2) the ancestry of the other genera of Glossograptidae can be found in Glossograptus; 3) cryptograptids evolved, from an early glossograptid or evolved independently from an isograptid ancestor; and 4) the, corynoidids evolved from Glossograptus. Flattened, non-isolated specimens of Isograptus lyra Ruedemann, which were also collected from the Athens Shale display a pericalycal arrangement of the proximal end and are referred to a new genus, which probably arose from Glossograptus through divergence of the stipes.

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