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Numerous well-preserved specimens referred to Glossograptus ciliatus Emmons and Cryptograptus
marcidus (Hall), ( = C. schaeferi 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, togehter with new data from studies of the detailed thecal morphopogy, 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 40 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.
|THE AFFINITIES OF ISOGRAPTUS, GLOSSOGRAPTUS, CRYPTOGRAPTUS, CORYNOIDES,
AND ALLIED GRAPTOLITES
STANLEY C. FINNEY
Acta Palaeontologica Polonica 23 (1978), 4, 481-495