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Department of Geology, Oklahoma State University, Stillwater, Oklahoma 74078, U.S.A.
Many graptolite taxa are important biostratigraphically because their first appearances in stratigraphic sequences represent unique events that are recognizable and correlative worldwide. However, because the direct ancestry of many of these taxa are cryptic, the biostratigraphical value of their first appearances may be questioned.
Astogenetic and ontogenetic studies of well-preserved, isolated specimens of Nemagraptus gracilis, Dicellograptus (four species), Leptograptus trentonenesis, Glossograptus ciliatus and Glyptograptus euglyphus from the middle Ordovician Athens Shale of Alabama show that considerable graptolite evolution occurs through heterochony. And, what are here considered to be small heterochronic changes, often produce large morphological discontinuities between ancestors and descendants. In turn the presence of these morphological discontinuities in graptolite sequences suggest that graptolite evolution follows the model of punctuated equilibrium, rather that that of phyletic gradualism. Whether nor not this is the case is here considered to be a matter of semantics. More importantly, heterochronic evolution is here considered to explain the cryptic ancestry of many graptolite taxa, and with this mode of evolution in mind, the graptolite biostratigrapher may find it easier to determine ancestor-descendent relations and thus recognize those first appearances that are valid for biostratigraphical correlation.
