ASTOGENY AND PHYLOGENY OF OF SOME LATE ORDOVICIAN GRAPTOLITES FROM OKLAHOMA: NEW EVIDENCE FROM THREE-DIMENSIONALLY PRESERVED SPECIMENS

The Late Ordovician Viola Group of the Arbuckle Mountains in Oklahoma is well known for containing three-dimensionally preserved specimens of graptolites. Viola Group graptolites have greatly improved our understanding of the astogeny and phylogeny of several graptolite groups. We isolated well-preserved specimens of Climacograptus caudatus, Neurograptus margaritatus, Corynoides americanus, and Dicranograptus hians from the Mountain Lake section (Alberstadt section I), and Climacograptus cruciformis, Dicranograptus spinifer, Corynoides calicularis, and Rectograptus n. sp. from the U.S. Highway 99 section near Fittstown (Alberstadt section D). The Mountain Lake fauna indicates a Coryoides americanus Zone age, and the US 99 specimens are probably Climacograptus bicornis Zone in age. Several of these taxa have not previously been described from three-dimensional material and need revised descriptions of their astogeny and phylogenetic relationships. Climacograptus caudatus Lapworth is a globally distributed taxon, usually recognized by its long virgella and parasicula. Our 3-D material reveals that the long spine is actually a theca 1¹ spine and that the parasicula hides a small, deflected virgella. C. caudatus has a Pattern E proximal development, is closely related to Diplacanthograptus spiniferus, and should be referred to Diplacanthograptus. Climacograptinids from the basal Viola Springs Formation at section D that have previously been referred to Climacograptus bicornis are actually C. cruciformis Vandenberg. These specimens have a Pattern D proximal development, a long virgella, and horizontal theca 1¹ and 1² spines. Neurograptus margaritatus has a Pattern G proximal development and very short sub-apertural walls from which branched ventral spines and lacinia develop. It’s periderm is greatly reduced, particularly in the sub-apertural region of the first two thecae where thinning results in a characteristic “birds-head” opening. Similar structures occur in Brevigraptus, Orthoretiolites, and Pipiograptus. Our specimens of Neurograptus margaritatus confirm its close relationship to other Lasiograptinae. A new species of Rectograptus retains the primitive character of thecal lappets, which reinforces its close phylogenetic relationship to Amplexograptus.