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Morphology and phylogeny of some early Silurian (Mid-Rhuddanian) monograptid graptolites from the South Urals of Russia

Published online by Cambridge University Press:  20 May 2016

Michael J. Melchin  and
Tatjana N. Koren'
Michael J. Melchin
Affiliation:
Geology Department, St. Francis Xavier University, P.O. Box 5000, Antigonish, Nova Scotia B2G 2W5, Canada,
Tatjana N. Koren'
Affiliation:
All-Russia Geological Research Institute (VSEGEI), Sredny Pr. 74, 199026, St. Petersburg, Russia,
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Abstract

An unusually well-preserved fauna of isolated monograptid graptolites has been recovered from the Sakmara Formation, in the Orenburg District of the South Urals of Russia. Seven different taxa belonging to three genera can be distinguished in these collections, including four new species, Pribylograptus orskensis n. sp., Monoclimacis? galeritus n. sp., Monoclimacis? orenburgensis n. sp., and Monoclimacis? oscitatus n. sp. Study of these new species using infrared video microscopy reveals two features never previously identified in monograptid graptolites on the three species of Monoclimacis?: thecal hoods constructed from both apertural and genicular fuselli; and, a resorption porus that shows some outward inflation or bulging as well as slight fusellar deflection. These two features may both represent stages of morphologic transition to the more “advanced” primary porus and apertural hoods and hooks seen in later monograptids. Phylogenetic analysis of these and some other Rhuddanian taxa suggests that species with “monoclimacid”” thecae arose once in the Rhuddanian from a Huttagraptus ancestor and these species later gave rise to the various groups of biform and uniform monograptids with apertural-hooded and hooked thecae. Those biform taxa with “monoclimacid” or hooked proximal thecae and lappeted (“pribylograptid”) distal thecae appear to have arisen from within this group independently of the lineage leading to Pribylograptus sensu stricto.

Type
Research Article
Information
Journal of Paleontology , Volume 75 , Issue 1 , January 2001 , pp. 165 - 185
Copyright
Copyright © The Paleontological Society

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