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The morphology and ultrastructure of Brevigraptus quadrithecatus n. gen., n. sp. (Diplograptacea), and Its convergence upon Dicaulograptus hystrix (Bulman)

Published online by Cambridge University Press:  02 September 2016

Charles E. Mitchell
Charles E. Mitchell*
Affiliation:
Department of Geology, State University of New York at Buffalo, Amherst 14226
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Abstract

Rhabdosomes of Brevigraptus quadrithecatus n. gen., n. sp. (Lasiograptinae, Diplograptacea), from the Upper Ordovician Viola Springs Formation of south-central Oklahoma, comprise four fully developed thecae. The sicula and the first two thecae are fully sclerotized. The ultrastructure of the fusellum is unusually dense but is overlaid by a typical diplograptacean bandaged cortex. The third and fourth thecae consist of clathria covered by a cortical sheet. Lacinia are absent. The cortical sheet comprises bandages deposited in a support dominated pattern that matches expectations of the pterobranch model of peridermal secretion. Lists are fusellar derivatives and exhibit traces of fuselli-like growth increments but no continuous fusellum is present. Lists are strongly thickened with cortical tissue. The fabricational pattern employed in list construction reveals the operation of strong historical constraints during the evolutionary reduction of the fusellum.

The thecal form and list architecture of Brevigraptus quadrithecatus are nearly identical to those of Pipiograptus hesperus Whittington. Brevigraptus quadrithecatus possesses a Pattern G astogeny and exhibits several derived astogenetic features that it shares with P. hesperus and Orthoretiolites hami Whittington. Both thecal and astogenetic similarities suggest the new taxon is a member of the Lasiograptinae, and is closely allied to the aforementioned species.

The thecae of B. quadrithecatus exhibit striking similarity with Dicaulograptus hystrix (Bulman). However, both the details of thecal construction and primordial astogeny differ markedly between these species. The thecal similarities appear to be convergent. Accordingly, D. hystrix is probably not closely allied to the Lasiograptinae.

Type
Research Article
Information
Journal of Paleontology , Volume 62 , Issue 3 , May 1988 , pp. 448 - 463
Copyright
Copyright © The Paleontological Society 

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