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Monograptus spiralis and its phylogenetic relationship to early cyrtograptids

Published online by Cambridge University Press:  19 May 2016

A. C. Lenz  and
M. J. Melchin
A. C. Lenz
Affiliation:
Department of Geology, University of Western Ontario, London N6A 5B7, Canada
M. J. Melchin
Affiliation:
Department of Earth Sciences, University of Waterloo, Waterloo N2L 3GI, Canada
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Abstract

Isolated, uncompressed specimens of Monograptus spiralis show that the thecae become increasingly asymmetric and twisted distally and that the lateral margins of the apertures possess lobate veins and extend laterally as spatulate rostral processes. These too become increasingly asymmetric distally.

Three species of Cyrtograptus, obtained uncompressed from the same and overlying strata, are compared with M. spiralis. They show little similarity in the details of proximal or distal thecal form. Cyrtograptus sakmaricus possesses more stout, overlapping proximal thecae, while those of C. cf. C. laqueus are slender and elongate. In both species, the thecae are moderately asymmetrical, become simpler and less strongly hooked distally, and have simple lateral margins. The third species, Cyrtograptus sp., possesses proximal thecae very like those of C. sakmaricus, but its distal thecae, while strongly twisted, are very much simpler than the type found in M. spiralis.

Despite superficial rhabdosomal similarities in flattened material, the thecal details clearly indicate that M. spiralis is very unlikely to be ancestral to any known species of Cyrtograptus owing to its unique and highly specialized thecal form. Comparison among species of Cyrtograptus suggests, however, that this genus might be polyphyletic, with its ancestors being other members of the “Spirograptus” group.

Type
Research Article
Information
Journal of Paleontology , Volume 63 , Issue 3 , May 1989 , pp. 341 - 348
Copyright
Copyright © The Paleontological Society 

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