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Polymorphic organization in a planktonic graptoloid (Hemichordata: Pterobranchia) colony of Late Ordovician age

Published online by Cambridge University Press:  09 August 2012

JAN A. ZALASIEWICZ ,
ALEX PAGE ,
R. BARRIE RICKARDS ,
MARK WILLIAMS ,
PHILIP R. WILBY ,
MICHAEL P. A. HOWE  and
ANDREA M. SNELLING
JAN A. ZALASIEWICZ*
Affiliation:
Department of Geology, University of Leicester, Leicester LE1 7RH, UK
ALEX PAGE
Affiliation:
Department of Geology, University of Leicester, Leicester LE1 7RH, UK
R. BARRIE RICKARDS
Affiliation:
Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK
MARK WILLIAMS
Affiliation:
Department of Geology, University of Leicester, Leicester LE1 7RH, UK
PHILIP R. WILBY
Affiliation:
Department of Geology, University of Leicester, Leicester LE1 7RH, UK British Geological Survey, Keyworth, Nottingham NG12 5GG, UK
MICHAEL P. A. HOWE
Affiliation:
British Geological Survey, Keyworth, Nottingham NG12 5GG, UK
ANDREA M. SNELLING
Affiliation:
British Geological Survey, Keyworth, Nottingham NG12 5GG, UK
*
*Author for correspondence: [email protected]
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Abstract

Graptolites are common fossils in Early Palaeozoic strata, but little is known of their soft-part anatomy. However, we report a long-overlooked specimen of Dicranograptus aff. ramosus from Late Ordovician strata of southern Scotland that preserves a strongly polymorphic, recalcitrant, organic-walled network hitherto unseen in graptoloid graptolites. This network displays three morphologies: proximally, a strap-like pattern, likely of flattened tubes; these transform distally into isolated, hourglass-shaped structures; then, yet more distally, revert to a (simpler) strap-like pattern. The network most likely represents a stolon-like system, hitherto unknown in graptoloids, that connected individual zooids. Its alternative interpretation, as colonial xenobionts that infested a graptoloid colony and mimicked its architecture, is considered less likely on taphonomic and palaeobiological grounds. Such polymorphism is not known in non-graptolite pterobranchs, which are less diverse and morphologically more conservative: a division of labour between graptoloid zooids for such functions as feeding, breeding and rhabdosome construction may have been the key to their remarkable evolutionary success.

Keywords

polymorphismgraptolitezooplanktonOrdoviciansoft anatomyxenobionts
Type
Original Articles
Information
Geological Magazine , Volume 150 , Issue 1 , January 2013 , pp. 143 - 152
Copyright
Copyright © Cambridge University Press 2012

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