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

Zalasiewicz, J. A. and Page, A. A. and Rickards, R. B. and Williams, M. and Wilby, P. R. and Howe, M. P. A. (2013) Polymorphic organization in a planktonic graptoloid (Hemichordata: Pterobranchia) colony of Late Ordovician age. Geological Magazine, 150 (1). pp. 143-152. DOI 10.1017/S0016756812000349

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Official URL: http://dx.doi.org/10.1017/S0016756812000349

Abstract

Graptolites are common fossils in Lower Palaeozoic strata, but little is known of their soft-part anatomy. However, we report a long-overlooked specimen of Dicranograptus aff. ramosus from Upper 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.

Item Type: Article
Uncontrolled Keywords: 2012AREP; IA63;
Subjects: 04 - Palaeobiology
Divisions: 04 - Palaeobiology
Journal or Publication Title: Geological Magazine
Volume: 150
Page Range: pp. 143-152
Identification Number: 10.1017/S0016756812000349
Depositing User: Sarah Humbert
Date Deposited: 09 Jul 2012 12:09
Last Modified: 23 Jul 2013 10:04
URI: http://eprints.esc.cam.ac.uk/id/eprint/2556

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