Geological Magazine

Articles

Graptolite ontogeny and the size of the graptolite zooid

Susan Rigbya1 and Margaret Sudburya2

a1 Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK

a2 Thorpe Cottage, Chestnut Avenue, Thornton Dale, Pickering, N. Yorkshire YO18 7RW, UK

Abstract

Two methods of estimating the size of the graptolite zooid are described and discussed. It is possible to size a zooid by reference to cortical bandages or with reference to modem Rhabdopleura and its tubes. These two methods give very different results, with the first suggesting small zooids relative to thecal size and the second suggesting zooids that filled their thecae completely. Comparison is made with the evidence available from rare cases of preserved zooids. When all these observations are considered in the light of the ontogeny of modem pterobranchs, an ontogenetic sequence for the graptolite zooid can be inferred which helps to reconcile the two methods of estimating zooid size. This sequence postulates that most skeletal building occurred early in the life of the zooid, before it developed the capacity to feed or to reproduce, and it implies that only the first of three stages in the zooid's life is recorded in the skeleton. The later stages occurred without normally leaving any trace on the preservable remains of the colony. Finally, there is discussion of the effects which different zooid sizes would have had on some aspects of the functional morphology of a theca and on the hydrodynamic behaviour of the rhabdosome as a whole.

(Received August 16 1994)

(Accepted April 12 1995)

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