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Bath Royal Literary and Scientific Institution, 16-18 Queen Square, Bath BA1 2HN
Their excellent fossil record and widespread distribution sees planktonic graptolites as one of the most important taxa in the history of geology. They have played a major role in the establishment of the stratigraphic divisions of the Early Palaeozoic, and their rapid evolution provides a high resolution timescale for this interval which is still used at the present day. Though they often only remain as faint impressions on the rock - Linnaeus coined the term graptolite for ‘things that resemble fossils, but are not fossils themselves’ - well-preserved graptolites contain a wealth of information pertaining to both their biological affinity and behaviour. Despite the long history of graptolite research and the increasing number of well-preserved graptolites, their fundamental palaeobiology has received little attention.
Graptolites themselves are unique in the fossil record, being colonial animals which actively constructed their exoskeleton by the coordinated behaviour of individual animals within the colony. Thus, in terms of animal architecture they are more akin to floating beehives than they are to mollusc shells or corals. An analysis of teratological specimens shows that this mode of construction may lead to the same species of graptolites fabricating the same structure in differing manners, showing that functional requirements outweigh constructional control. Fluid dynamic modelling suggests that many features of the graptolite exoskeleton developed as to the ambient hydrodynamic regime, and many subspecific morphological novelties may represent opportunistic characters developed late in a colony’s growth as a response to the local planktonic environment. Detailed investigation of graptolite synrhabsomes (radially-organised clusters of graptolite colonies) highlights the plasticity of graptolite morphology, showing clear evidence of cooperation between individual colonies to achieve an alternative life strategy. Given this pattern of opportunistic morphological development and the independent, convergent acquisition of many characters within their rapid evolution, it may be that that behaviourally-mediated skeletal construction provided an extra gear for graptolite evolution.
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