Dr Denis Bates (with Dr Nancy Kirk - Honorary College Fellow):
Research into graptolites commenced with a publication by Dr Kirk in 1969, setting out a radically new interpretation of the mode of life and evolution of the planktonic Graptoloidea, followed by further papers by her on these themes, and development of a model for the secretion of the graptolite skeleton.
In order to test these ideas, work commenced in 1976 with Dr Bates on the morphology and ultrastructure of particularly the retiolite graptoloids. From the first publication of results in 1978, progress in this work has been reported in a series of papers to both international conferences and to journals. This is culminating in a number of publications in which the detailed ultrastructure of a range of retiolite graptoloids is being described and interpreted (See papers)
Morphology of retiolites
The retiolite graptoloids have a complex skeleton which is normally
preserved as a meshwork of lists or bars, in contrast to the skeleton of
the “normal” graptolites which is formed of continuous walls. Our SEM work
has shown that many of these lists are thickenings of thin but continuous
walls or membranes (which are so thin and delicate that they are not normally
preserved). We have been able to demonstrate that the retiolite skeleton
was formed in different ways in a number of taxa: 1) Ordovician forms like
Reteograptus in which the thecal walls are strengthened by lists.
2) Ordovician forms including Phormograptus and Pipiograptus in which the thecal walls are formed of very thin membranes and lists, but outside which is a meshwork of lists.
3) The Ordovician genus Orthoretiolites which has thecal walls of membranes and lists, with elaborated spines projecting beyond.
4) Silurian forms with thecal walls of thin membranes and lists, and an outer ancora sleeve, also formed of thin membranes and lists. These forms evolved from diplograptid graptoloids which developed an ancora structure on the proximal spine, the virgella. Silurian genera differ in the details of the structures, withe the younger forms developing a strong ancora sleeve and weak thecal framework.
By the use of the SEM and the TEM, the ultrastructure of retiolite and normal graptolites has been examined to the limits of resolution of the microscopes. This has enabled us to determine the nature of the skeletal elements in a range of taxa, and particularly to determine the existence of the thin membranes. This work is progressing in both the planktonic graptoloids, and in some benthonic dendroids.
From this work we have concluded that the graptolites secreted their skeleton beneath soft tissue, both inside the thecae and outside (extrathecal tissue). This conclusion contrasts with that held by the majority of palaeontologists studying the group, who believe that the graptolites are closely related to the extant hemichordates Rhabdopleura and Cephalodiscus. These have no extrathecal tissue, but cement or mortar their coenoecium (not a skeleton) during excursions out of their thecae by the zooids. We maintain that this is not feasible in the case of the retiolites, and by extension in all graptolites.
Mode of Life
Dr Kirk in 1969 proposed that the planktonic graptoloids hovered or even moved vertically, on a diurnal basis like the rest of the plankton, through the water column as a reaction to a ciliary feeding current generated by the zooids (the theory of automobility). The very precise colony shapes in the graptoloids are the consequence of this; complex forms may have inhabited the deeper, more food-poor and quieter layers of the photic zone, and simple and smaller forms the upper layers.
The retiolites again form a good test of this hypothesis. The “double” skeleton found in both Ordovician and Silurian retiolites is only readily explicable in terms of harvesting efficiency.
As a result of this work, the taxonomy and evolution of the retiolite graptoloids can be more readily envisaged. It has been shown to be more complex than earlier envisaged. In general, the theory of automobility provides a means of interpreting the evolutionary record in terms of feeding strategies, depth zonation and sea-level changes.
This work has encountered a great deal of hostility among graptolite workers, since the ideas run counter to established views on the mode of life, and mode of secretion and affinities of the graptolites. In particular, papers have been subject to hostile, and cursory, criticism from referees, and even when accepted for publication, been subject to lengthy delays. Nevertheless, in recent years other workers have published versions of some of these ideas, particularly on their mode of life and feeding mechanism.
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