Ed Landing and Markes Johnson (eds) 1998. James Hall Centennial Volume, New York State Museum Bulletin 491, ISSN 0278-3355, ISBN 1-55557-206-5, ix+327pp.
Everything comes in cycles. Dean Martin has recently enjoyed a resurgence of appreciation, and members of my family tell me that even flares can again occasionally be worn without (too much) embarrassment. So it was in the Silurian – anyone looking for long at sedimentation or biotic turnover in rocks of this age can readily discern patterns of repetition at various scales. These simple observations have recently spawned a plexus of explanatory hypotheses, varying from eustatic sea-level changes to climatic oscillations to tectonic pulses, or involving various combinations of these factors.
This burgeoning interest clearly meant that the time was ripe for a get-together to discuss the patterns and their causes, and it was eminently appropriate that Markes Johnson, who has done much over the past decade to publicise and analyse Silurian cyclicity, should be the scientist to arrange the forum for the debate. And so it was that the Second International Symposium on the Silurian System, held at the University of Rochester, New York, in 1996, took cycles as its primary theme. The volume under review documents the science presented in a poster session and workshop on Silurian cyclicity mounted at the Rochester conference, supplemented by a few papers solicited by the editors to ’round out the collection’.
And it is, indeed, a pretty round set of articles. The papers cover all the major themes, and tackle cyclicity at local to global level. There is some variation in quality, but all the contibutors provide something of interest and there is quite a lot of new information. The volume is divided into four parts: Physical evidence for Silurian eustasy, Temporal faunal patterns related to eustasy, Short-term cycles, Isotope studies. As will be evident from these headings, there is an inclination towards hypotheses of (at least partially glacially driven) sea-level cyclicity, but climatic changes and tectonic influences also get a hearing. There is, perhaps, an over-enthusiasm to take the published sea-level curves as established, rather than as hypotheses for testing, with some authors bending backwards almost horizontally to make their data fit the expected pattern; but the discerning reader is usually provided with enough information to draw his/her own conclusions about how good the correspondence really is.
I’m not sure that this newsletter is the place to detail the contents of each paper; suffice it to say that anyone interested in the Silurian biota, Silurian biostratigraphy, and any aspect of palaeo-cyclicity will find this an indispensible volume. What I’m going to do instead is climb onto a familiar hobby-horse and rant about what really strikes me as a micropalaeontologist about this collection of papers – this is, of course, the near-absence of the consideration of any evidence from microfossils.
Actually, this is not strictly true, as ostracods get an occasional mention and conodonts are reasonably well represented, especially in Lennart Jeppsson’s summary of his ocean/atmosphere cyclicity model which is primarily based on an analysis of the conodont record (and also, diabolically, in Tesakov et al.’s error-strewn contribution on the East Siberian Basin, in which Silurian conodont biostratigraphy is completely re-written in an apparent attempt to make sea-level curves match). But, otherwise, the emphasis is clearly on the invertebrate macrofauna, as a quick glance at the titles of papers demonstrates: they mention graptoloids, cephalopods, gastropods, trilobites and reefs, while a flick through other papers shows that brachiopods continue to be regarded as especially significant. I do not decry the evidence that these elements of the biota undoubtedly provide, but I searched in vain for more than a sideways nod in the whole book to the importance of acritarchs, prasinophyte algae, chitinozoans or scolecodonts. This is not the fault of the symposium organisers, the editors, or the contributors; it’s ours. We are surely missing an opportunity here to champion the fundamental import of the phytoplankton, in particular, in reflecting, and probably in influencing, the development of cyclicity in ancient oceans. Sure, it’s difficult to interpret the record, but there’s a mass of information out there that certainly has something to tell us if we only make the effort to understand it. Having said this, I’m delighted to see that palynologists are now beginning to enter the debate in a major way, as is exemplified by the stimulating recent paper by Dorning and Harvey (1999) on Wenlock cyclicity.
So to a final comment. Another major message that comes through to me from the James Hall Centennial Volume is that the development and testing of our theories on Silurian cyclicity are currently severely handicapped by the limitations of the exisiting framework of biostratigraphical correlation. For example, in the very first paper, Johnson et al. present a very interesting approach to testing Silurian eustasy using the burial and erosion of coastal margins, but the results are manifestly marred by the uncertainties and inaccuracies of dating the inundations of rocky shorelines worldwide. Elsewhere, it is evident that difficulties in making accurate correlations between graptolitic and shelly sequences can confuse interpretations. However, we are unquestionably struggling successfully towards a greater understanding of Silurian cyclicity, and open debate on the issues raised in this volume will undoubtedly propel us further forwards in unravelling the complexity of factors that have influenced the record we have been bequeathed to study. An improving biostratigraphy will clearly help us in this endeavour, and, micropalaeontologists, we surely have a role to fulfil here as well!
Dorning, K. J., and Harvey, C. 1999. Wenlock cyclicity, palynology, and stratigraphy in the Buildwas, Coalbrookdale, and Much Wenlock Limestone formations, Shropshire, England. Bolletino della Societa Paleontologica Italiana, 38, 155-166
University of Leicester, UK