Fossil Dinoflagellate Cyst Systematics

Systematics

Abstract

Developments over the past few years in the field of fossil dinoflagellate cyst taxonomy have seen an increasingly phylogenetic philosophy brought to bear above the genus level. Although that work is still far from complete, the new frontier in our understanding of these organisms surely lies at the species level. We must pick apart the old form-genera which have been used as catch-all homes for species with superficial morphological resemblances, but no seriously implied kinship, and assemble a new taxonomy of genera which reflect true (or, at least, believed) phylogenetic relationships.

Keywords: fossil, dinoflagellate, cyst, dinocyst, systematics, taxonomy

Introduction

For many years, the taxonomy of fossil dinoflagellate cysts has been a strictly morphological science. Of course, fossils are characterised - one might almost say defined - by the fact we cannot access their DNA so, at root, the morphological approach is a necessity. However, dinoflagellate palynologists have been conspicuously less enthusiastic to grapple with the underlying evolutionary relationships of their subjects than have, say, paleozoologists. The systematics of trilobites, for example, while undoubtedly as prone to error as any other taxonomic endeavour, is strictly phylogenetic in philosophy, and has been so for more than 150 years.

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This situation has begun to change over the past twenty years or so, with the emergence of a new phylogenetic approach to dinoflagellate cyst systematics, signposted by the landmark treatise, Fensome et al. 1993. There were some earlier foreshadowings, of course: Dörhöfer & Davies 1980 particularly stands out in my memory for its explicitly evolutionary analysis of the rhaetogonyaulacineans. My own small contribution was to separate a new genus, Stoveracysta, purely on the basis of plate topology, consigning the nature of more conspicuous ornament to "infrageneric significance only" (Clowes 1985, p. 29). The state of our thinking at the time was such that this pronouncement induced a fit of apoplexy in one of my reviewers - and I paid the price by having my publication date delayed by a year.

Yet Fensome et al. 1993 appears to have captured the imagination of working palynologists to a degree not anticipated by earlier contributions. The comprehensive suprageneric classification is now all but ubiquitous in systematic treatments of fossil dinoflagellate cysts, even though, in many cases, it has been adopted simply as a framework within which to arrange lists of genera - an alternative to alphabetical order - which is no kind of adoption at all.

Whereas the phylogeny of trilobites has been studied for 150 years or so, that of dinoflagellates has not. As a consequence, phylogenetic approaches to the latter are still fraught with both systemic and interpretive difficulties. To begin with, although genome analysis may soon provide a more objective framework, in the absence of such data there is no a priori reason to suppose that thecal morphologies, long the object of neontological studies, are any more diagnostic of phylogenetic relationships than the cyst morphologies studied by paleontologists. Until we know better, we may be building our house upon the wrong framework entirely.

Moreover, the basic unit of this scheme is the genus, itself a form-taxon in most instances: The component species of a typical fossil dinoflagellate cyst genus have no demonstrated phylogenetic kinship with one another at all. This recognition points us to the obvious next task of applying phylogenetic principles down to the species level. We may regard monotypic genera as "done" already; provided any future additions to such genera are made with phylogenetic principles in mind, no more is required. At the opposite end of the spectrum are the very large genera, such as Cleistosphaeridium and Cordosphaeridium, which have traditionally found use as catch-alls for sundry cysts from all parts of the stratigraphic column, which almost certainly have little relationship to one another at all.

To further compound our difficulties, dinoflagellate cysts offer far fewer morphological features to scrutiny than do trilobites, or almost any animal. And, unfortunately, the same qualities that make Cleistosphaeridium an obvious target for dismantling also make it difficult. Many species assigned to such "rag-bag" genera reside there precisely because they lack obvious apomorphies - phylogenetically distinguishing characteristics - such as a clear tabulation. And with a stratigraphic range dating from at least Senonian to Neogene (in the case of Cleistosphaeridium), it is unlikely that any single or small group of palynologists has access to enough useful reference material to tackle the whole edifice head on. Instead, we may have to content ourselves with nibbling away at the edges - a less glamorous pastime, but good science all the same.

There are very pragmatic reasons to eschew a single-mindedly morphological taxonomy of dinoflagellates too. Neontological studies have shown intraspecific cyst morphology to be environmentally dependent and highly variable. Nevertheless, few palynologists have felt sufficiently emboldened to abandon the idea of defining genera on morphological grounds alone, thereby admitting the legitimacy of stratigraphy, geography, or other 'proximity' criteria - perhaps even paleoecology - as taxonomic diagnostics.

By way of example, consider Pyxidinopsis, a "hard-core" form-genus. Few would contend that the dozen or more Tertiary forms attributed to Pyxidinopsis have any close phylogenetic relationship to the genotype species, P. challengerensis , described by Habib (1976) from the North Atlantic Neocomian (lowermost Cretaceous), but rather that a phylogenetic taxonomy of these forms would place them elsewhere, possibly in one or more new genera - even if we cannot find consistent morphological characteristics by which to distinguish them.

Part of our reluctance can surely be attributed to deep misgivings about tautology creeping in to our interpretations, and possibly our American colleagues will feel this most acutely, they being exposed to rather more creationist nonsense than the rest of us must suffer. But all scientific interpretation requires care and judgement and the vigilance to avoid false assumption; we need not fear it.

I believe this broader view is required, right now, if we are to pay more than lip service to our calling. At the same time, however, we must also ensure that sound nomenclature does not become an unintended casualty of the exercise. Ironically, despite being guided by conceptual - one might almost say philosophic - principles, we will be required to focus even more closely than before upon type species and even type specimens, so that we are able to whittle the catch-all genera down to their true cores, without missing the mark. Thus, for example, the generic diagnosis for Cleistosphaeridium is, in some senses, actually outweighed by the original description of the type species, C. diversispinosum, insofar as the latter is an accurate description of the genoholotype.

References

Clowes, C.D. 1985: Stoveracysta, a new gonyaulacacean dinoflagellate genus from the Upper Eocene and Lower Oligocene of New Zealand. Palynology 9: 27-35.

Dörhöfer, Gunter; Davies, Edward H. 1980: Evolution of archeopyle and tabulation in rhaetogonyaulacinean dinoflagellate cysts. Life Sciences Miscellaneous Publications, Royal Ontario Museum, 91 pp.

Fensome, R.A.; Taylor, F.J.R.; Norris, G.; Sarjeant, W.A.S.; Wharton, D.I.; Williams, G.L. 1993: A Classification of fossil and living dinoflagellates. Micropaleontology Press Special Paper no. 7, 351 pp.

Habib, D. 1976: Neocomian dinoflagellate zonation in the western North Atlantic. Micropaleontology 21(4): 373-392.