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| The possible lifestyle of the eurypterid Onychopterella
augusti We welcome the contribution by Jeffrey Minicucci (Newsletter <comment39.html> 39, pp. 12-13 <comment39.html>) on the late Ordovician eurypterid Onychopterella augusti, from the Soom Shale of South Africa (Braddy et al. 1995). Research into the taphonomy (Gabbott 1998), biomechanics and ichnology (Braddy 1998, Braddy and Almond, in press) and respiratory palaeobiology (Braddy et al., in press) of O. augusti has continued during recent years, and it is pleasing to see that our work is generating interest. Minicucci’s comments concern the mode of life of this animal: he would have us believe that O. augusti, unlike most of its clan, took to burrowing in the substrate where it gobbled sediment and sought soft-bodied worms. However, any morphological evidence adduced for this hypothesis is vague. Minicucci suggests that the ‘flat, wide body seems appropriately shovel-like for plowing’ and that the posteriorly directed spines on appendages II-IV could have been useful for pushing aside mud. Maybe, maybe not. Similarly, broad comparisons of opisthosomal outline with the carcinosomatids cannot alone justify exclusion of Onychopterella from a nektonic or nektobenthic lifestyle, and the proportions of the prosomal appendages, which are typical for members of the family Erieopteridae, hardly point exclusively to an infaunal mode of life. The spiral-shaped alimentary canal, again, does not necessarily represent evidence for detritus feeding and may equally have served to increase the absorptive area of this part of the gut. Even if it were demonstrated that all these structures could have functioned as Minicucci suggests, it would, of course, not prove that they did. Analyses of functional morphology can provide constraints on ecological hypotheses, but the science of palaeoecology is potentially so tenuous that we must use all the evidence we can get. Therefore, we should look beyond morphology to any other available sources of information (e.g. associated organisms, taphonomic considerations and the sedimentary environment). In the case of the Soom Shale, the enclosing sediment provides a ready first test of lifestyle hypotheses. The shale itself is laminated on a millimetric scale, with the laminae persistent over metres. None of us working on the Soom have ever seen any evidence of bioturbation (Aldridge et al. 1994), which in itself rules out a burrowing hypothesis for any of the animals in the fauna. Geochemical analyses further demonstrate that the sediment was anoxic-euxinic with low pH (Gabbott 1998), providing exceptionally inhospitable conditions for infaunal organisms. These features of the shale serve to refute any hypothesis based on functional morphological criteria alone. It might be argued that the animals in the Soom were exotic, washed in from more hospitable regions, and that the local sedimentary conditions are relevant only to their preservation, not to their mode of life. However, the undisturbed fine laminae of the shale attest to a very low energy environment, and the strata extend in all directions for several tens of kilometres. There is no evidence at all of any currents that could have plucked infaunal organisms from marginal regions and carried them alive out into the Soom basin to be preserved with their soft tissues intact. Palaeoecology is perhaps one of the least exact of sciences, but we are drawn into it because we naturally want to know how our favourite organisms lived. It is often difficult, however, to gather sufficient evidence to frame well-formulated, testable hypotheses, and there is a temptation to retreat into vague scenarios, based on pretty flimsy functional morphological reasoning, Rudwickian paradigms notwithstanding. We must be wary of this, because there is also a wider point here. Those who occasionally denigrate palaeontology as a discipline are prone to point to some of the ‘weaknesses’ that they perceive in the science, including the widespread publication of historical narratives and unconstrained speculations. We need to be careful not to conform to these caricatures. It is certainly possible to produce rigorous, testable hypotheses in all aspects of our work and we must show that we can do so. This means we cannot ignore potential sources of crucial evidence, and in this respect palaeontology cannot do without geology. Richard J. Aldridge |
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| Eurypterid palaeobiology:
breathing life into fossil sea scorpions an oral presentation by PALAEONTOLOGICAL ASSOCIATION University of Manchester Recent research on the palaeophysiology of the eurypterids, extinct aquatic chelicerates, has revised our interpretations of their respiratory and reproductory capabilities. Exceptionally preserved fossils, notably Onychopterellaaugustifrom the Soom Shale Lagerstätte, and comparisons with extant taxa have been used to breath life into these extinct animals, revealing important implications for their palaeoecology and phylogeny. The nature of the eurypterid respiratory system has been unresolved for over a century, but is now interpreted as having involved four pairs of vertically orientated lamellate gills, housed within the branchial chambers (for aquatic respiration) and five pairs of accessory Kiemenplatten, situated on the roof of the branchial chambers (for accessory aerial respiration, by analogy with the pseudotracheae of some extant Crustacea). Eurypterid reproduction is interpreted as having involved indirect spermatophore transfer via the substrate; the female able to store a spermatophore in paired spermathecae until suitable environmental conditions prevailed for spawning. Cladistic analyses generally favour a monophyletic Arachnida, although similarities in their gross morphology and respiratory structures, infers a closer relationship between eurypterids and scorpions. This palaeobiological evidence also supports the view that some eurypterids were capable of limited amphibious excursions, perhaps associated with their life-cycle (mass-moult-mate events). |
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