Assessing the importance of drilling predation over the Palaeozoic and Mesozoic

Plausible predatory Palaeozoic and Mesozoic drillholes have been reported in the literature from 62 localities, mainly from North America and Western Europe, most of which were discovered in the last 20 years. Twenty-nine of these reports include sufficient data to be statistically valid. Analyses of these data suggest that there was a highly significant increase in drillhole, and hence predator, size in the Early Devonian which may reflect a change in predatory taxa as part of the Mid-Palaeozoic marine revolution. In this study frequency of drillholes in particular species has been examined. Examination of drilling frequency suffered by individual taxa from a range of Palaeozoic and Mesozoic sites showing that these are highly variable, just as they are between different Holocene localities. Nevertheless, levels of up to 50% in the Palaeozoic and 40% in the Mesozoic suggest that, at least locally, drillers did exert an important selection pressure. Analysis of the currently available data set reveals no significant change in drilling frequency over the Palaeozoic and Mesozoic. There are fewer records of drilling predation from the Palaeozoic and Mesozoic than from the Cenozoic. At present, however, it is difficult to assess whether drilling was truly less widespread than in the Cenozoic or whether at least part of the problem is taphonomic loss of information, in particular in the Late Palaeozoic and Mesozoic. It is striking that relatively few reports involve molluscan prey, despite their attractiveness as profitable prey with high flesh yield when compared with the apparently more frequently drilled brachiopods. However, study of the style of preservation of drilled molluscs, that were mostly originally aragonitic, suggests that drillholes are only preserved in faunas which are exceptionally preserved such that the original microstructure is still observable. By contrast, brachiopods, which were either originally phosphatic or calcitic, have much better preservation potentials and therefore may be more likely to show drillholes. An understanding of the preservation biases involved in the recognition of drillholes may suggest suitable localities for further research. More data, from a suite of localities world-wide, are required for a more detailed analysis of patterns of drilling predation in the Palaeozoic and Mesozoic. (C) 2003 Elsevier B.V. All rights reserved.