WATER STRIDERS FROM THE PALAEOGENE OF DENMARK WITH A REVIEW OF THE FOSSIL RECORD and EVOLUTION OF SEMIAQUATIC BUGS (HEMIPTERA: GERROMORPHA).

Fine-grained marine sediments from the Palaeocene/Eocene boundary in northern Jutland have recently yielded abundant compression fossils of non-marine organisms. The pondskaters and their relatives from this assemblage, together with a single specimen from strata of a similar age in northern Germany and three inclusion fossils in Baltic amber of Eocene/Oligocene age, are monographed in the opening section of this scholarly treatize. Apart from a monobasic genus of Hydrometridae previously described from these strata by Andersen, and a gerrid nymph in amber that was described in 1856, all are new to science. The new gerrid genus Palaeogerris is based on no fewer than thirty specimens assigned to three species, including the stately P. grandis which, at 3 cm is length, is exceeded in size only by the extant Oriental Gigantometra, to which the new genus is closely related. A new monobasic genus of Hydrometridae is described and a second species is added to that previously recognized in this material, all preserved as compression fossils. Another new genus is placed in Macroveliidae for lack of apomorphies. The amber fossils yielded two rather uninteresting gerrids and a new genus of Veliidae represented by a single specimen. The middle third of the work reviews and reinterprets all previously described Cenozoic and Mesozoic fossil Gerromorpha, providing descriptive notes or re-descriptions and examining their assignment to genera and higher taxa, reassigning or querying placements as necessary and describing a new Limnoporus species from the Middle Eocene of western Canada. Mesozoic fossils of Gerromorpha are unfortunately and unaccountably rare. Andersen accepts a Lower Cretaceous species from Brazil as a genuine hydrometrid and two more of similar age from Australia as possibly a veliid and a mesoveliid. He doubts the assignment of Engynabis to Gerromorpha, is silent as to the affinities of Karanabis (described in Nabidae but transferred to Gerridae by Popov in 1968), ignores Velia cornuta Weyenbergh (said by Handlirsch in 1907 to be merely an artefact), dismisses Chresmodidae as being non-hemipteran (with good reason) and overlooks Brodie's mention (in 1845), without descriptions, of ‘Hydrometra’ and ‘Velia(?)’ from the Jurassic of Britain. If genuine, Brodie's fossils would be both the earliest recognition of fossil Gerromorpha in print and the oldest known representatives of the group. In the concluding section of the work, Andersen discusses the implications of the fossil record in the areas of phylogeny, phylogenetic classification, palaeoecology, palaeobiology and historical biogeography, taking care always to define his terminology, explain his methodology and weigh his evidence. Thus, when he uses morphological evidence to make deductions about the relationships among the various gerromorphan taxa, the minimum ages of the various lineages, past climatic influences or the era when Hemiptera first ventured onto the water surface or initiated the practices of post-copulatory mate guarding and the maintenance of territories, the reader is impelled to share his conclusions and reservations alike. This is palaeontology as it should be done: the critical study of once-living organisms.

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