Ó Copyright, N. M. Andersen & Zoological Museum, University of
As is the case with most groups of insects, the fossil record of water striders is quite imperfect. Yet, recent findings of fossil water striders have provided useful insights into the evolutionary history of this group (Andersen, 1998, 2000, 2001; Andersen & Poinar, 1998 ).
The first fossil species of sea skaters, Halobates ruffoi, was described by Andersen et al. (1994) from the marine deposit "Pesciara di Bolca", in the province of Verona, northeastern Italy (Middle Eocene, age about 45 Myr). The fossil can be classified in the Gerridae - Halobatinae by a combination of characters of which the most significant are: Body relatively short and stout. Eyes large, partly overlapping prothorax. Prothorax shorter than head. Meso- and metathorax together much longer than prothorax. No traces of wings in adult. Middle leg longer than hind leg, both inserted towards the end of body. Abdomen distinctly shortened, much shorter than thorax. The presence of the fossil gerrid in a marine sediment favours its placement in one of the marine genera of Halobatinae. It is true that flying insects are frequently blown out from land and deposited on the sea surface, but the wingless state of the fossil specimen excludes this possibility.
The fossil does not reveal enough structural details
to permit a closer comparison with extant sea skaters. However, from its size
alone, Halobates ruffoi (body length 5.8 mm, middle femur 6.5 mm, hind
femur 5.1 mm) compares favourably with the coastal species H. formidabilis
(body length 4.6-5.5 mm) from India, Sri Lanka, and the Maldives, H. alluaudi
(5.0-5.8 mm) from the Seychelles, and H. princeps (6.0-6.6 mm) from the
Indo-Malayan Archipelago. In view of its geological age, however, it is unlikely
that the fossil Halobates belongs to any of these extant species. The
fossil species is larger than any open-ocean species, e.g., H. micans
(4.0-4.5 mm) or H. splendens (4.5-5.0 mm).
fossil originates from the deposit "Pesciara di Bolca", in the
province of Verona, northeastern Italy. It is imprinted in a whitish,
fine-grained limestone rich in fossils of marine animals. The fauna of marine
fish from this deposit is very similar to the present fauna of the Indo-West
Pacific region. Part of the Bolca formation contains also remains of plants,
jellyfish, a few mollusks, crabs, lobsters, terrestrial insects, crocodiles, and
bird feathers. Fossil coral reefs of more or less the same age are found in
deposits only a few kilometers away from Bolca. This indicates water surface
temperatures not lower than 20o C, that is, above the tolerance level
of extant Halobates species. it is estimated that the Eocene climate was
tropical in the Veneto area where Bolca is situated.
No living species of Halobates have so far been recorded from the Mediterranean. Three species, including the open-ocean species H. germanus White, are found in the Red Sea, but apparently none of these have managed to spread, actively or passively, through the Suez Canal to the Mediterranean as have other marine organisms. The record of a fossil sea skater, Halobates ruffoi, from the Middle Eocene of northern Italy may help understanding of the biogeographical history of sea skaters. In the Lower and Middle Eocene, the Mediterranean was connected with the Indo-Pacific Ocean by the Tethys Ocean, located in present day Turkey, Iraq, and the Persian Gulf. In this period, the Tethys was a wide and continuous ocean, probably with numerous islands.
Jaczewski (1972) hypothesized that the pantropical Halobates micans dispersed from the Indo-Pacific to the Atlantic Ocean through this Tethys sea connection. However, as pointed out by Jaczewski (1972) and Andersen (1999), , there have been other, more recent routes of dispersal to the Atlantic Ocean, e.g., from the Indian Ocean around southern Africa, or from the Pacific Ocean through Mesoamerica, before the closure of the Isthmus of Panama.
Andersen NM. 1999. The evolution of marine insects: phylogenetic, ecological and geographical aspects of species diversity in marine water striders. Ecography 22: 98-111.
Andersen, N.M., 2000. Fossil water striders in the Eocene Baltic amber (Hemiptera, Gerromorpha). – Insect Systematics & Evolution, 31: 257-284.
Andersen, N.M., 2001. Fossil water striders in the Oligocene/Miocene Dominican amber (Hemiptera: Gerromorpha). – Insect Systematics & Evolution, 31: 411-431.
Andersen NM, Farma A, Minelli A, Piccoli G. 1994. A fossil Halobates from the Mediterranean and the origin of sea skaters (Hemiptera, Gerridae). Zool. J. Linn. Soc. 112: 479-489.
Andersen NM, Poinar GO Jr. 1998. A marine water strider (Hemiptera: Veliidae) from Dominican amber. Entomol. Scand. 29: 1-9.
Jaczewski T. 1972. Geographical distribution of oceanic Heteroptera and the continental drift. Bull. Acad. Sci. pol., Ser. Biol. 201: 415-417.
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Last update: 04 september 2002