Biogeography

Europe and North America were connected by northern latitude land corridors via Greenland in the late Paleocene and early Eocene, whose existence was terminated by the opening of the North Atlantic (Smith et al. 1994). Until the early Oligocene,

Europe was further separated from Asia by the Turgai Strait, an epicontinental sea which divided the Eurasian land mass. Whereas overland dispersal between Europe and Asia was thus aggravated in the early Paleogene, intermittent land connections via Beringia existed during the whole Cenozoic (Rose 2006).

Given the occurrence of similar mammalian taxa in the Paleocene of North America and Europe (Rose 2006), it would be very surprising if there were not also significant concordances in the avifaunas of that time. However, the known Paleocene nonmarine avifaunas of Europe and North America share few closely allied taxa. Although strigiform birds occur on both continents, with Ogygoptynx in North America and Berruornis in Europe, they represent morphologically very different lineages (Sects. 15.1, 15.3). Whether there were representatives of the Lithornithidae in the Paleocene of Europe is uncertain, as identification of the fragmentary Walbeck coracoid is only tentative (see Sect. 5.1). The Gastornithidae seem to have been restricted to Europe in the Paleocene, whereas Paleocene Presbyornithidae have so far only been recorded from North America. Admittedly, it has to be kept in mind that the Paleocene fossil record of birds is still much less known than that of contemporary mammals; therefore, general conclusions cannot yet been drawn and the picture may change with future discoveries.

There was a maximum interchange of mammalian faunas between Europe and North America in the early Eocene (Janis 1993; Rose 2006). Previous authors (e.g., Houde and Olson 1989; Peters 1991; Blondel and Mourer-Chauvire 1998) recognized that the early Eocene avifaunas of Europe and North America were also very similar. The occurrence of the flightless Gastornithidae in the early Eocene of North America and Europe actually provides direct evidence for the existence of a land connection by that time or shortly before (Andors 1992). Other avian groups which occur in the early/middle Eocene of both Europe and North America are the Lithornithidae, Gallinuloididae, Messelornithidae, Juncitarsinae, Messelasturidae, Primobucconidae, stem group Leptosomidae (Plesiocathartes), Sandcoleidae and other stem group Coliiformes (Chascacocolius), Fluvioviridavidae, Halcyornithidae ("Pseudasturidae"), and Zygodactylidae. The representatives of most of these taxa were forest-dwelling birds, which is in concordance with the existence of extensive paratropical forests in the early Eocene of at least the western part of the Northern Hemisphere (Blondel and Mourer-Chauvire 1998).

Several taxa were only found in the early and middle Eocene of either North America or Europe, but because some of the most productive fossil localities on these two continents represent different paleoenvironments (Chap. 2), there remains a possibility that this is an artifact of the incomplete fossil record. So far exclusively known from the early Eocene of North America are the frigatebird Limnofregata, unambiguous Presbyornithidae, the Foratidae, stem group representatives of the Steatornithidae, and the Geranoididae. Taxa which were only reported from European deposits include the Palaeotididae, Remiornithidae, Ameghinornithidae, Idiornithidae, Archaeotrogonidae, stem group Nyctibiidae and Trochilidae, as well as unambiguous representatives of the Aegialornithidae.

Opening of the North Atlantic after the early Eocene limited overland dispersal between Europe and North America to land connections via Beringia. Nevertheless, the early Oligocene avifaunas of Europe and North America were still remarkably similar. Taxa which were reported from the latest Eocene/early Oligocene of North America and Europe include stem group Todidae (Palaeotodus), the putative stem group cuculiform Eocuculus, and stem group representatives of the Coliidae (Palaeospiza in North America and Primocolius and Oligocolius in Europe). Again, the avifaunal congruencies seem to be greatest for forest-dwelling taxa, whereas the terrestrial avifaunas of Europe and North America already appear to have been more different in the early Oligocene. The Bathornithidae, for example, are only known from North America, where they are a common avifaunal element. The Idiornithidae, on the other hand, have not been reported from North American fossil sites.

Because Europe and Asia were separated by the Turgai Strait in the early Eocene, it is likely that the flightless Gastornithidae dispersed into Asia from western North America rather than Europe, where the group seems to have originated (Sect. 6.2). An exact stratigraphic correlation of the Asian and North American records of these birds would, however, be required to confirm this hypothesis. If the North American Geranoididae are indeed most closely related to the Asian Eogruidae (Sect. 9.3.2), a dispersal route from North America to Asia via Beringia may also have been used by a species in the stem lineage of the latter.

Closure of the Turgai Strait at the beginning of the Oligocene facilitated an exchange between European and Asian faunal elements. Immigration of new taxa from Asia was probably one of the major factors which led to a marked faunal turnover in Europe, known as the "Grande Coupure" (Prothero 1994; Hooker et al. 2004). Little has so far been published on the impact of this event on the European avifauna at the Eocene-Oligocene boundary, and all observations were made for the Quercy taxa (Mourer-Chauvire 1980, 1988b). As yet, only a few avian groups with an extensive early Paleogene fossil record are known that disappear at the Eocene-Oligocene boundary, i.e., the Quercymegapodiidae, Quercypsittidae, the putative idiornithid Elaphrocnemus, and the Aegialornithidae. In contrast, there are a fair number of taxa which have their earliest European fossil record in the latest Eocene/earliest Oligocene, including the Anatidae, Phasianidae, Phalacrocoracidae, Palaelodidae, Gaviiformes (Colymboides), Rallidae, Parvigruidae, Charadrii-formes, Pici, and Passeriformes. Many of these latter groups are aquatic and semi-aquatic, so the Turgai Strait may not have formed an insurmountable barrier. Their occurrence in Europe thus cannot be convincingly ascribed to the disappearance of this epicontinental sea. The absence of some of these birds in earlier deposits may be an artifact of the fossil record, as most of our knowledge on late Eocene European avifaunas stems from the Quercy deposits, which mainly include terrestrial and arboreal forms. An immigration from Asia is, however, likely for the Passeriformes, which may well have originated in the Australian region (Sect. 17.2.1). Because extant Phasianidae and Pici have limited capabilities for crossing open sea, the occurrence of these taxa in the early Oligocene of Europe may also be due to immigration from Asia after closure of the Turgai Strait (see also Sect. 17.2.1).

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