Stratigraphic distribution of large flightless birds in the Palaeogene of Europe and its palaeobiological and palaeogeographical implications
The stratigraphic distribution of the three main groups of large flightless birds known from the Palaeogene of Europe, Gastornithidae, Phorusrhacidae and Ratitae, is reviewed. The huge, herbivorous gastornithids, represented by the single genus Gastornis, are known from the Selandian (Middle Palaeocene) to the late Lutetian (Middle Eocene), being recorded from reference levels MP5 to MP13. The carnivorous phorusrhacids are represented by a single species, Eleutherornis cotei, from the late Lutetian (MP14, late Middle Eocene). The ratites have a patchy distribution, being represented by two species of moderate size, Remiornis heberti from the Thanetian (MP6, Late Palaeocene) and Palaeotis weigelti from the Lutetian (MP11 to MP13, Middle Eocene). The stratigraphic distributions of large eggs referred to gastornithids in the Late Palaeocene and Early Eocene of southern Europe and the occurrence of enigmatic large avian footprints in the Late Eocene of France are discussed. Whereas gastornithids and ratites co-existed in both the Palaeocene and the Middle Eocene, phorusrhacids seem to have been the only large ground birds in Europe at the end of the Middle Eocene. The palaeobiogeographical and evolutionary implications of the stratigraphic distributions of those groups of large birds in Europe are discussed. As Gastornis first appears in North America and in Asia in the Early Eocene, it is likely that gastornithids originated in Europe and later spread to other land masses during a dispersal event close to the Palaeocene-Eocene boundary. Prior to that, gastornithids evolved on the European "island continent", where they were the largest terrestrial tetrapods during the Palaeocene. Gastornithids do not seem to have been significantly affected by the PETM. Ratitae have a more patchy record and relationships between Remiornis and Palaeotis remain unclear. Nevertheless, those European forms are among the earliest known ratites and this should not be overlooked in discussions of ratite evolution and palaeobiogeography. Phorusrhacids appear to have been present in Europe for only a short time and are interpreted as the result of dispersal from Africa followed by local extinction.