Cenizo et al.—Pseudo-toothed birds from Eocene of Antarctica
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Figure 1. Location map of the La Meseta Formation and the new Submeseta Formation outcrops in Seymour Island, Antarctic Peninsula, West Antarctica. Other stratigraphic units are also indicated: CVF = Cross Valley Formation; LBF = López de Bertodano Formation; SF = Sobral Formation; WF = Weddel Formation; sd = surficial deposits.
Zvonok, 2012, a relatively well-represented taxon from the middle Eocene of Ukraine; some fragmentary specimens from the middle Eocene of Togo (Bourdon and Cappetta, 2012), Belgium (Mayr and Smith, 2010; Mayr and Zvonok, 2012), and Nigeria (Andrews, 1916; Mayr et al., 2008); and other remains from the late Eocene of Antarctica (Cenizo, 2012), and the latest Eocene/?early Oligocene of North America (Goedert, 1989). Particularly, the Antarctic pelagornithid record is restricted to few and isolated remains from the Eocene of
Seymour Island in the Antarctic Peninsula (Fig. 1). The first pseudo-toothed birds known from this continent were represented by a very large rostral fragment (Tonni and Cione, 1978; Tonni, 1980), and two poorly preserved mandibular portions (Tonni and Tambussi, 1985). Stilwell et al. (1998) reported the first fossil bird from lower-middle Eocene outcrops of East Antarctica, which consists on an eroded humeral diaphysis described in detail by Jones (2000). A well preserved distal end of tarsometatarsus previously referred to the “terror birds” Phorusrhacidae (Case et al., 2006) was reallocated within Pelagornithidae (Cenizo, 2012). Furthermore, a complete humerus, still under study, was mentioned by
Rubilar-Rogers et al. (2011), and a tip of beak was recently figured by Tambussi and Degrange (2013) without comments about its affinities. The finding of the oldest Antarctic pelagornithid (a distal
end of humerus, MLP 12-I-20-4) in middle Ypresian levels (early Eocene), and the recent description of new and more complete Paleogene specimens (e.g., Bourdon et al., 2010; Mayr and Zvonok, 2011, 2012; Bourdon and Cappetta, 2012; Mayr et al., 2013) invites to reviewof the Antarctic record of the group, which represents the only pseudo-toothed birds known for the Paleogene of the Southern Hemisphere.
Geological setting
The sedimentary sequence exposed on Seymour (Marambio) Island is more than 2 kmthick and represents the uppermost part of the infill of the James Ross Basin (del Valle et al., 1992). The youngest strata constitute the Seymour Island Group that includes the Paleocene Cross Valley Formation at the bottom, and the early Eocene/?earliest Oligocene La Meseta Formation at the top. A geological map of the Seymour Island with new stratigraphic sequence and datings of the Eocene marine horizons was recently published (Montes et al., 2013). The La Meseta Formation was splitted into two geologic units, La Meseta and Submeseta formations which are separated and bounded by a prominent erosional surface covered by 20- to 40-m thick inclined heterolithic facies composed of estuarine, very fine sandstones, and mudstones (Marenssi, 2006). The La Meseta Formation (La Meseta Alloformation in
Marenssi et al., 1998a) is exposed along the slopes of the plateau of Seymour (Marambio) Island (Fig. 1). It is an unconformity-bounded unit (Elliot and Trautman, 1982; Ivany et al., 2008) deposited between the Thanetian and the Lutetian (i.e., between 58.7–40.4 Ma; Montes et al., 2013). This unit is approximately 560-m thick and fills a 7-km wide valley cut down into older sedimentary rocks constituting the island; its current location is the result of regional uplift and tilting of the Marambio Group beds (Reguero et al., 2013). The La Meseta Formation includes mudstones and
sandstones with interbedded conglomerates and is organized into six erosionally based units (Marenssi et al., 1998a). These are named, from base to top: Valle de Las Focas, Acantilados I, Acantilados II, Campamento, Cucullaea I, and Cucullaea II (Fig. 2). These lens-shaped units represent different sedimentation stages related to sea level fluctuations (Marenssi
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