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Journal of Paleontology 89(5):870–881
et al., 2002) deposited in deltaic, estuarine and shallow marine environments as part of a tectonically controlled incised valley system (Porębski, 1995; Marenssi et al., 1998b). An open marine, protected, and estuarine environment provided suitable habitat and good preservation potential, evidenced by a high
diversity and abundance of fossil remains (Marenssi, 2006). Provenance studies on sandstones of the La Meseta Formation demonstrated that the source rock was located west-northwest along the present day Antarctic Peninsula. The new unit, Submeseta Formation (Montes et al., 2013),
is the uppermost part of the former early/middle Eocene to ?earliest Oligocene La Meseta Formation (Elliot and Trautman, 1982; Ivany et al., 2008). This unit corresponds to the Facies Association III of Marenssi et al. (1998b), characterized by a uniform sandy lithology that represents a tidal shelf influenced by storms. The Submeseta Formation is organized into three allomembers (Fig. 2) named from base to top: Submeseta I, Submeseta II, and Submeseta III (Montes et al., 2013). Montes et al. (2013) placed the base of the Submeseta Formation at 43.4 Ma (upper Lutetian), and the top of this unit at 33.9 Ma (Priabonian/Rupelian).
Material and methods
Anatomical terminology follows Baumel and Witmer (1993). The pseudo-teeth were ranked following Mourer-Chauviré and Geraards (2008) and Louchart et al. (2013).
Institutional abbreviations.—LACM, The Natural History Museum of Los Angeles County, Los Angeles, USA; MCZ, Museum of Comparative Zoology, Harvard University, Cam- bridge, USA; MLP, Museo de La Plata, La Plata, Argentina; MNHN, Museo Nacional de Historia Natural, Santiago, Chile; NHMUK, Natural History Museum, London, United Kingdom;
OCP.DEK/GE, Office Chérifien des Phosphates, Direction des Exploitations deKhouribga, Service deGéologie,Morocco; SMF, Senckenberg Research Institute Frankfurt, Frankfurt am Main, Germany; UCR, University of California, Riverside, USA; USNM,NationalMuseumof NaturalHistory,Washington, USA.
Systematic paleontology
Order Odontopterygiformes Howard, 1957 Family Pelagornithidae Fürbringer, 1888 Pelagornithidae indet. morphotype 1 Figure 3
Figure 2. Stratigraphic section of the La Meseta Formation and the new Submeseta Formation unit proposed by Montes et al. (2013), Seymour Island, Antarctic Peninsula. Characteristic invertebrates are indicated as: t = Turritella;cu = Cuccullaea;v = veneroids.
Description.—The distal end of humerus MLP 12-I-20-4 is nearly complete but both condyli have lost their cranial surface, also the epicondyli dorsalis et ventralis and the sulcus scapulo- tricipitalis are partially eroded (Fig. 3.1–3.4). The assignment of MLP 12-I-20-4 to the pseudo-toothed birds is based on the following characters (Mourer-Chauviré and Geraads, 2008; Mayr and Smith, 2010; Bourdon et al., 2010; Bourdon and Cappetta, 2012): (1) craniocaudally compressed shaft, (2) distal condyli located at the same level and distally prominent, (3) roughly rounded condylus ventralis, (4) prominent epi- condylus dorsalis, (5) shallow fossa olecrani and sulcus humerotricipitalis, and (6) dorsal margin of the shaft narrow and ridge-like just proximal to the distal end. Although its size is somewhat larger, the Antarctic material
has strong similarities with a humerus coming from the middle Eocene of Togo and tentatively referred to Gigantornis
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