Cenizo et al.—Pseudo-toothed birds from Eocene of Antarctica
exhibit an intermediate condition. However, this trochlea is less plantarly projected (Fig. 5) than in Dasornis but more than in P. chilensis (Bourdon et al., 2010; Mayr and Rubilar-Rogers, 2010; Mayr, 2011; Mayr and Zvonok, 2012); the plantar extension in UCR 22176 is similar to that of L. tethyensis and the specimen tentatively assigned to Pelagornis (‘Palaeoche- noides’) mioceanus by Hopson (1964; see also Mayr et al., 2013). The processus medioplantaris (Fig. 4.11, 5) of the trochlea metatarsi II is more medially extended than in Dasornis and L. tethyensis (it is absent in P. chilensis). As in Dasornis and L. tethyensis, the trochleae metatarsorum II and IV are narrower and less excavated than those of Pelagornis. Likewise, as in Dasornis (although in a lesser degree), the plantar surface of trochlea metatarsi III is narrow, elongate, and with proximally convergent margins (a condition also observed in Lutetodontopteryx); whereas in Pelagornis this trochlea is plantarly wider, shorter and its margins are more parallel each other (the only exception is the late Oligocene/early Miocene Pelagornis sp. from Oregon, Mayr et al., 2013, fig. 5). UCR 22176 shares with Dasornis the lateral tilting of the trochlea metatarsi III (Bourdon, 2005; Bourdon et al., 2010). The dorsal opening of the foramen vasculare distale in UCR 22176 is proximodistally elongated, like that of Dasornis (Bourdon et al., 2010; a similar condition would be present in L. tethyensis); whereas it is subcircular in Pelagornis. On the other hand, the foramen vasculare distale in UCR 22176, is recessed plantarly and opens close to the canalis interosseus distalis like in Pelagornis (Bourdon, 2005; Mayr and Rubilar-Rogers, 2010; Mayr, 2011).
Material.—MLP 08-XI-30-42, rostral end of rostrum maxillare (Fig. 4.2, 2.3); MLP 78-X-26-1, most rostral narial region of rostrum maxillare (Fig. 4.4–4.8); UCR22176, distal end of right tarsometatarsus (Fig. 4.9–4.11).
Occurrence.—All the specimens come from the Submeseta II Allomember (level 38 in Montes et al., 2013, fig. 2) of the Submeseta Formation (Montes et al., 2013), DPV 13/84 (MLP 08-XI-30-42 and MLP 78-X-26-1) and RV 8702 (UCR 22176) localities, Seymour Island, Antarctic Peninsula, West Antarctica. Strontium dating yielded an age between 41.1–37.8 Ma for both localities (Bartonian, middle Eocene; Montes et al., 2013).
Measurements.—See Table 1.
Remarks.—An almost complete humerus (SGO.PV 22001) assigned to a pelagornithid was recently found in Bartonian/? Priabonian levels (middle/late Eocene, Submeseta Formation) of Seymour Island (Rubilar-Rogers et al., 2011). Its morphology resembles more the Neogene Pelagornis than the Paleogene Dasornis. It belongs to a bird similar-sized to the largest know pelagornithid Pelagornis sandersi Ksepka, 2014 (Table 1), but unfortunately, it is still under study and cannot be directly compared. Giant size, morphological affinities (Pelagornis-like features) and the stratigraphical provenance of the specimen (Rubilar-Rogers et al., 2011) are consistent with assignment to morphotype 2.
877
Figure 6. Specimens previously referred to Pelagornithidae and here reassigned to other taxa. (1, 2) Fragmentary mandible (MLP 83-V-30-1) reassigned to Sphenisciformes indet. coming from the middle Eocene of Seymour Island (locality DPV 13/84, level 38) in lateral and medial views; (3–5) fragmentary dentary of fish (MLP 83-V-30-2) coming from the late Eocene of Seymour Island (DPV 16/84 locality, level 39) in lateral, cranial and medial views, respectively. Abbreviations: cl = cotyla lateralis; p = prominence. Scale bar =20mm.
Reassignment of other alleged Antarctic Pelagornithids
An incomplete articular portion of a mandible (MLP 83-V-30-1, Fig. 6.1, 6.2) found in the middle levels of the Submeseta Formation (DPV 13/84 locality, level 38, Submeseta II Allomember, middle Eocene; Fig. 2) has previously been assigned to Pelagornithidae (Tonni and Tambussi, 1985). However, such a taxonomic assignment was based on characters that are present in other birds, such as (1) a straight ventral margin of the mandible (shared with penguins), (2) the caudal edge forming an angle of almost 90° with the ventral margin (shared with Eocene penguins; Acosta Hospitaleche and Haidr, 2011), and (3) an elongated articular surface, oblique with respect to the ramus mandibulae (shared with many other birds, including penguins). This mandible is strong and robust, but unfortunately, it
shows a badly weathered external surface. Neither the processus retroarticularis nor the fossa caudalis are preserved. The cotyla lateralis is larger than the cotyla medialis; they are both merged with a small prominence at the medial side (Fig. 6.2). MLP 83-V-30-1 exhibits the morphology that is congruent with that expected for a penguin mandible (Ksepka and Bertelli, 2006, fig 3; Acosta Hospitaleche and Haidr, 2011, fig. 2A–C, and E). Regrettably, the incomplete nature of the material does not allow an accurate assignment. However, the external cortex of MLP 83-V-30-1 is relatively thick, unlike in pelagornithids. Based on that and the features commented on above, MLP 83-V-30-1 should be considered as Sphenisciformes indet., belonging to a robust and giant species, within the size range of Palaeeudyptes or Anthropornis (Jadwiszczak and Acosta Hospitaleche, 2013, 2015; Acosta Hospitaleche and Reguero, 2015). A second specimen (MLP 83-V-30-2, Fig. 6.3–6.5) was
previously described by Tonni and Tambussi (1985) as a pela- gornithid. This fossil comes from the upper levels of the Sub- meseta Formation (DPV 16/84 locality, level 39, Submeseta III Allomember, late Eocene; Fig. 2). This specimen consists of a
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