Institutional abbreviations

LACM, The Natural History Museum of Los Angeles County, Los Angeles, USA; MCZ, Museum of Comparative Zoology, Harvard University, Cambridge, 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 de Khouribga, Service de Géologie, Morocco; SMF, Senckenberg Research Institute Frankfurt, Frankfurt am Main, Germany; UCR, University of California, Riverside, USA; USNM, National Museum of Natural History, Washington, USA.

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 scapulotricipitalis 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, Reference Mourer-Chauviré and Geraads2008; Mayr and Smith, Reference Mayr and Smith2010; Bourdon et al., Reference Bourdon, Amaghzaz and Bouya2010; Bourdon and Cappetta, Reference Bourdon and Cappetta2012): (1) craniocaudally compressed shaft, (2) distal condyli located at the same level and distally prominent, (3) roughly rounded condylus ventralis, (4) prominent epicondylus 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 Andrews, Reference Andrews1916 by Bourdon and Capetta (2012, fig. 2L–N and P). However, both specimens are included in the size range observed for Dasornis emuinus (Bowerbank, Reference Bowerbank1854) from the late Paleocene/early Eocene of Morocco (Bourdon et al., Reference Bourdon, Amaghzaz and Bouya2010; Table 1). The wide intraspecific size variability reported for some pelagornithids (i.e., D. emuinus; D. toliapica [Owen, Reference Owen1873]; Bourdon et al., Reference Bourdon, Amaghzaz and Bouya2010; and Pelagornis miocaenus Lartet, Reference Lartet1857; Mourer-Chauviré and Geraards, 2008) suggests that the Antarctic and Togo specimens may belong to the same species.

Table 1

Skull and limb dimensions of pseudo-toothed birds of Seymour Island compared with those of other Pelagornithidae

All measurements are in millimeters.

Data taken from: ^aBourdon et al. (Reference Bourdon, Amaghzaz and Bouya2010), ^bHarrison and Walker (1976), ^cGonzález-Barba et al. (Reference González-Barba, Scwennicke, Goedert and Barnes2002), ^dthis work, ^eRubilar-Rogers et al. (Reference Rubilar-Rogers, Yury-Yáñez, Mayr, Gutstein and Otero2011), ^fMayr and Zvonok (2012), ^gMayr and Smith (Reference Mayr and Smith2010), ^hBourdon and Cappetta (2013), ⁱGoedert (Reference Goedert1989), ^jMourer-Chauviré and Geraads (2008), ^kHopson (Reference Hopson1964), ^lOlson (1985), ^mWetmore (1928), ⁿOkazaki (Reference Okazaki1989), ^oMayr et al. (Reference Mayr, Goedert and McLeod2013), ^pHoward (1957), ^qHoward and Warter (Reference Howard and Warter1969), ^rMayr and Rubilar-Rogers (2010), ^sChávez et al. (2007), ^tOlson and Rasmussen (Reference Olson and Rasmussen2001), ^uOno (Reference Ono1980), ^vBoessenecker and Smith (2011), ^wLambrecht (1930), ^xKsepka (2014).

* Preserved.

D-dist=depth of distal end; H-an=height at aperturae nasi ossea; TL=total length; W-dist=width of distal end; W-nf=width of naso-frontal hinge; W-prox=width of proximal end.

Although the epicondylus dorsalis of MLP 12-I-20-4 is only partially preserved, its morphology is similar to that of cf. Gigantornis sp. from Togo. It is more robust than that of Dasornis and less projected and rounded than the corresponding structure in Pelagornis (Bourdon et al., Reference Bourdon, Amaghzaz and Bouya2010; Bourdon and Cappetta, Reference Bourdon and Cappetta2012). According to the preserved outline, the condylus ventralis of MLP 12-I-20-4 was smaller than that of Pelagornis and similar to those of Dasornis and cf. Gigantornis sp. In dorsal view (like in Dasornis and cf. Gigantornis sp.), MLP 12-I-20-4 exhibits two deep concavities for the insertion of the musculus extensor carpi ulnaris surrounded by a thick and smooth ridge (Fig. 3.3). In Pelagornis these concavities are shallower and the ridge is missing or poorly developed (Bourdon et al., Reference Bourdon, Amaghzaz and Bouya2010; Bourdon and Cappetta, Reference Bourdon and Cappetta2012). A well-defined tuberculum supracondylare ventrale is present in MLP 12-I-20-4, which is more vertically oriented (subparallel to the main axis of the diaphysis), and more proximally extended than in Dasornis.

The fragmentary distal shaft of the humerus USNM 494035 from East Antarctica has been figured by Stilwell et al. (Reference Stilwell, Jones, Levy and Harwood1998, fig. 2) and Jones (Reference Jones2000, fig. 1). Although the surface of the diaphysis is highly crushed and broken at the proximal level of the condyli, the fossa musculi brachialis is preserved as a cranial depression recognizable in distal view. The size of the diaphysis, its cranio-caudal compression, and the extremely thin wall of cortical bone (evidencing a high pneumaticity not present in others large sized coetaneous Antarctic birds; i.e., Ratites, Sphenisciformes) allow the inclusion of USNM 494035 in the Pelagornithidae (see similarities with specimens described by Boessenecker and Smith, Reference Boessenecker and Smith2011; and Fitzgerald et al., Reference Fitzgerald, Park and Worthy2012). Moreover, the wide fossa musculi brachialis, the narrow ridge-like margo dorsalis, the shallow sulcus humerotricipitalis, and the shape and extension of the tuberculum supracondylare ventrale (Jones, Reference Jones2000) are also similar to that of pelagornithids. Size and morphological characters of USNM 494035 are similar to those observed in the humerus MLP 12-I-20-4 here described.

Material

MLP 12-I-20-4, distal end of right humerus (Fig. 3.1–3.4); USNM 494035, fragmentary distal portion of shaft of right humerus (Stilwell et al., Reference Stilwell, Jones, Levy and Harwood1998, fig. 2; and Jones, Reference Jones2000, fig. 1).

Occurrence

MLP12-I-20-4 comes from the Cucullaea I Allomember (level 35 in Montes et al., Reference Montes, Nozal, Santillana, Marenssi and Olivero2013, fig 2) of the La Meseta Formation (Marenssi et al., Reference Marenssi, Santillana and Rinaldi1998a), IAA 1/95 locality, Seymour Island, Antarctic Peninsula, West Antarctica. Strontium dating yielded ages of 52.8–49 Ma for this locality (middle Ypresian, early Eocene; Montes et al., Reference Montes, Nozal, Santillana, Marenssi and Olivero2013).

The specimen USNM 494035 was found in an erratic glacial (A303) collected from moraine deposits on the NW side of Mount Discovery, McMurdo Sound, East Antarctica. The associated micro and macro invertebrate fauna indicates an early/middle Eocene age (Jones, Reference Jones2000; and references therein).

Measurements

MLP 12-I-20-4: see Table 1. USNM 494035 (Jones, Reference Jones2000): preserved length, 85 mm; maximum dorso-ventral width, 32 mm; maximum cranio-caudal depth, 22 mm; thickness of bone wall, 1.5 mm.

Pelagornithidae indet. morphotype 2

Figure 4

Figure 4

Pelagornithid specimens assigned to morphotype 2 from the middle Eocene of Seymour Island (West Antarctica). (1) Comparative proportions of the cranial remains from Antarctica (MLP 08-XI-30-42, MLP 78-X-26-1) and the holotype of the giant species Pelagornis chilensis (after Mayr and Rubilar-Rogers, Reference Mayr and Rubilar-Rogers2010). (2, 3) rostral end of rostrum maxillare (MLP 08-XI-30-42) in lateral and dorsal views; (4–8) most rostral narial region of rostrum maxillare (MLP 78-X-26-1) in dorsal, lateral, ventral, and cranial (7, 8) views, respectively; (9–11) distal end of right tarsometatarsus (UCR 22176, cast MLP) in dorsal, plantar, and distal views, respectively. Abbreviations: cid=canalis interosseus distalis; cr=culmen ridge; ct=cristae tomiales; ftp=fossa for mandibular tooth-like processes; fvd=foramen vasculare distale; dc=dorsolateral constriction; pmp=processus medioplantaris; mvs=medioventral sulcus; ls=longitudinal sulcus; pr=palatal ridge; tp=toothlike processes (2–4=rank); tII, tIII, and tIV=trochleae metatarsorum II, III, and IV. Scale bars=20 mm; except in 1=50 mm.

Description

The rostral end of the rostrum maxillare MLP 08-XI-30-42 is congruent with those of other pseudo-toothed birds (Rincón and Stucchi, Reference Rincón and Stucchi2003; Stidham, Reference Stidham2004; Bourdon et al., Reference Bourdon, Amaghzaz and Bouya2010; Mayr and Rubilar-Rogers, Reference Mayr and Rubilar-Rogers2010; Mayr and Zvonok, Reference Mayr and Zvonok2012; Ksepka, Reference Ksepka2014). Its size exceeds the medium-sized D. toliapica and L. tethyensis, and even the larger D. emuinus, corresponding in dimensions with the giant Neogene Pelagornis species (Fig. 4.1). The cristae tomiales preserve the base of the first rostral-most large tooth-like process (Fig. 4.2). The rostral end is slightly ventrally curved, and the transverse sulcus before the tip of the beak is absent in MLP 08-XI-30-42, as well as in D. toliapica and L. tethyensis, whereas is typically present and well developed in Pelagornis (Olson, Reference Olson1984; Stidham, Reference Stidham2004; Bourdon et al., Reference Bourdon, Amaghzaz and Bouya2010; Mayr and Rubilar-Rogers, Reference Mayr and Rubilar-Rogers2010; Mayr and Zvonok, Reference Mayr and Zvonok2012; Ksepka, Reference Ksepka2014). The longitudinal sulcus of MLP 08-XI-30-42 is less defined than in Pelagornis.

The rostrum maxillare MLP 78-X-26-1 (Fig. 4.4–4.8) has a deep longitudinal sulcus running upward along the dorsal third of the beak. In pelagornithids, these lateral sulci are longitudinally extended along the dorsal margin of the beak from apex rostri to aperturae nasi, whereas posteriorly, these sulci are more ventrally located (Bourdon et al., Reference Bourdon, Amaghzaz and Bouya2010; Mayr and Rubilar-Rogers, Reference Mayr and Rubilar-Rogers2010). Because of that, MLP 78-X-26-1 is referred to the sector immediately developed cranially to the aperturae nasi ossea. A dorsolateral constriction on the caudal section of the preserved culmen (Fig. 4.4) corresponds to that one developed cranially to the aperturae nasi ossea reported for other pelagornithids (Mayr and Rubilar-Rogers, Reference Mayr and Rubilar-Rogers2010; see also Ksepka, Reference Ksepka2014, fig.1.a). A dorsally located ridge (Tonni, Reference Tonni1980, fig. 4.4, 4.5) is very well defined as in Dasornis, whereas it is weaker or absent in Pelagornis. The maxilla from Antarctica is less compressed than in Dasornis, but narrower than in Pelagornis. On the contrary, the longitudinal sulcus is more dorsally located than in Dasornis and similar to the condition of Pelagornis (Harrison and Walker, Reference Harrison and Walker1976; Bourdon et al., Reference Bourdon, Amaghzaz and Bouya2010). As is typical of Pelagornithidae, the ventral surface of MLP 78-X-26-1 shows two longitudinal sulci for the location of the cristae tomiales and deep fossae for the tooth-like projections of the mandible (Lambrecht, Reference Lambrecht1930; Harrison and Walker, Reference Harrison and Walker1976; Tonni, Reference Tonni1980; Stidham, Reference Stidham2004; Mayr and Rubilar-Rogers, Reference Mayr and Rubilar-Rogers2010). Between both, there is a palatal ridge (central palatal region sensu Harrison and Walker, Reference Harrison and Walker1976, fig. 4.7) extended ventrally almost half of the rostrum in cross section. This condition is similar to that of Dasornis, which shows a well-defined medioventral sulcus (interpalatal groove sensu Harrison and Walker, Reference Harrison and Walker1976). In contrast, Pelagornis has a palatal ridge that is strongly convex and prominent (Harrison and Walker, Reference Harrison and Walker1976; Bourdon et al., Reference Bourdon, Amaghzaz and Bouya2010), and the sulcus is missing (Harrison and Walker, Reference Harrison and Walker1976; see Mayr and Rubilar-Rogers, Reference Mayr and Rubilar-Rogers2010, fig. 1D). On both cristae tomiales, MLP 78-X-26-1 shows three poorly preserved tooth-like processes. The largest one is on the middle, separated from the other two by an equidistance of 9.7 mm (Tonni, Reference Tonni1980), corresponding to pseudo-teeth of rank 2 and 3, respectively. Between them, small protuberances probably represent the basis of very thin “needles” of rank 4. The largest tooth-like processes of rank 1 are not preserved. A striking feature is the dorsoventral height of the beak measured at the level of the apertura nasi ossea, which is highest than in any other pelagornithid species known (Table 1). A polishing of the cross section (Fig. 4.8) permits the first detailed observation of its inner structure, which evidences an extreme bone pneumaticity.

The tarsometatarsus UCR 22176 (Fig. 4.9–4.11, 5) has the largest distal transverse width known for pseudo-toothed birds (Table 1, Cenizo, Reference Cenizo2012). The general morphology is similar to Dasornis, although it shares some features with Pelagornis (Cenizo, Reference Cenizo2012). The corpus of UCR 22176 is mediolaterally narrower than Pelagornis, and wider than Dasornis (Bourdon et al., Reference Bourdon, Amaghzaz and Bouya2010, fig.5) whereas is similar to the preserved proximal shaft of L. tethyensis (Mayr and Zvonok, Reference Mayr and Zvonok2012). The shaft of UCR 22176 is nearly square in cross-section, like in Dasornis; in Pelagornis the section is rectangular and dorsoplantarly depressed. Like in Dasornis, the trochlea metatarsi II of UCR 22176 is less distally displaced (Fig. 5) than in Pelagornis chilensis Mayr and Rubilar-Rogers, Reference Mayr and Rubilar-Rogers2010; whereas L. tethyensis and ?late Oligocene/early Miocene Pelagornis specimens 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., Reference Bourdon, Amaghzaz and Bouya2010; Mayr and Rubilar-Rogers, Reference Mayr and Rubilar-Rogers2010; Mayr, Reference Mayr2011; Mayr and Zvonok, Reference Mayr and Zvonok2012); the plantar extension in UCR 22176 is similar to that of L. tethyensis and the specimen tentatively assigned to Pelagornis (‘Palaeochenoides’) mioceanus by Hopson (Reference Hopson1964; see also Mayr et al., Reference Mayr, Goedert and McLeod2013). 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., Reference Mayr, Goedert and McLeod2013, fig. 5). UCR 22176 shares with Dasornis the lateral tilting of the trochlea metatarsi III (Bourdon, Reference Bourdon2005; Bourdon et al., Reference Bourdon, Amaghzaz and Bouya2010). The dorsal opening of the foramen vasculare distale in UCR 22176 is proximodistally elongated, like that of Dasornis (Bourdon et al., Reference Bourdon, Amaghzaz and Bouya2010; 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, Reference Bourdon2005; Mayr and Rubilar-Rogers, Reference Mayr and Rubilar-Rogers2010; Mayr, Reference Mayr2011).

Figure 5

Comparative morphology of the tarsometatarsus in Pelagornithidae. Selected specimens are listed from left to right: Dasornis toliapica OCP.DEK/GE 1146 and Dasornis emuinus OCP.DEK/GE 1106 (after Bourdon et al., Reference Bourdon, Amaghzaz and Bouya2010, reversed for comparison), D. emuinus NHMUK 894 (after Harrison and Walker, 1977), Lutetodontopteryx tethyensis SMF Av 553a+b (holotype, after Mayr and Zvonok, Reference Mayr and Zvonok2012), Pelagornithidae indet. morphotype 2 UCR 22176 (MLP cast), cf. Pelagornis (‘Palaeochenoides’) mioceanus MCZ 2514 (after Hopson, Reference Hopson1964, reversed for comparison), Pelagornis sp. LACM 128424 (after Mayr et al., Reference Mayr, Goedert and McLeod2013), Pelagornis chilensis MNHN SGO.PV 1061 (holotype, after Mayr and Rubilar-Rogers, Reference Mayr and Rubilar-Rogers2010). (1–3) Dorsal, plantar, and distal views, respectively. Abbreviations: DP=distal projection of the trochleae metatarsorum II and IV; mp=processus medioplantaris; PP=plantar projection of the trochleae metatarsorum II and IV; tII, tIII, and tIV=trochleae metatarsorum II, III, and IV. Unscaled images for comparison.

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); UCR 22176, 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., Reference Montes, Nozal, Santillana, Marenssi and Olivero2013, fig. 2) of the Submeseta Formation (Montes et al., Reference Montes, Nozal, Santillana, Marenssi and Olivero2013), 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., Reference Montes, Nozal, Santillana, Marenssi and Olivero2013).

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., Reference Rubilar-Rogers, Yury-Yáñez, Mayr, Gutstein and Otero2011). 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, Reference Ksepka2014 (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., Reference Rubilar-Rogers, Yury-Yáñez, Mayr, Gutstein and Otero2011) are consistent with assignment to morphotype 2.

Pelagornithid size-types

Otherwise, considering the worldwide pelagornithid record and according to the estimated wingspan, four approximate size-ranges were identified (Fig. 7). The small gannet-sized Dasornis abdoun Bourdon et al., Reference Bourdon, Amaghzaz and Bouya2010 (1.5–1.7 m wingspan; Bourdon et al., Reference Bourdon, Amaghzaz and Bouya2010), the medium albatross-sized D. toliapica and L. tethyensis (2–3 m wingspan; Bourdon et al., Reference Bourdon, Amaghzaz and Bouya2010), the large D. emuinus (3.5–4.5 m wingspan; Mayr, Reference Mayr2009; Bourdon et al., Reference Bourdon, Amaghzaz and Bouya2010), and finally, the giant middle/late Eocene and Neogene Pelagornis-related taxa (5–6 m wingspan; Olson, Reference Olson1985; Mayr, Reference Mayr2009; Mayr and Rubilar Rogers, 2010; Boessenecker and Smith, Reference Boessenecker and Smith2011; Ksepka, Reference Ksepka2014).

Figure 7

Temporal distribution, phylogenetic affinities, and size-types in selected pelagornithid species (modified from Mayr et al., Reference Mayr, Goedert and McLeod2013). The internal nodes are supported by the following presumable derived features (Mayr et al., Reference Mayr, Goedert and McLeod2013, and present work): Clade 1 (humerus with ventral portion of proximal end narrow and tuberculum dorsale strongly projected, ulna with very short olecranon, trochlea metatarsi II at the same level or slightly more plantarly projected than the trochlea IV); Clade 3 (femur with trochlea fibularis lacking sulcus). The Clade 2 is still uncertain, but according to general similarities it considered by Mayr et al. (Reference Mayr, Goedert and McLeod2013) as Pelagornis s.l. (a presumable derived character noted here is the presence of the transverse furrow near the tip of the rostrum). The size-types represent the wingspan inferred by extrapolated dimensions of the known elements in incomplete specimens compared with the homologous elements in the P. chilensis holotype, the most completely known pelagornithid specimen. The gray areas indicate possible cladogenetic event. (Af) Africa, (E) Europe, (NA) North America, (Ant) Antarctica, (NZ) New Zealand, (SA) South America, (Au) Australia.

The Antarctic material assigned to morphotype 1 (middle Ypresian), including the humerus MLP 12-I-20-4 and the humeral shaft USNM 494035 (Jones et al., 2000) correspond to the large size-type (i.e., equivalent in size to D. emuinus, Table 1). The material assigned to morphotype 2 (Bartonian/?Priabonian, including the humerus recently described by Rubilar-Rogers et al., Reference Rubilar-Rogers, Yury-Yáñez, Mayr, Gutstein and Otero2011) correspond to the giant size-type. As stated previously (see also Tonni, Reference Tonni1980; Cenizo, Reference Cenizo2012), they may belong to birds larger than the huge Neogene taxa (i.e., P. chilensis; similar-sized to P. sandersi; Table 1), constituting one of the largest pelagornithid known so far.

In this sense, two Antarctic morphotypes were recognized by previous authors. However, it was thought that both morphs coexisted during the middle/late Eocene of Seymour Island (Tonni and Tambussi, Reference Tonni and Tambussi1985; Cenizo, Reference Cenizo2012). This idea was conceived from the finding of the mandible MLP 83-V-30-1 assigned—at that time—to a pelagornithid. Its removal from pelagornithids implies that only giant pseudo-tooth birds (i.e., morphotype 2) are known for the Bartonian/?Priabonian strata of the Submeseta Formation.

Across the world, a large diversity of body sizes of pelagornithids has been recorded between the late Paleocene and middle Eocene (Mayr, Reference Mayr2009). Since the middle/late Eocene, only giant pelagornithids are known. These forms (more than 5 m wingspan) show a trend toward acquiring huge sizes, reaching a maximum specialization during the late Neogene (Fig. 7). Even taking into account the incompleteness of the fossil record, it seems that large, medium, and small taxa (Fig. 7) become extinct after middle Eocene times. An ecological competition for food or breeding sites could be the main cause (Mayr, Reference Mayr2009). A large list of candidates for such a competition may include the oldest Procellariformes (Noriega and Tambussi, Reference Noriega and Tambussi1996; Tambussi and Acosta Hospitaleche, Reference Tambussi and Acosta Hospitaleche2007; Tambussi and Degrange, Reference Tambussi and Degrange2013; Reguero et al., Reference Reguero, Goin, Acosta Hospitaleche, Dutra and Marenssi2013) described for the Cucullaea II Allomember (Ypresian/Lutetian), and the extremely diversified and widely extended early penguin fauna (Acosta Hospitaleche and Reguero, Reference Acosta Hospitaleche and Reguero2010; and references therein). The onset and diversification of giant forms occurred at the same time both in penguins (Clarke et al., Reference Clarke, Ksepka, Stucchi, Urbina, Giannini, Bertelli, Narváez and Boyd2007; Ksepka and Clarke, Reference Ksepka and Clarke2010), and pseudo-toothed birds. Giant penguins are recorded until the latest Eocene/earliest Oligocene in Antarctica (Acosta Hospitaleche, 2013, Reference Acosta Hospitaleche2015; Acosta Hospitaleche and Reguero, Reference Acosta Hospitaleche and Reguero2015), and the late Oligocene in New Zealand (Ksepka and Ando, Reference Ksepka and Ando2011). After that, giant penguins disappear from the fossil record, suggesting an important ecological segregation favoring medium and small penguins, in contrast with pseudo-toothed birds, in which giant size was maintained until their extinction in the late Neogene.

Highlights in pelagornithid evolution

The phylogenetic affinities within Pelagornithidae are not well understood, and thus, reliable conclusions are difficult to assess. The following inferences about the evolutionary history of the group should be considered as preliminary, and must be confirmed with new and more complete findings. On this basis, and having in mind that the pelagornithid record is patchy, three main evolutionary events can be recognized.

MLP 12-I-20-4, assigned to morphotype 1, is the oldest pseudo-toothed birds from Antarctica, and is more derived than Dasornis in several features. It indicates in the middle Ypresian, the presence of a more specialized ‘clade’ than that of Dasornis. This first event probably includes the disappearance of Dasornis and would be the starting point of the dominance of more advanced forms (“Clade” 1, Fig. 7) with a trend toward the extreme specialization of soaring capacity.

The second event would be linked to the origin of the Pelagornis s.l. species (hypothetical Clade 2, Fig. 7; see Mayr et al., Reference Mayr, Goedert and McLeod2013). It would have occurred during the early Oligocene, although unfortunately, the fossil record of pseudo-toothed birds is very scarce for that time. Finally, Mayr et al. (Reference Mayr, Goedert and McLeod2013) recognized some presumable synapomorphies that allow to join the younger Mio/Pliocene taxa in a highly specialized group of forms (Clade 3, Fig. 7), that probably constituted the third and more recent evolutionary event in pelagornithids.