Conflicto antarcticus, Tambussi & Degrange & De Mendoza & Sferco & Santillana, 2019

CONFLICTO ANTARCTICUS SP. NOV.

FIGURES 2–13

urn:lsid:zoobank.org:act:F39B7797-D2F8-49AA-B912-59887F2ACA1D

Holotype: MLP 07 View Materials -III-1-1, a three-dimensionally preserved, partly complete skeleton, including cranium, both mandibular rami, ten vertebrae, a portion of the sternum, the cranial portion of the right coracoid, furcula, distal left humerus, proximal end of right humerus, both carpometacarpi, wing phalanx, left femur, left tibiotarsus lacking the proximal end, pelvis, synsacrum and ribs and five tracheal rings, including the pessulus ( Fig. 2). Housed at La Plata Museum ( MLP).

Derivation of name: Relative to the geographical area of origin, Antarctica.

Type locality and stratigraphy: The holotype was collected in Seymour Island at GPS 64°18 ′ 3.6 ″ S, 56°44 ′ 19.2 ″ W, from level 10 of Montes et al. (2012a, b) in the López de Bertodano Formation, lower Palaeocene (Danian, ~66–61 Myr) in age GoogleMaps .

widely separated; facies articularis clavicularis wide and smooth; processus procoracoideus long, curved, ending in a sternally directed hook. Incisura or foramen nervi supracoracoidei lacking. Carpometacarpus with the processus extensorius distally elongated and the dorsal rim of the trochlea carpalis slightly shorter than the ventral rim. Humerus without prominent capital shaft ridge directed toward the caput humeri. Well-marked incisura capitis only in the ventral edge of the caput humeri. Crista spinosa synsacri, very well demarcated; both fenestrae intertransversariae caudad of acetabulum and tuberculum preacetabulare lacking. Thin pubis extended caudally beyond the spina dorsolateralis ilii; foramen ilioischiadicum broad, craniocaudally elongated; subcircular foramen acetabuli located approximately at the mid-length of the ilium; large antitrochanter, concave in transverse section, located at the dorsocaudal rim of the acetabulum. Femoral shaft long, straight with parallel margins; well-marked tuberculum m. gastrocnemialis; lateral condyle diverges slightly from the axis of the diaphysis. Sulcus patellaris mediodistally narrower than the condylus medialis.

Distribution: Known only from the type locality.

Diagnosis: Conflicto antarcticus is diagnosed by the following combination of characters. A long-legged, medium-sized volant anseriform. Skull domed, with a pair of frontal eminences separated by the midline, slightly developed processus postorbitalis, reduced sessile processus basipterygoidei, os lacrimale not fused with the os frontale, fonticuli occipitales present. Os quadratum with a well-marked crista orbitalis; two condyli with condylus medialis more extended caudally than condylus lateralis. Three pneumatic foramina: (1) the foramen pneumaticum rostromediale caudal to the crista medialis; (2) the foramen basiorbitale (conspicuous and round) located on the orbitocondylar angle, on the caudal aspect at the confluence between the processus oticus and processus mandibularis; and (3) the foramen caudomediale on the processus oticus. Mandible with short, compressed, elongated and slightly dorsally curved processus retroarticularis. Heterocoelous cervical vertebrae. V-shaped furcula. Coracoid shaft depressed, dorsal surface flat, sulcus m. supracoracoidei broad, with the processus acrocoracoideus and the processus procoracoideus

DESCRIPTION AND COMPARISONS

Measurements of the new taxon are provided in Table 1. Conflicto antarcticus is a medium-sized waterfowl, with a body mass estimation of 2184.72 g, similar to that of Anseranas semipalmata Latham , the sole extant member of Anseranatidae , and considerably higher than that of any species of Anas ( Dunning, 2008) . Smooth bone surfaces indicate that the individual reached skeletal maturity ( Tumarkin-Deratzian et al., 2006).

Cranium ( Figs 3–5; Supporting Information,

Fig. S1)

The skull is partly preserved; it is broken in the nasofrontal hinge area, with the cranium displaced rostrodorsally of the caudal end of the rostrum so that the two parts overlap. The arci jugales are not preserved.

In lateral view, the overall shape of the skull does not resemble that of any living waterfowl ( Fig. 3; Supporting Information, Fig. S1). The preserved portion of the bill is not spatulate but rostrally tapered; the interorbital region of the cranium at mid-length is narrower than the rostral portion, and the processus postorbitales are very short. These last two features are shared with the anseriform Presbyornis and several oxyurines. Two eminences, symmetrical about the axial plane, are present in the dome of the skull, but they are not similar to the large protuberances that make up the casque characteristic of the magpie goose. MLP 07-III-1-1 exhibits some key galloanserine apomorphies ( Mayr, 2017; Mayr et al., 2018), such as sessile processus basipterygoidei and an elongated, compressed processus retroarticularis on the caudal portion of the mandible (although much shorter and less dorsally curved than in extant waterfowl).

The preserved part of the rostrum is stout, not especially as wide as in living ducks or swans, though no less wide than it is in mergines or dendrocygnines. Although the tip of the bill is missing, the beak seems to be longer than the braincase. The rostrum is abraded along part of its tomial margin, but the portions preserved prohibit the possibility of a great ventrolateral development, as in all anseriforms except anhimids. The tomium is directed laterally, with a low dorsal slope similar to that in the magpie goose, Anseranas semipalmata . The ventral surface of the beak is concave, with a well-defined groove in its midline. This ventral groove arises from the rostral end of the narial openings and is directed towards the tip of the bill. It is present in recent anseriforms (and in Procellariiformes). Presence of lamellae could not be confirmed.

Although it is partly covered by the jaw and sediment, the apertura nasi ossea are almost completely exposed when viewed ventrally ( Fig. 4C), a condition shared with Anseranas but not with Anatidae . Only the left narial opening is visible in lateral view. It has a scalene subtriangular contour. Narial openings are wider than in any other anseriform; they seem to occupy near half of the entire length of the beak (i.e. maximal length of the nostril is one-third of the preserved length of the skull; see Table 1) and extend dorsoventrally for almost the lateral wall of the beak. This condition differs greatly from that in Presbyornis , Anhimidae and in extant ducks. In most Anatidae and in Presbyornis , the lateral wall ventral to the apertura nasi ossea is greater than the height of the nostril. In C. antarcticus , the ventral margin of the nostril is narrower. The narial opening is caudally limited by a wide column, supposedly consisting of the os nasale and the ascending process of the os maxillare, but the margins of the bones cannot be perceived. The holorhynal condition is clear. The form and arrangement of the nostrils are noticeably different from those of Anatidae , resembling the condition seen in Presbyornis . The pila supranasalis is very narrow and slightly convex dorsally (flat in Presbyornis ). Both sides of the pila are subparallel, without caudal expansion.

There is no large bony casque characteristic of Anseranatidae on the ossa frontales ( Olson, 1999a). The interorbital bridge (fused ossa frontales) is narrow, much narrower than in Anseranas . It has a shallow medial furrow. The rostralmost portion of the ossa frontales is markedly concave, V-shaped in appearance. The most peculiar feature of the dome of the skull is the presence of two symmetrical protuberances. We do not know of any anseriform with this feature. The correspondence of these to any structures on the external surface of the encephalon is currently under study. A preliminary virtual three-dimensional model of the endocast allowed us to discard the presence of well-developed Wulsts as causing these features. Given that skull USNM299486 of Presbyornis is slightly distorted by lateral compression, the apparent frontal eminences seen on this specimen might be unnatural and not homologous to those on C. antarcticus . In some Charadriiformes (i.e. Larus ), it is possible to denote similar elevations, but in these cases they are rostrally demarcated by the fossae glandularum nasales.

The fossae temporales are triangular, shallow and broad, with a wide dorsal extension (although they do not contact apically; Figs 3, 4). Both the crista temporalis and the crista nuchalis transversa are very distinct, especially the former, which reaches the processus paroccipitalis. In this aspect, C. antarcticus looks more like Presbyornis than Anas . The crista nuchalis sagittalis is absent, a condition shared with all ducks. The processus paroccipitales are robust and less ventrally extended than in Anas . In lateral view, the otic region forms an arch between the processus paroccipitales and postorbitales, a condition similar to that of presbyornithids. The processus zygomaticus is absent and the processus suprameaticus reduced.

Both the cotylae quadratica otici and squamosi show very well-defined limits. The recessus tympanicus rostralis is large, and dorsally located to the ostium canalis tubae auditivae (small foramen).

The foramen magnum has a pentagonal shape, with its dorsal point at the base of the vertical prominentia cerebellaris, which forms a particularly sharp ridge that runs towards the crista nuchalis transversa ( Fig. 5A). The fossa subcondylaris is deep and small. The condylus occipitalis is heart shaped in caudal aspect (with a deep incisura condylaris), a character not shared with Presbyornis .

The fonticuli occipitales are present (only the right side is preserved). These fonticuli are also present in many anatids and presbyornithids ( Olson & Feduccia, 1980; Ericson, 2000), but the size and presence or absence of these may even be variable individually ( Iwaniuk et al., 2009).

T h e l a m i n a p a r a s p h e n o i d a l i s i s p e n t a g o n a l i n s h a p e, s h a l l o w l y c o n c a v e. T h e p r o c e s s u s laterales parasphenoidales are very prominent, and the processus mediales parasphenoidales, easily recognizable, are elongated. The processus basipterygoidei are sessile and elongated and are located on the rostrum parasphenoidalis rostral to the lamina parasphenoidalis. Reduced processus basipterygoidei are typical of Galliformes and Anatidae ( Elzanowski, 1977) .

The ossa lacrimales have not been preserved, but the presence of clear facies articularis lacrimalis shows that they were present and must have been unfused to the ossa frontales. This is considered as a plesiomorphic condition shared with Presbyornis , Anseranas and Anhimidae and several basal anatids (such as Dendrocygna and Coscoroba ), whereas in most other Anathidae the lacrimals are fused to the frontals ( Olson, 1999a; Worthy, 2009; Worthy et al., 2017).

The orbit is longer than the braincase. On the ventrocaudal part of the thin septum interorbitale it is possible to discern a large opening, which corresponds to the exit of nerve II. Rostrally, a smaller, rounded fonticulus interorbitalis can be noticed. A large fonticulus orbitocranialis opens dorsally to the septum. Caudally, this fonticulus connects with the cranial cavity. The inferior border of this fenestra is provided by the thick dorsal limit of the septum. The fenestra merges rostrally with the sulcus olfactorius, the groove for the olfactory nerve (nerve I). The groove converges with a conspicuous fossa, excavated in the internal face of the frontal and the dorsal margin of the os mesethmoidale that would be covered laterally by the lacrimal.

The processus postorbitales are short, blunt and ventrorostrally directed, unlike any living anseriforms, in which these processes are much longer. There is no sulcus glandulae nasalis. Along the entire supraorbital margin of the orbits, a scar a few millimetres in width and higher than the rest of the frontals develops; this scar is linked with the nasal gland and resembles that of Anseranas , anatids and Presbyornithidae (but not Anhimidae ). This shallow scar reaches the processus postorbitalis. In contrast, a well-marked scar marks the presence of a salt gland and characterizes the skull of most Charadriiformes, Gaviiformes , Sphenisciformes and Procellariiformes ( Bertelli et al., 2010). Below this ridge, rostrally and internally, the region of articulation with the os lacrimale is clearly evident.

Owing to deformation, the ossa palatina are displaced to the right of the cranium ( Fig. 4C, D). In their general aspect, they resemble the palatines of ducks. They are unfused along their midline, curved ventrally (in ducks, ventrolaterally); the pars lateralis is broad, expanded lateromedially, leaving the lateral margin positioned more ventrally than the medial (consequently, the sheet of the palatine is arranged obliquely), and the maxillary processes are flat, narrow (wider than in anatids) and long. They do not diverge at a wide angle rostrally. The palatines are very narrow in Anseranas semipalmata (see Miller, 1919). The processus pterygoideus is robust and elongated caudally. The angulus caudolateralis is slightly pronounced, and the crista lateralis is robust (more than in living anatids). The lamina choanalis is not visible.

The vomer is not preserved. A disarticulated os pterygoideum that we identify as the right element was recovered ( Fig. 4E). It is a short, rod-like, stout bone (although less stout than that of Presbyornis ), and not fused with the palatine. The facies articularis basipterygoidea is absent; however, the surface of the shaft is so poorly preserved that this absence could be a taphonomic artefact. A large basipterygoid facet is present in the Maastrichtian Vegavis iaai Clarke et al. the latter processus, a delicate crest is directed forward to the pars palatina. The facies articularis palatina is unique, wide and flattened. No pneumatic foramen is visible, which contrasts with the fact that pneumaticity of the pterygoid was interpreted as a Galloanserinae plesiomorphy ( Elzanowski & Stidham, 2010).

(2005), and its presence is considered as a plesiomorphic condition, whereas in most Neoaves these processes on the pterygoid are vestigial or absent ( Clarke et al., 2016). The processus quadraticus is stout, and the facies articularis quadratica is elongated, formed by a deep furrow bounded laterally and medially by two processes: the medial one constitutes a lip, whereas the lateral one is stout and blunt, laterocaudally projected. From Os quadratum: Only the right quadrate was preserved ( Fig. 6). In general, the corpus quadratum is slender, much more than that of the presbyornithids and anseriforms, in which the corpus is short and stout. In fact, the os quadratum resembles that of extant Galliformes (e.g. Megapodiidae ).

The processus oticum is high and robust, whereas in most anseriforms it is mediolaterally narrow. The very well-marked incisura intercapitularis delimits two heads of articulation. As in Presbyornis ( Elzanowski & Stidham, 2010) , the capitulum oticum is slightly larger than the capitulum squamosum and is less projected dorsally. A tuberculum subcapitulare is present but small. Elzanowski (2014) describes the tuberculum subcapitulare of Presbyornis as poorly defined or continuous with the capitulum squamosum.

The crista lateralis is sharp and caudally oriented. There is a well-edged crista medialis. A ventrocaudally oriented canal, the depressio caudomedialis, is delimited between the crista supraorbitalis (which extends from the rostral aspect of the capitulum oticum to the processus orbitalis) and the crista medialis.

The processus orbitalis is long and curved dorsally, thin throughout its length. The orbitocondylar angle (between the axes of the mandibular articulation and processus orbitalis) is 125°, being greater than in Presbyornis ( Elzanowski & Stidham, 2011) .

The caudal margin of the os quadratum, between the processus oticus and the condylus lateralis of the processus mandibulae, is more markedly concave in the Antarctic fossil than in the examined extant species of anatids. There are three pneumatic foramina ( Fig. 6C): a foramen basiorbitale (conspicuous and round), located on the orbitocondylar angle, on the caudal aspect at the confluence between the processus oticus and mandibularis, caudally delimited by the crista medialis and separated from the basiorbital fossa by a distinct ridge; a foramen caudomediale on the processus oticus, rostrally delimited by the crista medialis; and a foramen rostromediale, placed rostrally to the caudomedial foramen and the crista medialis. The presence of the first is a plesiomorphic condition for the galloanserines, whereas the second is characteristic of all living anseriforms ( Elzanowski, 2014), and the rostromedial foramen is absent in non-presbyornithid anseriforms but variably present in Chauna torquata ( Elzanowski & Stidham, 2010) . In Presbyornis , the caudomedial foramen is absent ( Elzanowski, 2014; Worthy et al., 2017).

The processus mandibularis has two sturdy mandibular condyli, very well separated by a shallow notch. The condylus lateralis is longer than the medialis, but the latter is more extended caudally. The presence of two condyles (rather than three that characterize Neoaves), is a distinctive morphology of the galloansere quadrate ( Mayr & Smith, 2001; Worthy et al., 2017; Mayr et al., 2018). The fovea quadratojugalis has not been preserved. The condylus pterygoideus is elongated and low, poorly marked.

Following Elzanowski & Stidham (2010), several characteristics of the os quadratum of C. antarcticus are plesiomorphic for anseriforms and conform with those of the ancestral quadrate for galloanserines: presence of a tuberculum subcapitulare, a crista orbitalis located on the ventral surface of the processus orbitalis, bicondylar mandibular articulation and a large orbitocondylar angle. Characters such as the presence of a foramen pneumaticum rostromediale located rostrally to the crista medialis, a very strong crista orbitalis and the condylus medialis extending more caudally than the condylus lateralis fit with those features of the quadrate in Anseriformes , but the presence of three pneumatic foramina (foramen pneumaticum rostromediale, caudomediale and basiorbitale) is exclusive among Neornithes. The os quadratum of the new taxon shares with presbyornithids the presence of an orbitocotylar crest extending on the corpus, the fact that the orbital angle is> 90°, and the presence of at least two pneumatic foramina.

Mandible

The lower jaws can be fully appreciated in lateral view ( Fig. 3). They are disjointed, and the pars symphysialis is missing. The rami mandibulae are higher at the level of the orbitae, with the dorsal margin strongly deflected dorsally just rostral to the articular region, a condition observed in Galloanseres. They are elongated, taller and thinner on the pars caudalis, shorter and stouter towards the pars symphysialis, with a blunt crista tomialis, and with a marked angulus mandibulae that separates both pars, as in Anatidae .

The os dentale has a ventrolateral projection, shorter than in extant ducks. On its lateral surface, a deep groove can be appreciated. Only one small fenestra mandibulae, interpreted here as the fenestra mandibulae rostralis, is present, ventrally delimited by the projection of the os dentale and dorsally by the os angulare. This fenestra is classically large in Galliformes . Caudal to this fossa, the prominent processus coronoideus of the mandible is present, a character that is also present within the clade Anseres ( Dyke, 2001). The configuration of the fossa articularis quadratica is, in general, similar to that of living anatids when viewed dorsally ( Fig. 4B). The cotyla medialis is deep and elongated, whereas the cotyla lateralis is flat and bordered laterally by a strong edge, the processus lateralis mandibulae. This processus is directed towards a lateral stout tubercle that is ventrocranially slanted, located on the lateral aspect of the jaw, beneath the fenestra mandibulae rostralis. The processus medialis mandibulae is sturdier than that of modern ducks, although shorter and endowed with a dorsal pneumatic foramen on its base, medial to the cotyla medialis. As in Anseranas , a shallow recessus conicalis is present. The processus retroarticularis is present, transversely compressed, gently curved dorsally, shorter than in Anatidae , and delimits laterally the deep fossa caudalis. Short or absent retroarticular processes also characterized some members of the stem group Anseriformes , such as V. iaai ( Clarke et al., 2016; Mayr et al., 2018), and pseudotoothed birds ( Mayr & Rubilar-Rogers, 2010). The crista transversa fossae is stout.