Keywords: Miocene, fossil birds,Wilaru tedfordi, Gondwana, palaeobiogeography

2.1. Fossil comparative material

The following specimens ofPresbyornis pervetus were examined at USNM: skull—USNM 299846, USNM 618166, USNM 618202; premaxilla—USNM 510082, USNM 299845 (six nose slab); mandible—USNM 299847, USNM 618169, USNM 618215; quadrate—USNM 498770; thoracic vertebrae—USNM 616555, USNM 616556, USNM 618205, USNM 618207; sternum—USNM 618212, USNM 618214; scapula—USNM 616557–60; coracoid—USNM 618183, USNM 616561–67 (four sternal and three omal parts); humerus—USNM 483163 (cast), USNM 616568, USNM 618204, USNM 618180; ulna—USNM 616569–74; carpometacarpus—USNM 618168, USNM 618226, USNM 618227; femur—USNM 618228–35; tibiotarsus—USNM 483165 (cast), USNM 618192–96, USNM 618236; tarsometatarsus—USNM 483166 (cast), USNM 618175–76, USNM 618177, USNM 618178, USNM 618213, USNM 618237; pelvis—USNM 618167, USNM 618172, USNM 618198. Casts ofPresbyornis isoni were examined at USNM and PIN. The material attributed toT. gobiensis, as well as the holotype ofPresbyornis mongoliensis and the collection ofPresbyornis sp. specimens from the early Eocene of Mongolia [20] were examined at PIN.Telmabates antiquus was assessed based on casts at PIN, as well as the original description and images [13] and on Ericson’s diagnosis of the species [17]. Lithornithidae (Palaeognathae) were also considered for comparative purposes.

2.2. Extant comparative material

The following specimens were examined: Anhimidae:Anhima cornuta (MV B.12574; USNM 345208);Chauna chavaria (PIN Osteology collection 43-2-1);Chauna torquata (CM Av.21208). Anseranatidae:Anseranas semipalmata (SAM B.36790; USNM 621019). Anatidae:Anser caerulescens (SAMB36868);Biziura lobata (CM Av.7116);Cereopsis novaehollandiae (CM Av.21198; SAMB39638, 49165);Hymenolaimus malacorhynchos (CM Av.5217);Tadorna tadornoides (SAM B.39583; 39872);Tadorna variegata (CM Av.12424). Burhinidae:Burhinus grallarius (SAM B.48793; B.49554);Burhinus capensis (MV B.13648);Esacus magnirostris (SAM B.5052).

Fossil and living phoenicopteriforms (flamingos and allies) were also examined.

2.3. Remarks on the material previously attributed toWilaru tedfordi by Boleset al.

Paratype humeri AMNH 11407 and AMNH 11406 of the type speciesW. tedfordi are part of the same bone and now joined. Coracoid AMNH 11414 is a left one, contra Boleset al. [36]. Right coracoid SAM P.23625 is now a paratype of a new species (see below) and is here removed from the referred material ofW. tedfordi. Scapulae SAM P.48923 (fig. in [36]), AMNH 10990, AMNH 11434 and AMNH 11477 are here removed from the referred material ofW. tedfordi, as they are attributable to the Palaelodidae (Phoenicopteriformes). The unnumbered piece of mid-shaft associated with ulna AMNH 11456 is of a carpometacarpus, not an ulna. Distal ulna AMNH 10995 (not listed in [36]) may be associated with the unnumbered proximal right ulna mentioned by Boleset al. as they were found in the same box. Carpometacarpus AMNH 11474 (distal left) and AMNH 10998 (proximal left) are part of the same bone. Right carpometacarpus missing proximal end AMNH 10999 and proximal left carpometacarpus SAM P.42004 are here added to the referred material ofW. tedfordi. AMNH 10986 was erroneously listed as a proximal tibiotarsus, it is a distal end and belongs to Psittaciformes; the proximal tibiotarsus considered by Boleset al. is AMNH 11416 (we note that this bone was not formally referred toW. tedfordi, and we consider it to be Aves indet.). UCMP57152, a distal left tibiotarsus, is removed from the referred material as it is referrable toAustralotadorna alecwilsoni. Two specimens, a distal right tibiotarsus (SAM P.53134), from SAM North Locality, Lake Palankarinna, and a distal left tibiotarsus (SAM P.53135) from White Sands Basin, Lake Palankarinna, are here added to the referred material. The accompanying fragment of the left tarsometatarsus SAM P.48931 is a right proximal tarsometatarsus, not a left one.

3.1. Order Anseriformes Wagler, 1831

3.2. GenusWilaru Boles, Finch, Hofheins, Vickers-rich, Walters, & Rich, 2013

Type species: Wilaru tedfordiBoles, Finch, Hofheins, Vickers-Rich, Walters, & Rich, 2013

Type locality and age:Lake Pinpa (=Pine Lake), Site C, South Australia; Namba Formation, late Oligocene (24–26 Ma).

3.3.Wilaru prideauxi sp. nov.

Holotype: Right tarsometatarsus SAM P.53136 (formerly UCMP 108052) (figure 2q,r,b^′).

Type locality and age: Leaf Locality, Lake Ngapakaldi, South Australia (UCMP locality V6213); Wipajiri Formation; Kutjamarpu Local Fauna;ca 23.4–22 Ma [45,46].

Etymology: After vertebrate palaeontologist Gavin Prideaux (1969–), who has worked extensively on Oligo-Miocene mammalian faunas from South Australia, including the formations bearing fossils of species ofWilaru.

Paratypes: Right coracoid SAM P.23625 (figure 1i,j), Mammalon Hill, Lake Palankarinna, South Australia; Etadunna Formation Zone D; Ngama Local Fauna;ca 22 Ma [45]. Left carpometacarpus SAM P.41255 (figure 2a,b), Mammalon Hill, Lake Palankarinna, South Australia.

Measurements (mm): Coracoid: medial length: 38.9; length from angulus medialis to proc. procoracoideus: 24.7; length from cotyla scapularis to processus acrocoracoideus: 19.6; maximum omal length of sulcus m. supracoracoideus: 11.1 (shorter in Phoenicopteriformes); depth of facies articularis clavicularis: 8.8; length of facies articularis humeralis: 12.8. Carpometacarpus (table 1). Tarsometatarsus: maximum length: 65.9; proximal width: 12.7; shaft width (mid-shaft): 4.7; shaft depth: 4.7; width of trochlea metatarsi III: 4.9; depth of trochlea metatarsi III: 6.9.

Differential diagnosis:Only slightly larger thanW. tedfordi but considerably stouter (figure 1i–j;figure 2a,b,q,r). Differs fromW. tedfordi in: tarsometatarsus with (i) sulcus extensorius shallower (figure 2q); (ii) plantarly, rounded ligamental scar between trochleae metatarsi II and IV deeper and closer to foramen vasculare distale (figure 2r); (iii) fossa metatarsi nearly absent (figure 2r). Carpometacarpus with (iv) synostosis metacarpalis distalis proximodistally shorter (figure 2a,b); (v) facies articularis digitalis minor projecting further distally (figure 2b).

4.1.Teviornis gobiensis, a Cretaceous presbyornithid

Teviornis gobiensis, known primarily from its carpometacarpus (figure 2f,g,m,o), was attributed to the Anseriformes mainly based on its straight os metacarpale minus, and to the Presbyornithidae based on (i) the caudal part of the dorsal rim of the trochlea carpalis being well developed and connecting with the dorsal edge of the os metacarpale majus, (ii) the presence of well-developed scars for lig. ulnocarpometacarpale dorsale (fossa supratrochlearis) and lig. radiocarpometacarpale dorsale (fossa infratrochlearis), and (iii) the presence of a small canalis interosseus distalis in the fossa infratrochlearis [28]. Clarke & Norell [29] challenged presbyornithid, and even anseriform affinities ofT. gobiensis, noting that of the diagnostic characters listed a straight minor metacarpal may be a plesiomorphic feature of Neornithes, that features (ii) and (iii) are present in other anseriforms, and (i) is also present in anhimids. Clarke and Norell did not, however, consider all the distinguishing features mentioned in the description. A recent study [49] supported anseriform affinities of this taxon, noting marked differences from other taxa with a non-curved carpometacarpus (e.g. Gallinuloididae, Lithornithidae). Additional features confirm the identity ofT. gobiensis as a presbyornithid, namely (iv) the dorsal and ventral rims of the trochlea carpalis extend caudally and distally to about same level, (v) the sulcus tendineus is very elongate, extending just about to the distal end of the scar for M. extensor carpi ulnaris, (vi) in distal view, the facies articularis digitalis minor is considerably smaller than the facies articularis digitalis major (as inWilaru andTelmabates), and (vii) the synostosis metacarpalis proximalis is longer than it is craniocaudally wide. Within anseriforms, feature (iv) is present only in presbyornithids, (v) in presbyornithids and anhimids, (vii) is present in presbyornithids, anhimids, anseranatids, and only few anatids, whereas (vi) is widely distributed within Anseriformes but the alternate condition is present inP. pervetus.

Kurochkinet al. [28] noted thatT. gobiensis differed from other presbyornithids in having a fossa infratrochlearis stretched markedly craniocaudally. A shallow, craniocaudally elongated fossa is nonetheless also present in species ofWilaru (figure 2d). Similarly, the dorsoventrally and craniocaudally widened proximal portion of the os metacarpale minus are present in species ofWilaru but also in anhimids and other anseriforms, suggesting they could be plesiomorphic features for Presbyornithidae that are absent inP. pervetus.T. gobiensis, therefore, displays a combination of features of the carpometacarpus present uniquely in presbyornithid genus-level taxa.

4.2. Palaeobiology of species ofWilaru: terrestrial and territorial

The cranially elongated extensor process of the carpometacarpus of some specimens ofW. tedfordi forms a conspicuous rugose enlargement (figure 2c,d,k), known as a carpal knob or spur. Carpal knobs are projecting bony cores used primarily in fighting, which in some taxa may have an outer layer of horn [50] but can be bare in others [51]. These rugose structures arise from the deposition of bone on the extensor process, to which they are fused. Carpal knobs and spurs occur in several anseriforms [51], most notably in steamer ducks [52] and anhimids [50]. Within anseriform species that bear them, they are better developed in males, but are still present in females [50]. Well-developed carpal knobs, similar to those of the male paradise shelduckT. variegata, were present in three specimens of our sample, which were also slightly larger compared to the rest (table 1). Both sexual dimorphism and age probably explain these differences [51]. Some of the specimens with the less protruding extensor process still displayed a form of rugose enlargement. From this we infer that, following the pattern in other anseriforms, those individuals ofW. tedfordi with the well-developed carpal knobs are males. Anseriform species that bear prominent structures tend to engage in aggressive behaviour and hold year-round feeding and breeding territories [51]. The lack of well-developed carpal knobs in most of the specimens in our sample suggests that predominantly males may have engaged in aggressive behaviour. Howard [13] observed a ‘slight excrescence’ on the tip of the extensor process ofT. antiquus, and illustrations clearly show [13], fig. 6, p. 16, a small carpal knob. This structure is not present inP. pervetus or thePresbyornis specimens from the Eocene of Mongolia [20], which closely resemble each other.

Compared with other presbyornithids, the morphology of the tarsometatarsus, with mediolaterally splayed trochleae for the articulation of the toes, a less plantarly retracted trochlea for the second digit, and a relatively low hallux, suggests that species ofWilaru were more terrestrial. Screamers, which have a similar tarsometatarsal morphology, are birds of predominantly terrestrial habits, frequenting open savannahs and wetlands (meadowlands, marshes, swamps and lakes with abundant vegetation) [53]. The tarsometatarsus is proportionally longer and more gracile inP. pervetus, in which the narrowly divergent trochleae and the markedly retracted trochlea for the second digit indicate more aquatic adaptations, as in most anseriforms (e.g. [17,44]). The length and most detail of the tarsometatarsus are not known forT. antiquus, but it appears to have resembled that ofP. pervetus [17].

Terrestrial habits have evolved independently several times within Anseriformes [54,55], and even possibly within screamers [17,44] (§4.3). The terrestrial habits of species ofWilaru may, therefore, reflect a trophic specialization (such as herbivory) derived within Presbyornithidae. These differences are not surprising given the apparent temporal separation ofca 25 Ma between species ofWilaru and other presbyornithids, and may have been the key to the longevity of the presbyornithid lineage in Australia (§4.3).

The younger, larger and more robustW. prideauxi represents a further step from the morphology ofW. tedfordi down the path of terrestriality. As such, species ofWilaru provide the first example for Australia of two successive species within an avian lineage in the Oligo-Miocene. Multiple lineages of mammals are known over this time period in the Namba and Etadunna Formations in Australia and form the foundation of the biochronological understanding of the different faunas [1,56]. The occurrence ofW. prideauxi in the Ngama Local Fauna from Zone D of the Etadunna Formation at Lake Palankarinna, and in the Kutjamarpu Local Fauna from the Wipajiri Formation at Lake Ngapakaldi, South Australia, provides further evidence of the contemporaneity of these local faunas, otherwise linked by mammals, and supports their early Miocene age and distinction from underlying Etadunnan faunal zones [1].

4.3. The role of Gondwana and the evolutionary history ofWilaru

Several species of anatids were described from late Oligocene and early Miocene deposits of the Namba and Etadunna formations in South Australia [57], supporting an already established diversity of crown-group anatids by the late Oligocene. Having been recovered from the same late Oligocene and early Miocene localities, the survival of presbyornithids in Australia into the Neogene indicates they were living alongside crown-group anatids. Presbyornithids seem to have disappeared from the rest of the world during the Eocene [19], coinciding with the earliest records of stem group anatids. However, the more terrestrial adaptations of species ofWilaru suggest that in Australia, they may not have been in direct resource competition with coeval waterfowl. Similarly, at least two palaelodids and two species of flamingo cornered the wading niche in the lakes in which these deposits were formed [5,58]. The causes of the ultimate demise of species ofWilaru after the early Miocene are unknown, but as in the case of much of Australia’s fauna, climate change and the progressive aridification of the continent may have played a role, especially if species ofWilaru were territorial and dependent on specific habitats for breeding.

Fossils from the early Eocene Tingamarra Local Fauna, Queensland, were tentatively referred to the form-taxon Graculavidae (‘transitional shorebirds’ [59]) [32], but it has been acknowledged that some of the material may in fact be presbyornithid [30,32]. Indeed, the coracoid, fragment of humerus, and one distal tibiotarsus tentatively attributed to the ‘Graculavidae’ were recognized by Boles [32] as remarkably similar toP. pervetus, and we further note the marked similarity with the corresponding elements ofW. tedfordi and other members of the Presbyornithidae. Further assessment of this material will help establish if presbyornithids have been in Australia since at least the early Eocene.

Australia’s long period of geographical isolation, from complete separation from Antarctica to a close proximity to the Indo-Malayan region (ca 50–15 [60,61]), has certainly promoted the extended temporal continuance of its fauna. The presence of presbyornithids in the early Miocene of Australia, therefore, ought not to appear all that surprising. Within mammals, marsupials and monotremes have survived in Australia since at least the early Eocene [62] and late Oligocene [63], respectively, long after most lineages disappeared elsewhere in the world (e.g. [64]). A similar pattern can be observed among birds, as the globally distributed Palaelodidae, which first appear in the fossil record during the early Oligocene, survived in Australia until the mid-Pleistocene [5], and the plains-wanderer lineage, which was once more widespread but has been on the continent since at least the late Oligocene [8,11], still remains in Australia with a sole representative,Pedionomus torquatus. Anseranatids, arguably known from the early Eocene and late Oligocene of Europe [19] and nowadays represented only byA. semipalmata, survive in Australia and New Guinea, having been recorded in Australia since the late Oligocene [7]. We note that although members of Anseranatidae are known from similar-aged deposits in Australia, they differ from presbyornithids in the morphology of most skeletal elements, but especially the humerus and coracoid.

Within Presbyornithidae, the postcranial morphology ofW. tedfordi andW. prideauxiagrees with the South AmericanT. antiquus in nearly all elements (§i), the tarsometatarsus being the exception. On the other hand,P. pervetus resemblesT. antiquus in some features more than it doesW. tedfordi, but mainly in the morphology of the tarsometatarsus. The overall close similarity betweenW. tedfordi andT. antiquus may lend support to the hypothesis that at least some aspects of the morphology ofP. pervetus may be derived within Presbyornithidae (see also [19]). However, the uncertainty, at least for the time being, as to whether these similarities are derived or plesiomorphic both within Anseriformes and Presbyornithidae precludes a well-informed phylogenetic hypothesis. Assessing potential presbyornithid material from the Eocene of Australia, the discovery of cranial material of species ofWilaru, and the assessment of early-diverging anhimids (thought to be morePresbyornis-like [40]), may all contribute to clarifying the phylogenetic relationships between the different presbyornithid taxa.

Although conceiving a historical biogeographical scenario with the evidence at hand may be premature, the morphological similarity between species ofWilaru andT. antiquus emphasizes the role of Gondwana during the evolutionary history of the Presbyornithidae. This raises the possibility that members of the generaWilaru,Telmabates andPresbyornis had a common ancestry on the southern landmasses, or at least that there was one Gondwanan radiation within Presbyornithidae includingT. antiquus and species ofWilaru. Because only very few elements are known for the CretaceousT. gobiensis, assessment of its position within Presbyornithidae, and the role Laurasia played during the early evolutionary history of presbyornithids will need to await the discovery of additional material. The morphology ofP. mongoliensis and that of most elements attributed toPresbyornis sp. by Kurochkin & Dyke [20], some of which are probably attributable toP. mongoliensis, agrees well with the morphology ofP. pervetus. A close relationship between the two can be explained by the geographical proximity of North America and Mongolia during the early Cenozoic [65].

4.4. Comments on presbyornithid relationships

There are marked differences between screamers and presbyornithids in major skeletal elements that extend beyond the highly pneumatic nature of anhimid limb bones, e.g. the humerus of anhimids has an inflated shaft and a pneumatic fossa pneumotricipitalis, the coracoid has a very reduced acrocoracoid with an unusually shallow sulcus supracoracoideus, and the carpometacarpus has a unique spur-like development of the processus extensorius. Despite this, many of the postcranial features we used in this study reflect interesting similarities with the Anhimidae, despite the derived aberrant morphology of this taxon [44]. Within anseriforms, characters that are shared between Anhimidae and Presbyornithidae, and not present in other Anseriformes (i.e. Anseranatidae and Anatidae), include (1) the presence of a deep fossa in the incisura capitis of the humerus; (2) both articular rims of the trochlea carpalis of the carpometacarpus extending caudally and distally to about same level; (3) a very elongate sulcus tendineus, extending to the distal end of the scar for m. extensor carpi ulnaris; (4) a very prominent linea intermuscularis cranialis of the femur, separated from the lateral margin of the crista trochanteris (inP. pervetus this line runs closer to crista trochanteris); (5) a much smaller, mediolaterally and proximodistally, medial condyle of the tibiotarsus compared to the lateral condyle; and (6) a hypotarsus with one well-developed sulcus hypotarsi and one well-marked crista intermedia hypotarsi with the rest reduced to a near flat embossment in the lateral portion of the hypotarsus (not present inP. pervetus andT. antiquus). Ericson [40] further noted the presence of pleurocoelous thoracic vertebrae in presbyornithids and screamers, but because of the pronounced pneumaticity in the skeleton of extant anhimids whether this feature is indeed homologous for the two remains to be ascertained.

Some of the characters listed above may be plesiomorphic for Anseriformes (e.g. characters 2, 3, and 5), but whether that is true of all features or whether some could be synapomorphic for a clade including (Anhimidae + Presbyornithidae) needs to be established in a cladistic framework. A yet undescribed Eocene representative of the Anhimidae may shed some light on the subject [66]. Although not formally described, this early Eocene specimen from Wyoming was briefly assessed by Ericson [40], who noted that several elements of the postcranial skeleton closely match those of presbyornithids, including the carpometacarpus, coracoid, furcula and tibiotarsus (see also [32]).Chaunoides antiquus from the late Oligocene–early Miocene of Brazil [67] is known from several fragmentary bones, and despite it being morphologically very similar to extant anhimids, the extreme pneumatisation of the skeleton that characterizes modern anhimids is absent. There is, therefore, a strong possibility that screamers are derived from presbyornithid-like birds or that they had a common ancestor. Although living screamers have a galliform-like hooked bill, it has been proposed that the rudimentary lamellae on the bill of anhimids indicates a secondary loss of the filter-feeding ability [44,68], and therefore the skull of early-diverging anhimids may have been more ‘anseriform-like’ than that of extant anhimids [40]. The occurrence of lamellae-like structures, however, may not necessarily be linked to filter-feeding, as aquatic herbivores may use these structures for grasping and cutting plants [54].

In any case, the recognition ofW. tedfordi as a presbyornithid certainly calls for a reassessment of the phylogenetic position of Presbyornithidae within Anseriformes (see also [69–71]), which may be closer to the base of the anseriform tree than previously assumed [40,47,72]. Similarly, the phylogenetic placement of anseriform fossil taxa that have been based on cladistics analyses with limited taxon sampling and includingP. pervetus alone, such as that of the Late CretaceousVegavis iaaifrom Antarctica [72], should be revised in the light of these new findings.

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