Abstract
The giant flightless bird Sylviornis neocaledoniae (Aves: Sylviornithidae) existed on La Grande Terre and Ile des Pins, New Caledonia, until the late Holocene when it went extinct shortly after human arrival on these islands. The species was generally considered to be a megapode (Megapodiidae) until the family Sylviornithidae was erected for it in 2005 to reflect multiple cranial autapomorphies. However, despite thousands of bones having been reported for this unique and enigmatic taxon, the postcranial anatomy has remained largely unknown. We rectify this deficiency and describe the postcranial skeleton of S. neocaledoniae based on ~600 fossils and use data from this and its cranial anatomy to make a comprehensive assessment of its phylogenetic affinities. Sylviornis neocaledoniae is found to be a stem galliform, distant from megapodiids, and the sister taxon to the extinct flightless Megavitiornis altirostris from Fiji, which we transfer to the family Sylviornithidae. These two species form the sister group to extant crown-group galliforms. Several other fossil galloanseres also included in the phylogenetic analysis reveal novel hypotheses of their relationships as follows: Dromornis planei (Dromornithidae) is recovered as a stem galliform rather than a stem anseriform; Presbyornis pervetus (Presbyornithidae) is the sister group to Anseranatidae, not to Anatidae; Vegavis iaai is a crown anseriform but remains unresolved relative to Presbyornis pervetus, Anseranatidae and Anatidae. Sylviornis neocaledoniae was reconstructed herein to be 0.8 m tall in a resting stance and weigh 27–34 kg. The postcranial anatomy of S. neocaledoniae shows no indication of the specialised adaptation to digging seen in megapodiids, with for example, its ungual morphology differing little from that of chicken Gallus gallus. These observations and its phylogenetic placement as stem galliforms makes it improbable that this species employed ectothermic incubation or was a mound-builder. Sylviornis neocaledoniae can therefore be excluded as the constructor of tumuli in New Caledonia.
Highlights
Avian evolution on islands has resulted in the evolution of large flightless forms in multiple groups [1, 2], such as the nine species of ratite moa (Dinornithiformes) of New Zealand [3], giant waterfowl (Anatidae) including moa-nalos in Hawaii [4] and geese in New Zealand [3], pigeons (Columbidae) in the Mascarenes [5, 6] and Fiji [7], and enigmatic gruiforms (Aptornithidae) in New Zealand (e.g., [8, 9])
Sylviornis neocaledoniae is found to be a stem galliform, distant from megapodiids, and the sister taxon to the extinct flightless Megavitiornis altirostris from Fiji, which we transfer to the family Sylviornithidae
The broad confidence interval for tmtL/FemL on loadings plot probably relates to the fact that much of the separation relates to the single point for Sylviornis neocaledoniae relative to the remaining individuals
Summary
Avian evolution on islands has resulted in the evolution of large flightless forms in multiple groups [1, 2], such as the nine species of ratite moa (Dinornithiformes) of New Zealand [3], giant waterfowl (Anatidae) including moa-nalos in Hawaii [4] and geese in New Zealand [3], pigeons (Columbidae) in the Mascarenes [5, 6] and Fiji [7], and enigmatic gruiforms (Aptornithidae) in New Zealand (e.g., [8, 9]). They considered the shared features with megapodes to be symplesiomorphic and on the basis of its many autapomorphies, they established the monotypic Sylviornithidae for S. neocaledoniae Notable features these authors highlighted included: the broad flattened cranium; a massive, dorsoventrally deep, laterally compressed rostrum with a large bony ornament; mandible with an elongated symphysis; and a zona flexoria craniofacialis that forms a synovial joint, hereafter termed a craniofacial hinge, that transects the nasals early in ontogeny. Such autapomorphies might not preclude this taxon from being embedded within an established family as, for example, some ratites (e.g., Casuarius sp.) and galliforms (e.g. Numididae) have bony ornament on their skull, and some galliforms have such on their rostrum (e.g. some, but not all, cracids). These observations were augmented by Balouet with a simple skeletal reconstruction and some sketchy details, including that the pelvis had developed transverse processes and a large ilioischiadic foramen, the ribs lacked uncinate processes, the clavicles were unfused so there was no furcula, the clavicle and coracoid were fused, and that there were a large number of synsacral and caudal vertebrae [14]
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