Osteodontornis - Distribution

Distribution

This species is well documented from various locations of generally Miocene age, although usually by much fragmented remains due to the thin and tender bones it had. Most importantly, it was found on both sides of the North Pacific. It is not certain whether all Osteodontornis remains belong to a single species; size differences suggest that some evolution took place during the timespan in which the genus existed. Thus, some fossils are referred to Osteodontornis only, without further assigning them to this species.

The type specimen of O. orri, SBMNH 309, is a rather comprehensive fossil preserved mostly as imprint, with some bone pieces and even feather impressions in addition; it was found in Clarendonian (Late Miocene) shale of California (USA). Subsequently, for example in the Barstovian (Middle Miocene) Round Mountain Silt or in Late Miocene deposits of the Monterey Formation, quite a few additional specimens dating from about the same time were found in California. Roughly contemporary specimens were described from the Haranoyan-Tozawan boundary in Japan – a complete right quadrate bone (NSM PV-18696) from the Middle Miocene Nagura Formation at Chichibu, Saitama, an Early Miocene right mandible piece (MFM 28351) found in the Oi Formation at Misato, Mie, and some additional material of about the same age from the Mizunami Group at Mizunami, Gifu. From the Early Miocene Nye Formation and the Middle Miocene Astoria Formations of Oregon a handful of specimens that appear to be Osteodontornis are known. Similar fossils have been found in the Middle Miocene Capadare Formation of Venezuela and from the Late Miocene of the Pisco Formation of Peru; they might rather be of a distinct but closely related genus, and it must be remembered that at that their time the Isthmus of Panama had not been formed yet so that an affiliation with the Atlantic Pelagornis cannot be discounted. The former, specimen MBLUZ-P-5093 from Cueva del Zumbador in Falcón State, is a premaxilla tip of immense dimensions; its bearer might have exceeded a wingspan of 7 metres (23 ft) in life. Though some of the Miocene North American material was initially (and sometimes is still) assigned to Pelagornis, recent authors generally place them in the present genus.

Some wing bone fossils from the Eo-Oligocene boundary of Oregon (USA), though assigned to Argillornis (= Dasornis), do not differ much from those of Osteodontornis (as far as can be told in their fragmented state), and may be from an older relative. They are the oldest known remains of large North Pacific pseudotooth birds, but if the enigmatic Cyphornis magnus from the same region dates back to the Paleogene they may well be assignable to that taxon, whatever their systematic affiliations might be beyond that. As few directly comparable bones of sufficient quality exist, the relationship of Osteodontornis to other pseudotooth birds is not completely resolved. As noted above, the entire genus (regardless of how many species can be recognized) may be a junior synonym of Cyphornis. Generally, recent authors have tended to place large Neogene pseudotooth bird fossils from the Atlantic in Pelagornis, and those from the North Pacific in Osteodontornis. It remains to be seen if this east-west division can be upheld, but biogeographically it seems quite sensible at least as a working hypothesis until sufficient well-preserved material has been found to make an in-depth study. It is less clear what to make of the Southern Hemisphere pseudotooth birds fossils, none of which are complete enough for more than the most tentative identification. Many seabirds of our time, such as albatrosses and other Procellariiformes, show a phylogenetic division between Northern and Southern Hemisphere lineages, separated by the Equatorial currents. Whether this also held true in the warmer climate of the Miocene is not known, but the general phylogenetic patterns found in Procellariiformes suggests that the north-south division is rather ancient and evolved even before the Miocene.

From the Neogene of New Zealand "Pseudodontornis" stirtoni has been described, which unlike the rest of its (doubtfully valid) genus is not from the Paleogene Atlantic region. It has been proposed as a monotypic genus Neodontornis, but this has not been widely accepted. It may be valid still, as the bones are of a rather small pseudotooth bird; though apparently too small for Osteodontornis detailed comparisons could be insightful. Its jugal arch is indeed short and very stout behind the orbital process of the prefrontal bone, like in Osteodontornis but apparently unlike in the type species of its supposed genus, P. longirostris. A larger proximal (initially misidentified as distal) humerus piece (CMNZ AV 24,960), probably from the Waiauan (Middle/Late Miocene) and found near the Waipara River mouth, is little if any distinct from O. orri in shape and size; it has a flange at the side and is less straight, but whether these features are natural or due to the damaged state of the specimens is unclear. It also agrees more with Pelagornis than with Paleogene remains from Oregon mentioned above. A distal left humerus end and some wing bone fragments from the Late Oligocene Yamaga Formation of Kitakyūshū (Japan) might be the oldest remains of an Osteodontornis, but their assignment to the present genus is just as uncertain as in the case of the New Zealand fossil. Also from Japan are one or two of the youngest pseudodontorn fossils – a fragmentary right humerus from the Early Pliocene Yushima Formation at Maesawa, and probably also a distal right femur (MFM 1801) from the Early Pleistocene Dainichi Formation at Kakegawa that was initially believed to be from an albatross. These might represent the last survivors of Osteodontornis – the Kakegawa fossil at least is a good match in size –, but require more study before they can be assigned there.