| Osteodontornis | |
|---|---|
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| Artist's impression | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Aves |
| Order: | †Odontopterygiformes |
| Family: | †Pelagornithidae |
| Genus: | †Osteodontornis Howard, 1957 |
| Species: | †O. orri |
| Binomial name | |
| †Osteodontornis orri Howard, 1957 | |
Osteodontornis is anextinctseabirdgenus. It contains a single namedspecies,Osteodontornis orri (Orr's bony-toothed bird, in literal translation of itsscientific name), which was described quite exactly one century after the first species of thePelagornithidae (Pelagornis miocaenus) was.O. orri was named after Santa Barbara Museum of Natural History paleontologist Phil C. Orr, for his recognition of the importance of the specimen.[1]
The bony-toothed orpseudotooth birds were initially believed to be related toalbatrosses in theProcellariiformes, but actually they seem to be rather close relatives of eitherpelicans andstorks, or ofwaterfowl, and are here placed in theorder Odontopterygiformes to account for this uncertainty. Also, their internaltaxonomy is not well-resolved. An earlier-described pseudotooth bird,Cyphornis magnus fromVancouver Island (Canada), was believed to be ofEocene age but is nowadays assumed to have lived about twenty million years ago in theEarly Miocene, not too long before theClarendonian (Middle/Late Miocene)O. orri. It may be thatOsteodontornis is ajunior synonym ofCyphornis.[2]
With a wingspan of 5.5 to 6 metres (18 to 20 ft) and a height of 1.2 metres (3.9 ft) when on the ground,Osteodontornis orri and similar giant pseudotooth birds[3] were the second-largest flying birds known, surpassed only by theteratornArgentavis magnificens. The head, from neck to bill-tip, measured about 40 centimetres (1.3 ft), and theeyesockets were about 5.3 centimetres (2.1 in) wide. Thehumerus, though about as long as a human's, was only about 3.5 centimetres (1.4 in) wide at the shoulder end. The skull'squadrate bone measured almost 30 millimetres (1.2 in) at its widest and was nearly 45 millimetres (1.8 in) high.[4]
Like its relatives,O. orri had a stout but extremely light-boned body, feet that were presumably webbed as in its aquatic relatives, and long and probably very narrowwings resembling those of analbatross. Itsbeak made up about three-quarters of the head's length and had bonytooth-like serrations that were hollow or maybe filled withcancellous bone. The beak was so heavy the creature probably held it between its shoulders while in flight, just like modernpelicans do. The arrangement of itsbony serrations is characteristic for this genus: one small "tooth", flanked by small points or even smaller "teeth", is placed between each pair of large ones. However, the "tooth" pattern of pseudotooth birds changed along the length of the beak, and is not considered a very reliable way to distinguishgenera.[5]
In general lifestyle, it was probably most similar to thealbatrosses,tropicbirds andfrigatebirds of today, with long slender wings adapted forsoaring vast distances over the open seas. Due to its size, the bird is presumed to have been an excellentlyadapteddynamic soarer. It probably built itsnest on highplateaus or similar places, where it could easily take flight by simply walking into the wind with wings spread. It was a seabird that apparently lived mainly offsquid and other soft-bodied prey; the "teeth" were less saw-like than thehorny serrations on the beak of the fish-eating saw-billed ducks (Merginae), pointing straight downwards instead and in the fossils often very abraded or broken. The downward-pointing "teeth" were ideal for digging into and holding slippery, soft-skinnedpelagic animals such ascephalopods that were probably snatched out of the water in flight or while swimming. Lightly built as it was,O. orri was probably not a good diver and may have found it impossible to dive at all.[6]
Osteodontornis is one of the pseudotooth birds of which rather comprehensive remains are known, but the lack of good fossils of most other Odontopterygiformes allows for few direct comparisons between genera. Still, thedistalhumerus of the present genus (e.g. theBarstovian specimenLACM 50660 fromKern County, California) can be compared to that of a smaller and older fossil[7] tentatively assigned toOdontopteryx.Osteodontornis has a wider and deeper notch between the externalcondyle and the ectepicondylar prominence, with the pit between these farther from the bone's end, than did the smallishPaleogene species. Itsquadrate bone differed from that ofOdontopteryx toliapica in a more narrowly grooveddorsal head, and a larger and less forward-pointingorbital process. The forward center of theventral articulation ridge extends upwards and forward, and thepterygoid process is conspicuously expanded to the upper center inOsteodontornis. The socket for thequadratojugal has an intermediate position and thelateral ridge of the slender main shaft is straight and fairly thin. The quadrate of the mysteriousPseudodontornis longirostris skull (which some consider to belong inPelagornis) is not very well preserved; it agees withOdontopteryx in a broad main shaft and withOsteodontornis in the straight main shaft ridge and its upward-directed ventral articulation ridge's forward center. Otherwise, it differs from both.[8]
Thisspecies is well documented from various locations of generallyMiocene 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 theNorth Pacific. It is not certain whether allOsteodontornis remains belong to a single species; size differences suggest that someevolution took place during the timespan in which thegenus existed. Thus, some fossils are referred toOsteodontornis only, without further assigning them to this species.[9]
Thetype specimen ofO. 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 inClarendonian (Late Miocene)shale ofCalifornia (USA). Subsequently, for example in theBarstovian (Middle Miocene)Round Mountain Silt[10] or in Late Miocene deposits of theMonterey Formation,[11] quite a few additional specimens[12] dating from about the same time were found in California. Roughly contemporary specimens were described from the Haranoyan-Tozawan boundary in Japan – a complete rightquadrate bone (NSM PV-18696) from the Middle MioceneNagura Formation atChichibu, Saitama, anEarly Miocene rightmandible piece (Mizunami Fossil Museum (MFM) 28351) found in theOi Formation atMisato, Mie, and some additional material of about the same age from the Mizunami Group atMizunami, Gifu. From the Early MioceneNye Formation and theMiddle MioceneAstoria Formations ofOregon a handful of specimens[13] that appear to beOsteodontornis[14] are known. Similar fossils have been found in the Middle MioceneCapadare Formation ofVenezuela and from the Late Miocene of thePisco Formation[15] ofPeru; they might rather be of a distinct but closely related genus, and it must be remembered that at that their time theIsthmus of Panama had not been formed yet so that an affiliation with the AtlanticPelagornis cannot be discounted. The former, specimenMBLUZ-P-5093 from Cueva del Zumbador inFalcón State, is apremaxilla 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 toPelagornis, recent authors generally place them in the present genus. [16]
Some wing bone fossils[17] from theEo-Oligocene boundary ofOregon (United States), though assigned toArgillornis (=Dasornis), do not differ much from those ofOsteodontornis (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 enigmaticCyphornis magnus from the same region dates back to thePaleogene they may well be assignable to thattaxon, whatever theirsystematic affiliations might be beyond that. As few directly comparable bones of sufficient quality exist, the relationship ofOsteodontornis to other pseudotooth birds is not completely resolved. As noted above, the entire genus (regardless of how many species can be recognized) may be ajunior synonym ofCyphornis. Generally, recent authors have tended to place largeNeogene pseudotooth bird fossils from theAtlantic inPelagornis, and those from the North Pacific inOsteodontornis. It remains to be seen if this east–west division can be upheld, butbiogeographically it seems quite sensible at least as aworking hypothesis until sufficient well-preserved material has been found to make an in-depth study. It is less clear what to make of theSouthern Hemisphere pseudotooth birds fossils, none of which are complete enough for more than the most tentative identification. Manyseabirds of our time, such asalbatrosses and otherProcellariiformes, show aphylogenetic division between Northern and Southern Hemisphere lineages, separated by theEquatorial currents. Whether this also held true in thewarmer climate of theMiocene is not known, but the general phylogenetic patterns found in Procellariiformes suggests that the north–south division is rather ancient andevolved even before the Miocene.[18]
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 amonotypic genusNeodontornis, 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 forOsteodontornis detailed comparisons could be insightful. Itsjugal arch is indeed short and very stout behind theorbital process of theprefrontal bone, like inOsteodontornis but apparently unlike in thetype species of its supposed genus,P. longirostris. A largerproximal (initially misidentified asdistal) humerus piece (CMNZ AV 24,960), probably from theWaiauan (Middle/Late Miocene) and found near theWaipara River mouth, is little if any distinct fromO. 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 withPelagornis than with Paleogene remains from Oregon mentioned above. A distal left humerus end and some wing bone fragments from theLate OligoceneYamaga Formation ofKitakyūshū (Japan) might be the oldest remains of anOsteodontornis, 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 theEarly PlioceneYushima Formation atMaesawa, and probably also a distal rightfemur (MFM 1801) from theEarly Pleistocene[19]Dainichi Formation atKakegawa that was initially believed to be from an albatross. These might represent the last survivors ofOsteodontornis – the Kakegawa fossil at least is a good match in size –, but require more study before they can be assigned there.[20]
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