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18.12.2015 18:53 - Largest prehistoric animals Vol.1 Vertebrates part2 Birds Ch.3 Bony-toothed birds-giant wings over seas and oceans
Автор: valentint Категория: Забавление   
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Последна промяна: 03.12.2016 02:22


Bony-toothed birds Pelagornithidae (Odontopterygiformes)

The largest in this group – which has been variously allied with Procellariiformes, Pelecaniformes and Anseriformes – and the largest flying birds of all time other than Argentavis were the huge Cyphornis, Dasornis, Gigantornis and Osteodontornis. They had a wingspan of 5.5–6 m (18–20 ft) and stood about 1.2 meters (3.9 feet) tall. Exact size estimates and judging which one was largest are not yet possible for these birds, as their bones were extremely thin-walled, light and fragile, and thus most are only known from very incomplete remains.


Osteodontornis

Osteodontornis orri (Orr's Bony-toothed Bird, in literal translation of its scientific name), which was described quite exactly one century after the first species of the Pelagornithidae (Pelagornis miocaenus) was. O. orri was named after then-recently deceased naturalist Ellison Orr.

The bony-toothed or pseudotooth birds were initially believed to be related to albatrosses in the Procellariiformes, but actually they seem to be rather close relatives of either pelicans and storks, or of waterfowl, and are here placed in the order Odontopterygiformes to account for this uncertainty. Also, their internal taxonomy is not well-resolved. An earlier-described pseudotooth bird, Cyphornis magnus from Vancouver Island (Canada), was believed to be of Eocene age but is nowadays assumed to have lived in the Early Miocene, not too long before the Clarendonian (Middle/Late Miocene) O. orri. It may be that Osteodontornis is a junior synonym of Cyphornis.

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 were the second-largest flying birds known, surpassed only by the teratorn Argentavis magnificens. The head, from neck to bill-tip, measured about 40 centimetres (1.3 ft), and the eyesockets were about 5.3 centimetres (2.1 in) wide. The humerus, though about as long as a human's, was only about 3.5 centimetres (1.4 in) wide at the shoulder end. The skull's quadrate bone measured almost 30 millimetres (1.2 in) at its widest and was nearly 45 millimetres (1.8 in) high.

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 narrow wings resembling those of an albatross. Its beak made up about three-quarters of the head's length and had bony tooth-like serrations that were hollow or maybe filled with cancellous bone. The beak was so heavy the creature probably held it between its shoulders while in flight, just like modern pelicans do. The arrangement of its bony 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 distinguish genera.

In general lifestyle, it was probably most similar to the albatrosses, tropicbirds and frigatebirds of today, with long slender wings adapted for soaring vast distances over the open seas. Due to its size, the bird is presumed to have been an excellently adapted dynamic soarer. It probably built its nest on high plateaus 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 off squid and other soft-bodied prey; the "teeth" were less saw-like than the horny 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-skinned pelagic animals such as cephalopods 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.

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, the distal humerus of the present genus (e.g. the Barstovian specimen LACM 50660 from Kern County, California) can be compared to that of a smaller and older fossil tentatively assigned to Odontopteryx. Osteodontornis has a wider and deeper notch between the external condyle and the ectepicondylar prominence, with the pit between these farther from the bone's end, than did the smallish Paleogene species. Its quadrate bone differed from that of Odontopteryx toliapica in a more narrowly grooved dorsal head, and a larger and less forward-pointing orbital process. The forward center of the ventral articulation ridge extends upwards and forward, and the pterygoid process is conspicuously expanded to the upper center in Osteodontornis. The socket for the quadratojugal has an intermediate position and the lateral ridge of the slender main shaft is straight and fairly thin. The quadrate of the mysterious Pseudodontornis longirostris skull (which some consider to belong in Pelagornis) is not very well preserved; it agees with Odontopteryx in a broad main shaft and with Osteodontornis in the straight main shaft ridge and its upward-directed ventral articulation ridge's forward center. Otherwise, it differs from both.



Cyphornis

Cyphornis is a genus of the prehistoric pseudotooth birds. These were probably rather close relatives of either pelicans and storks, or of waterfowl, and are here placed in the order Odontopterygiformes to account for this uncertainty.

Only a single species, Cyphornis magnus, is known to date. It is only known with certainty from a single specimen, the rather abraded proximal part of a left tarsometatarsus which was found at Carmanah Point on Vancouver Island (Canada), where the Juan de Fuca Strait opens into the Pacific. The deposits from which it originated were initially dated to the Eocene; subsequent authors have usually assigned them to the Early Miocene though certainly rocks from around the Eo-Oligocene boundary also occur in the region where it was found. At the time of its discovery, it "probably represent[ed] the largest known bird of flight." Even today it is one of the largest (though not heaviest) flying birds known.

Some huge pseudotooth wing bone fossils have been found in Oregon. Specimen LACM 128462, a mostly complete proximal end of a left ulna, originates from the Keasey Formation of Washington County. LACM 127875 are fragments of the proximal humerus ends, the proximal right ulna and the right radius of a single individual presumed to be of the same species; they were found in the Pittsburg Bluff Formation near Mist. These remains all date from the Eo-Oligocene boundary, and considering their size they may well be of C. magnus if it is in fact that old, or of its ancestor or older relative.


Gigantornis

Gigantornis eaglesomei was a giant prehistoric bird, described from a fragmentary specimen from the Eocene of Nigeria. It was considered to be a representative of the albatross family (Diomedeidae), but was later referred to the bony-toothed birds, (Pelagornithidae). One of the largest pseudotooth birds, with an inferred wingspan of about 6 m (20 ft) it is among the largest birds ever.

Its identified remains consist of a broken sternum found in Middle Eocene Ameki Formation deposits at Ameki (Nigeria). The fossil bird was considered to belong the albatross family (Diomedeidae), as no sterna of pseudotooth birds were known until its discovery, and it remained the only such specimen for decades. Only in the 1970s its true affinities were realized, after it had become clear that although it must have been from a dynamic soarer with wings like an albatross, it resembled pelicans (order Ciconiiformes) rather than tubenoses (order Procellariiformes, to which albatrosses belong) in its details.

It is not known whether this bird belongs to a distinct genus; it might even be the very same species as the similar-sized Dasornis emuinus, whose fossils are not uncommon in the Ypresian (Early Eocene) London Clay of the Isle of Sheppey (England). Remains of a large pseudotooth bird were also found in the Middle Eocene of Kpogamé-Hahotoé (Togo) and have been provisionally termed "Aequornis traversei"; their analysis is likely to provide at least some insight on the taxonomic status of G. eaglesomei.

The Gigantornis sternum is of the typical short and deep-crested shape found in dynamic soarers. Compared to LHNB (CCCP)-1, a Middle to Late Miocene pseudotooth bird sternum found in Portugal and tentatively assigned to Pelagornis, its articular facet for the furcula consists of a flat section at the very tip of the sternal keel and a similar one set immediately above it at an outward angle, and the spina externa is shaped like an Old French shield in cross-section. The slightly smaller LHNB (CCCP)-1 has a less sharply protruding sternal keel, the articular facet for the furcula consists of a large knob at the forward margin, and the spina externa is narrow in cross-section.

Dasornis

Dasornis is a genus of the prehistoric pseudotooth birds. These were probably rather close relatives of either pelicans and storks, or of waterfowl, and are here placed in the order Odontopterygiformes to account for this uncertainty.

Almost all known material of this bird is from some 50 million years ago (Ma) and has been recovered from the Ypresian (Early Eocene) London Clay of the Isle of Sheppey (England). The exception are a few approximately 45 Ma-old remains from the Lutetian (Middle Eocene, MP11-13) of Etterbeek (Belgium) that are only tentatively included here, and some even more conjectural remains from outside Europe (see below).

Like those of its relatives, the thin-walled bones of Dasornis broke easily and thus very few fossils – though still far more than of the average pseudotooth bird genus – are in decent condition. Among these is a superbly preserved partial skull that has been of crucial importance in sorting out the convoluted synonymy of this genus. Apart from that and another not quite as well-preserved partial skull, however, a number of beak and cranium pieces as well as a few broken remains of wing and tarsometatarsus bones make up the known remains of Dasornis. The most tell-tale characteristic of the present genus are the combination of Paleogene age and huge size. But given the fragmented state of these, it is not at all clear whether the genus was restricted to the North Atlantic (and perhaps the adjacent Paratethys) or occurred also in the Pacific and in the Southern Hemisphere, where fossils of a similar size were found (see below).

This genus belongs to the group of huge pseudotooth birds, with wingspans in excess of 5 m (16 ft), and probably as much as 6 m (20 ft). The complete head and bill probably measured almost 45 cm (1.48 ft) in life, the eye socket had a diameter of 55 millimetres (2.2 in) and the humerus at its distal end was about 35 millimetres (1.4 in) wide. The well-preserved skull fossil shows deep grooves along the underside of the upper bill, with pits to accommodate the lower bill's "teeth". Thus, only the upper "teeth" were visible when the bird closed its bill. Dasornis resembles the much smaller Odontopteryx in having a jugal arch that is mid-sized, tapering and stout behind the orbital process of the prefrontal bone, unlike in the large Neogene Osteodontornis. Also, its paroccipital process is much elongated back- and downwards, again like in Odontopteryx but unlike in Pseudodontornis longirostris. Further traits in which Dasornis agreed with Odontopteryx – and differed from Pelagornis (a contemporary of Osteodontornis) are a deep and long handward-pointing pneumatic foramen in the fossa pneumotricipitalis of the humerus, a latissimus dorsi muscle attachment site on the humerus that consists of two distinct segments instead of a single long, and a large knob that extends along the ulna where the ligamentum collaterale ventrale attached. As the traits as found in Odontopteryx and Dasornis are probably plesiomorphic, they cannot be used to argue for a closer relationship between the two Paleogene genera than either had with Osteodontornis and/or Pelagornis.

















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Автор: valentint
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