Pelagornithidae

The Pelagornithidae, commonly called pelagornithids, pseudodontorns, bony-toothed birds, false-toothed birds or pseudotooth birds, are a prehistoric family of large seabirds. Their fossil remains have been found all over the world in rocks dating between the Late Paleocene and the Pliocene-Pleistocene boundary. Most of the common names refer to these birds' most notable trait: tooth-like points on their beak's edges, which, unlike true teeth, contained Volkmann's canals and were outgrowths of the premaxillary and mandibular bones. Even 'small' species of pseudotooth birds were the size of albatrosses; the largest ones had wingspans estimated at 5–6 metres (15–20 ft) and were among the largest flying birds ever to live. They were the dominant seabirds of most oceans throughout most of the Cenozoic, and modern humans apparently missed encountering them only by a tiny measure of evolutionary time: the last known pelagornithids were contemporaries of Homo habilis and the beginning of the history of technology. Apart from the giant teratorn Argentavis magnificens, the biggest of the pseudotooth birds were the largest flying birds known. Almost all of their remains from the Neogene are immense, but in the Paleogene there were a number of pelagornithids that were around the size of a great albatross (genus Diomedea) or even a bit smaller. The undescribed species provisionally called 'Odontoptila inexpectata' – from the Paleocene-Eocene boundary of Morocco – is the smallest pseudotooth bird discovered to date and was just a bit larger than a white-chinned petrel (Procellaria aequinoctialis). The Pelagornithidae had extremely thin-walled bones widely pneumatized with the air sac extensions of the lungs. Most limb bone fossils are very much crushed for that reason. In life, the thin bones and extensive pneumatization enabled the birds to achieve large size while remaining below critical wing loadings. Though 25 kg/m2 (5 lb/ft2) is regarded as the maximum wing loading for powered bird flight, there is evidence that bony-toothed birds used dynamic soaring flight almost exclusively: the proximal end of the humerus had an elongated diagonal shape that could hardly have allowed for the movement necessary for the typical flapping flight of birds; their weight thus cannot be easily estimated. The attachment positions for the muscles responsible for holding the upper arm straightly outstretched were particularly well-developed, and altogether the anatomy seems to allow for an ability of holding the wings rigidly at the glenoid joint unmatched by any other known bird. This is especially prominent in the Neogene pelagornithids, and less developed in the older Paleogene forms. The sternum had the deep and short shape typical of dynamic soarers, and bony outgrowths at the keel's forward margin securely anchored the furcula. The legs were proportionally short, the feet probably webbed and the hallux was vestigial or entirely absent; the tarsometatarsi (anklebones) resembled those of albatrosses while the arrangement of the front toes was more like in fulmars. Typical for pseudotooth birds was a second toe that attached a bit kneewards from the others and was noticeably angled outwards. The 'teeth' were probably covered by the rhamphotheca in life, and there are two furrows running along the underside of the upper bill just inside the ridges which bore the 'teeth'. Thus, when the bill was closed only the upper jaw's 'teeth' were visible, with the lower ones hidden behind them. Inside the eye sockets of at least some pseudotooth birds – perhaps only in the younger species – were well-developed salt glands. Altogether, almost no major body part of pelagornithids is known from a well-preserved associated fossil and most well-preserved material consists of single bones only; on the other hand the long occurrence and large size makes for a few rather comprehensive (though much crushed and distorted) remains of individual birds that were entombed by as they lay dead, complete with some fossilized feathers. Large parts of the skull and some beak pieces are found not too infrequently. In February 2009, an almost-complete fossilized skull of a presumed Odontopteryx from around the Chasicoan-Huayquerian boundary c. 9 million years ago (Ma) was unveiled in Lima. It had been found a few months earlier in Ocucaje District of Ica Province, Peru. According to paleontologist Mario Urbina, who discovered the specimen, and his colleagues Rodolfo Salas, Ken Campbell and Daniel T. Ksepka, the Ocucaje skull is the best-preserved pelagornithid cranium known as of 2009. Unlike the true teeth of Mesozoic birds like Archaeopteryx or Aberratiodontus, the pseudoteeth of the Pelagornithidae do not seem to have had serrated or otherwise specialized cutting edges, and were useful to hold prey for swallowing whole rather than to tear bits off it. Since the teeth were hollow or at best full of cancellous bone and are easily worn or broken off in fossils, it is surmised they were not extremely resilient in life either. Pelagornithid prey would thus have been soft-bodied, and have encompassed mainly cephalopods and soft-skinned fishes. Prey items may have reached considerable size. Though some reconstructions show pelagornithids as diving birds in the manner of gannets, the thin-walled highly pneumatized bones which must have fractured easily judging from the state of fossil specimens make such a mode of feeding unlikely, if not outright dangerous. Rather, prey would have been picked up from immediately below the ocean surface while the birds were flying or swimming, and they probably submerged only the beak in most situations. Their quadrate bone articulation with the lower jaw resembled that of a pelican or other birds that can open their beak widely. Altogether, the pseudotooth birds would have filled an ecological niche almost identical to that of the larger fish-eating pteranodontian pterosaurs, whose extinction at the end of the Cretaceous may well have paved the way for the highly successful 50-million-year reign of the Pelagornithidae. Like them as well as modern albatrosses, the pseudotooth birds could have used the system of ocean currents and atmospheric circulation to take round-track routes soaring over the open oceans, returning to breed only every few years. Unlike albatrosses today, which avoid the tropical equatorial currents with their doldrums, Pelagornithidae were found in all sorts of climate, and records from around 40 Ma stretch from Belgium through Togo to the Antarctic. It is conspicuous that penguins and plotopterids – both wing-propelled divers that foraged over the continental shelf – are almost invariably found in the company of pseudotooth birds. Thus, pseudotooth birds seem to have gathered in some numbers in upwelling regions, presumably to feed but perhaps to breed nearby also (see below). It is sometimes claimed that as with some other seabirds (e.g. the flightless Plotopteridae), the evolutionary radiation of cetaceans and pinnipeds outcompeted the pseudotooth birds and drove them into extinction. While this may be correct for the plotopterids, for pelagornithids it is not all too likely for two reasons: First, the Pelagornithidae continued to thrive for 10 million years after modern-type baleen whales evolved, and in the Middle Miocene Pelagornis coexisted with Aglaocetus and Harrison's whale (Eobalaenoptera harrisoni) in the Atlantic off the Eastern Seaboard, while the Pacific Osteodontornis inhabited the same seas as Balaenula and Morenocetus; the ancestral smallish sperm whale genus Aulophyseter (and/or Orycterocetus) occurred in both Northern Hemisphere oceans at that time, while the mid-sized sperm whale Brygmophyseter roamed the North Pacific. As regards Miocene pinnipeds, a diversity of ancient walruses and ancestral fur seals like Thalassoleon inhabited the north-east, while the ancient leopard seal Acrophoca is a remarkable species known from the south-east Pacific. Secondly, pinnipeds are limited to near-shore waters while pseudotooth birds roamed the seas far and wide, like large cetaceans, and like all big carnivores all three groups were K-strategists with moderate to very low population densities.