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.[1]
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).[2]
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).[3]
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.[4]
Only a single species, Dasornis emuinus, is accepted today. However, it has a very convoluted synonymy, with its fossil remains assigned to no less than six genera (of which two were invalid junior homonyms) and divided between at least four species – excluding spelling errors and invalid "corrections" – that were variously moved between these genera for almost 150 years:[5]