Dasornis

Dasornis is a genus of prehistoric pseudotooth birds. These were probably close relatives of either pelicans and storks or waterfowl; they are 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.

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:

1854-1890: "Lithornis" emuinus, "Megalornis" of Seeley, Dasornis and Argillornis
The first fossil of D. emuinus, a piece of right humerus shaft, was found in the Ypresian (Early Eocene) London Clay of the Isle of Sheppey (England). It was misidentified as a tibiotarsus of the paleognath Lithornis and described as L. emuinus by James Scott Bowerbank in 1854. Harry Govier Seeley recognized this error in 1866 and established the genus Megalornis, though he misspelled the specific name as emuianus. However, the genus name he chose had already been used for some of the great herons (Ardea). Richard Owen established the genera Dasornis (in 1870) and Argillornis (in 1878) for, respectively, a broken skull and two humerus ends that were found in the same deposits. Some authors^[who?] claim he had already erected the former genus in 1869, but in that year he only used the names informally in his brief initial report on the newly discovered skull. Misled by the skull's large size and perhaps overly eager to be the first to describe the remains of a "European moa" (Owen was the foremost authority on these New Zealand endemics at that time), he placed Dasornis in the Dinornithidae. Argillornis, on the other hand, was recognized early on as some sort of aquatic bird, but its immense size puzzled paleontologists to no little extent.

1891-1985: spelling errors, "Neptuniavis" and "completely unrealistic" taxonomy
Subsequent authors, noting that it was quite obviously not a paleognath ratite, placed Dasornis in the Gastornithidae. Richard Lydekker in 1891 proposed to rename Owen's D. londinensis to D. londiniensis, and later that year wanted to change Dasornis to Dasyornis. But the altered specific name was not in accordance with the rules of zoological nomenclature, and neither was the genus name he chose – and which, moreover, had already been used earlier by Nicholas Aylward Vigors and Thomas Horsfield for the bristlebirds. In 1921, Kálmán Lambrecht "corrected" Seeley's Megalornis emuianus to emuinus and in 1933 he misspelled Owen's A. longipennis as longipes. Pierce Brodkorb resolved the Megalornis homonymy in 1963 by merging "M." emuinus with A. longipennis, combining the older specific name emuinus and the then-valid genus name Argillornis. However, he rather inexplicably allied Argillornis with the enigmatic Mesozoic Elopteryx nopcsai – a sort of "wastebin taxon" for Late Cretaceous maniraptoran theropod remains from Romania that might not even be of birds – and the mid-late Eocene Eostega (probably a primitive gannet). In 1976, Colin James Oliver Harrison and Cyril Alexander Walker finally determined all those remains to be of pseudotooth birds. They also proposed that part of the supposed A. longipennis remains was actually from a distinct and slightly smaller genus and species, which they described in a monotypic genus as Macrodontopteryx oweni. In 1977, the same authors erected the genus Neptuniavis for supposed procellariiform tarsometatarsi also found on the Isle of Sheppey; they included two species there. Already however, eminent avian paleontologists such as Storrs L. Olson were voicing their reservation about this proliferation of taxa in no uncertain terms.

2008: just Dasornis emuinus after all
Almost 150 years after the description of "L." emuinus, at the start of the 21st century, a rather well-preserved skull (lacking the beak) was discovered, once again in the Isle of Sheppey London Clay. This specimen – SMNK-PAL 4017 – was studied by Gerald Mayr at the Senckenberg Museum. He determined that all the large seabird bones from the London Clay bones belonged to a single species of pelagornithid. To this, the scientific name Dasornis emuinus applies, a novel combination of the oldest valid genus and species names ever used for these fossils. Indeed, the importance of this specimen can hardly be underestimated, for the holotype skull of Dasornis "londinensis" (which was used to establish the genus Dasornis) is so badly preserved that its status as a pseudotooth bird was debated as recently as 1985. Only the fossils named "Neptuniavis" minor were not of D. emuinus, but of the much smaller contemporary and sympatric pseudotooth bird Odontopteryx toliapica.