Argillornis

''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).

Description

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 , and probably as much as . The complete head and bill probably measured almost in life, the eye socket had a diameter of and the humerus at its distal end was about 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''.

Systematics and taxonomy

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 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''.

"Dasornithidae"

The family Dasornithidae was established by Harrison and Walker in 1976 for ''Dasornis'' and its presumed relatives, which are however nowadays included in the former. As current scientists generally try to avoid monotypic taxa unless required by phylogeny, the Dasornithidae never were widely accepted; they are generally considered a junior synonym of the Pelagornithidae instead. And this seems to be quite correct indeed – as noted above, ''Pelagornis'', the type genus of the Pelagornithidae, probably belongs to the same pseudotooth bird lineage as ''Dasornis'' and may even be descended from it. Thus, even if several families were recognized in the Odontopterygiformes, ''Pelagornis'' and ''Dasornis'' would almost certainly remain in the Pelagornithidae.

Synonyms

The junior synonyms of the genus ''Dasornis'' are thus:
* ''Argillornis'' Owen, 1878
* "Dasornis" Owen, 1869 (''nomen nudum'')
* ''Dasyornis'' Lydekker, 1891 (''non'' Vigors & Horsfield, 1836: preoccupied)
* ''Megalornis'' Seeley, 1866 (''non'' Gray, 1841: preoccupied)
* ''Neptuniavis'' Harrison & C.A.Walker, 1977

The junior synonyms of the species ''D. emuinus'' are:
* ''Argillornis emuinus'' (Bowerbank, 1854)
* ''Argillornis longipennis'' Owen, 1878
* ''Argillornis longipes'' Lambrecht, 1933 ('' lapsus'')
* "Dasornis londinensis" Owen, 1869 (''nomen nudum'')
* ''Dasornis londinensis'' Owen, 1870
* ''Dasornis londiniensis'' Lydekker, 1891 (unjustified emendation)
* ''Lithornis emuinus'' Bowerbank, 1854
* ''Megalornis emuianus'' Seeley, 1866 (''lapsus'')
* ''Megalornis emuinus'' Lambrecht, 1921 (''lapsus'')
* ''Neptuniavis miranda'' Harrison & C.A.Walker, 1977
*''Odontopteryx'' Owen, 1873

Other fossils perhaps belonging in ''Dasornis''

'' "Pseudodontornis" longidentata'', described from a beak piece and a damaged atlas vertebra of what appears to have been a single individual, is yet another supposed pseudotooth bird species from the Early Eocene London Clay of the Isle of Sheppey. It may well be synonymous with ''D. emuinus'' too, or with '' Macrodontopteryx oweni'' if that is indeed a distinct species. This also applies to the Lutetian ( Middle Eocene) material from Etterbeek (Belgium) which was at first assigned to ''Argillornis'' (as was the holotype skull of ''M. oweni''); at least part of the supposed ''A. longipennis'' remains – though not its syntype humerus pieces – does seem to be rather small for ''D. emuinus''. Perhaps ''Gigantornis'' which is only known from pieces of a sternum found in Middle Eocene rocks in Nigeria also belongs in ''Dasornis''; the sternum of ''D. emuinus'' remains unknown, but its size would have been a close match of the Nigerian fossil. Analysis of the unidentified large pelagornithid fossils from the Middle Eocene of Kpogamé-Hahotoé ( Togo) which are provisionally termed " Aequornis traversei"Published in a thesis and hence a ''nomen nudum'': ICZN (1999) might shed light on this issue. The fairly large undescribed remains from the Late Paleocene/Early Eocene of the Ouled Abdoun Basin ( Morocco) which have been provisionally termed " Odontopteryx gigas" may in fact be from a small or juvenile ''Dasornis''. The same applies to ''M. oweni'' – nonwithstanding that it is sometimes placed in ''Odontopteryx'' – considering it was for long included in ''Argillornis''.

Also provisionally assigned to ''Argillornis'' were some pelagornithid wing bone remains, specimens LACM 128462 and presumably also LACM 127875 from the Keasey and Pittsburg Bluff Formations of the Eocene/Oligocene boundary of Oregon. Whether this Pacific species was the same as the Atlantic ''D. emuinus'' is undetermined, but considering the age difference it is not all too likely and they may well belong to different genera. In that respect, the enigmatic '' Cyphornis magnus'' from the same region is most often assigned a Miocene age, but might actually be from around the Eo-Oligocene boundary as initially assumed; it or (if of Miocene age) an ancestor, or perhaps an ancestor of the Miocene genus ''Osteodontornis'', make a more plausible candidate for the Oregon fossils. Lack of sufficient well-preserved remains have prevented more detailed study however. Similar in size and age to the present genus are some pseudotooth bird remains from Antarctica, namely a jaw piece from the Middle/ Late Eocene of the La Meseta Formation of Seymour Island near the Drake Passage, and a Middle Eocene piece of a humerus shaft from Mount Discovery on the continent's Pacific side. Separated from the North Atlantic by a wide distance and the equatorial currents, even in the case of the Seymour Island specimen it is doubtful whether they could be referred to ''Dasornis'', because the fossils are simply too fragmentary.Olson (1985: pp.196,199), Tonni & Tambussi (1985), Goedert (1989), Stilwell ''et al.'' (1998), González-Barba ''et al.'' (2002), Mayr (2009: pp.57-58)

Footnotes

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!-- This should be treated with extreme caution as regards merging of species. Splits are usually good though. See also critical review in Auk121:623-627 here http://www.findarticles.com/p/articles/mi_qa3793/is_200404/ai_n9396879 -->
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{{Taxonbar, from=Q16991804

Prehistoric bird genera
Eocene birds
Eocene birds of Europe
Fossils of England
Fossil taxa described in 1870
Taxa named by Richard Owen