[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).

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).

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.

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.

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] 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).

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.

[7] 1891-1985: spelling errors, "Neptuniavis" and "completely unrealistic"[8] taxonomy Subsequent authors, noting that it was quite obviously not a paleognath ratite, placed Dasornis in the Gastornithidae.

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).

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[14] – though not its syntype humerus pieces – does seem to be rather small for D. emuinus.

Analysis of the unidentified large pelagornithid fossils from the Middle Eocene of Kpogamé-Hahotoé (Togo) which are provisionally termed "Aequornis traversei"[15] 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"[16] may in fact be from a small or juvenile Dasornis.

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.