| Palaeochenoides Temporal range:Late Oligocene (see text) | |
|---|---|
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Aves |
| Order: | †Odontopterygiformes |
| Family: | †Pelagornithidae |
| Genus: | †Palaeochenoides Shufeldt, 1916 |
| Species: | †P. mioceanus |
| Binomial name | |
| †Palaeochenoides mioceanus Shufeldt, 1916 | |
| Synonyms | |
Palaeochenoides miocaenus(lapsus) | |
Palaeochenoides is agenus of theprehistoricpseudotooth birds of somewhat doubtful validity. These were probably rather close relatives of eitherpelicans andstorks, or ofwaterfowl, and are here placed in theorder Odontopterygiformes to account for this uncertainty.[1]
Only a singlespecies,Palaeochenoides mioceanus, is known to date. The firstfossil assigned to it – adistal rightfemur piece – was found near the source of theStono River inCharleston County, South Carolina (United States). At first it was believed to be from theEarly MioceneHawthorne Formation – itsspecific name, seemingly referring to the "Mioceneocean" as presumedhabitat but actually a simple spelling error for "miocaenus", "from the Miocene", that was never corrected and hence became valid – alludes to this. But in fact no Hawthorne Formation rocks were known in the Charleston region when the fossil was found, and consequently modern authors consider aChattian (Late Oligocene) age more likely and suggest the fossil came from theCooper orChandler Bridge Formation. SpecimenMCZ 2514, a distal lefttarsometatarsus fragment from theAshley River, was more tentatively assigned toP. mioceanus later on; it was also erroneously believed to be from the Hawthorne Formation.[2]
Theholotype femur's classification mirrors the recently renewed uncertainties about the pseudotooth birds' placement. At the time of its description, when it was still much surrounded bymatrix, it was believed to be from a giantgoose orswan. This is referred to in thegenus name, which means "ancient goose-like [bird]". But only one year later, the bone had been prepared from the matrix and was submitted to an improvisedphenetic analysis of its details. It was compared to that ofAnserinae andDendrocygninae (otherAnseriformes were either similar to these or too unlikeP. mioceanus), as well as withPelecanidae,Phaethontidae andPhalacrocoraciformes[3] of the "higher waterbird" radiation, and found to resemble the former in one, the latter in 4 out of 5 traits. Some minor details of the femur were also unlike in the Anseriformes. The study proposed the fossil bird to have been "a largesteganopod [...] somewhat larger thanPelecanus erythrorhynchos orP. onocrotalus,"[4] and not securely assignable to a knownfamily. But ascladistic analysis had not yet been developed, it was of course not studied whether the similarities betweenP. mioceanus and the "higher waterbirds" areapomorphies. It has been noted, however, thatadaptation to oceanic habits, has induced in pseudotooth bird legs and feet featuresconvergent to otherseabirds. Still, the fossil is a rather good specimen, considering how little humerus material known from pseudotooth birds; a recent cladistic analysis (which did not includeP. mioceanus) did not put much emphasis on humerus traits, for example, as so few good fossils are known.[5]
WhenWitmer Stone, then editor ofThe Auk, commented on thetaxonomic confusion that the bird caused during less than 25 months after its description, he noted:
"It would seem desirable that those who name fossil birds should not fashion their generic names on those of existing birds as it is embarrassing to find them shifted into other families or orders where the name becomes somewhat of a misnomer!"[6]
Of thetarsometatarsus, only the end remains, but this is complete and even not very much abraded. Only on the edges of the outertrochleae does significant material seem to be lost. Altogether, the bone is very similar to that of thesympatric and probably contemporaryTympanonesiotes wetmorei, only appearing a bit lessalbatross-like. The spread of the toes must have resembled that found in afulmar quite a lot, by contrast. The thin-walled bone has a secondtoe trochlea that attaches notably kneewards from the others and is angled slightly outwards while thehallux was vestigial or missing, as is typical for the pseudotooth birds. The fossil is about one-quarter larger than that ofTympanonesiotes, with a maximum end width of 34.7 mm (1.37 in) – probably a bit more in life, as the trochlea rims are eroded away. The shaft is 22.3 mm (0.88 in) wide at the point where it flares into the trochleae. It was thus more than twice as large as "Odontoptila inexpectata"[7] from theLate Paleocene/Early Eocene of theOuled Abdoul Basin (Morocco), and – likeOsteodontornis orri – thus belonged to the large pseudotooth birds, with a wingspan of more than 5, perhaps as much as 6 m (16–20 ft). Its bill was presumably long and massive as in its relatives, and in length the bird exceeded the largestpelicans.[8]
The earlysystematic uncertainties have been noted again in recent times, in the scope of the debate whether the pseudotooth birds wereGalloanseres closely related to waterfowl orNeoaves related to pelicans. Meanwhile, as a presumed pelican relative,P. mioceanus was allied withCyphornis (another pseudotooth bird, known only from aproximal lefttarsometatarsus) and placed in afamily Cyphornithidae. This was subsequently assigned to a "pelecaniform"suborderCladornithes. But the enigmatic Late OligoceneCladornis – thetype genus of that supposed suborder – from theArgentinian part ofPatagonia is known from adistal right tarsometatarsus only, and thus was not directly comparable toPalaeochenoides andCyphornis.Tympanonesiotes wetmorei, yet another pseudotooth bird, known from a distal right tarsometatarsus fragment, was initially presumed to be similar toCladornis however, and also toCyphornis; consequently, the three genera were allied simply because of their size and some similarities to the corresponding bones of pelicans. Today however,Cladornis is more generally held to be aterrestrial bird rather than aseabird.[9]
As regards the supposed Cyphornithidae, most if not all pseudotooth birds placed there are probably closely related to the better-knownPelagornis,type genus of the family Pelagornithidae. And even ifCyphornis is thesenior synonym ofPalaeochenoides andTympanoneisiotes (which is not overly likely due to size differences), according to the rules ofzoological nomenclature the family name Pelagornithidae would not be affected. Cyphornithidae would almost certainly be ajunior synonym of Pelagornithidae even if the pseudotooth birds are (as some have proposed) divided into several families – rather than being all placed in the Pelagornithidae as is usual nowadays – asCyphornis,Osteodontornis,Palaeochenoides,Pelagornis and perhaps the smallerTympanoneisiotes appear to be very closely related and are probably part of amonophyletic lineage of (usually) giant pseudotooth birds. As at the time of its description no adequate comparative material was known, it was explicitly cautioned thatP. mioceanus might includePseudodontornis – which may actually be synonymous withPelagornis.[10]
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