Polarornis

Taxobox
name = "Polarornis"
fossil_range = Late Cretaceous
regnum = Animalia
phylum = Chordata
classis = Aves
ordo = Gaviiformes?
genus = "Polarornis"
genus_authority =
species = "P. gregorii "
binomial = "Polarornis gregorii"
binomial_authority =

"Polarornis" [It is sometimes suggested that this taxon was never formally described correctly and thus a "nomen nudum", but this is not true.Fact|date=April 2007] is a controversial (see Mayr, 2002) genus of prehistoric bird. It contains a single species "Polarornis gregorii" known from incomplete remains of one individual [Often claimed to be found in association, i.e. laid out in the pattern of a living bird. This is possibly incorrect, as indicated by the fact that e.g. two leg bones were found but the entire pelvis and most of the vertebral column was not.] found on Seymour Island, Antarctica, in rocks which are claimed to be of Late Cretaceous (López de Bertodano Formation, c. 76 mya) origin by supporters of the bird's antiquity, but may be no older than Eocene or about 50 mya.

The relationships of this taxon are unclear. It is not infrequently claimed to be an ancestor of modern loons (divers), but this is not necessarily correct as the material is not very diagnostic and the reconstructions take very much leeway to depict this bird as a primitive loon. What can be said with a reasonable degree of certainty is that it was in all likelihood aquatic and fed on fish and large invertebrates, probably being an ecological equivalent of loons, grebes, or the Cretaceous Hesperornithes of the Northern Hemisphere. Analysis of the structure of a femur (TTU P 9265) indicates that "Polarornis" was a flightless or near-flightless diving bird along the lines of the Hesperornithes or penguins, making a direct connection with modern looks highly unlikely (Chinsamy "et al." 1998).

The theory has been proposedFact|date=February 2007 that "Polarornis" represents a Southern Hemisphere radiation of loon ancestors which filled the same ecological niche as the Hesperornithes and might even have prevented them from spreading southwards. Although conjectural, as clearly recognizable loons do not appear in the fossil record until the Late Eocene (some 35 mya), this theory is not entirely without merit: the Late Cretaceous genus "Neogaeornis" and "Lonchodytes" may be ancestral loons - indeed, the former is sometimes considered synonymous with "Polarornis" - which are known from South and North American locations, respectively. However, both are alternatively considered to be Hesperornithes or other seabirds; at any rate, there is no unequivocal proof that loons "ever" occurred in the Southern Hemisphere, and the overwhelming majority of evidence clearly points to a Western European origin of the order (see Loon for details).

It is notable that Eocene penguins from Seymour Island are somewhat intermediate between modern penguins and loons in the characters of their skull (Olson, 1985), and the least controversial interpretation of "Polarornis" is to consider it a basal neognathe bird of a lineage which later evolved into some sort(s) of seabirds like loons and/or penguins. The possibility that it is a non-neornithine cannot be ruled out completely at present.

Biogeography of avian survivorship of the Cretaceous–Tertiary extinction event coupled with new data on avian phylogeny (e.g. Fain & Houde 2004) suggests that survival of at least one, more probably some, southern Pacific lineage(s) of Neoaves is indeed likely. The possibility that "Polarornis" represents a valid Mesozoic taxon in some kind of basal relationship to any or all of such birds as plotopterids, cormorants, albatrosses, penguins, storks and/or loons is likely enough to warrant more research of the remains.

References

* Chinsamy, A.; Martin, Larry D. & Dobson, P. (1998): Bone microstructure of the diving "Hesperornis" and the volant "Ichthyornis" from the Niobrara Chalk of western Kansas. "Cretaceous Research" 19(2): 225-235. DOI|10.1006/cres.1997.0102 (HTML abstract)

* Fain, Matthew G. & Houde, Peter (2004): Parallel radiations in the primary clades of birds. "Evolution" 58(11): 2558-2573. DOI|10.1554/04-235 [http://biology-web.nmsu.edu/houde/Parallel_radiations.pdf PDF fulltext]

* Mayr, Gerald (2004): A partial skeleton of a new fossil loon (Aves, Gaviiformes) from the early Oligocene of Germany with preserved stomach content. "Journal of Ornithology" 145: 281–286. DOI|10.1007/s10336-004-0050-9 [http://www.senckenberg.de/files/content/forschung/abteilung/terrzool/ornithologie/colymboides.pdf PDf fulltext]

* Olson, Storrs L. (1985): The fossil record of birds. "In:" Farner, D.S.; King, J.R. & Parkes, Kenneth C. (eds.): "Avian Biology" 8: 79-238. Academic Press, New York.

Footnotes