| Gaviiformes | |
|---|---|
.jpg) | |
| Common loon (Gavia immer) | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Aves |
| Clade: | Phaethoquornithes |
| Clade: | Aequornithes |
| Order: | Gaviiformes Wetmore &Miller, 1926 |
| Genera | |
| Synonyms | |
ColymbiformesSharpe, 1891 | |
Gaviiformes (/ˈɡævi.ɪfɔːrmiːz/) is anorder ofaquatic birds containing theloons or divers and their closest extinct relatives. Modern gaviiformes are found in many parts ofNorth America and northernEurasia (Europe, Asia and debatably Africa), though prehistoric species were more widespread.
There are five living species, and all are placed in the genusGavia.[1] The loons were formerly often considered to be the most ancient of the northern hemisphere bird families; this idea grew basically out of the perceived similarity of shape and (probably) habits between loons and the entirely unrelated extinctCretaceous orderHesperornithiformes. In particularEnaliornis, which was apparently an ancestral andplesiomorphic member of that order, was sometimes used to support claims ofAlbian (Early Cretaceous) Gaviiformes.[2][3]
More recently, it has become clear that theAnseriformes (waterfowl) and theGalliformes are the most ancient groups of modern birds, and these being distinct by the end of the Albian 100million years ago (Ma), while just possible, is not at all well-supported. Loons belong to a more modern radiation. They were once believed to be related togrebes, which are also foot-propelled diving birds, and both groups were once classified together under the orderColymbiformes or in even older classifications as the Urinatores.[4] The family name Urinatoridae was used for the family Gaviidae.[5] This was derived from LatinUrinator meaning "diver".[6] However, as recently as the 1930s, it was determined that the two groups (grebes and divers) are not that closely related at all and are merely the product ofconvergent evolution and adapted in a similar way to a similarecological niche. The similarity is so strong that even the most moderncladistic analyses of generalanatomical features are easily misled into grouping loons and grebes.[2][7][8]
TheSibley-Ahlquist taxonomy still allied the loons with the grebes in its massivelypolyphyletic "Ciconiiformes", and it is almost certain that the relationships of loons lie withsome of the orders placed therein. Namely, other recent authors have considered loons to share a rather close relationship withseabirds such aspenguins (Sphenisciformes), tubenoses (Procellariiformes),waders (Charadriiformes) – and perhaps the newly discoveredcladeMirandornithes which unites grebes (Podicipediformes) and their closest living relatives, theflamingos (Phoenicopteriformes). It is perhaps notable that some early penguins had skulls and beaks that were in many aspects similar to those of the known living and fossil Gaviiformes.[7][9]
In prehistoric times, the loons had a more southerly distribution than today, and their fossils have been found in places such asCalifornia,Florida andItaly. The conflicting molecular data regarding their relationships is not much resolved by thefossil record; though they seem to have originated at the end of theLate Cretaceous like their presumed relatives, modern loons are only known with certainty since theEocene. By that time almost all modern bird orders are at least strongly suspected to have existed – if not known from unequivocally identified specimens – anyway.[8]
The oldest known stem-gaviiform isNasidytes from theEarly Eocene agedLondon Clay of England, dating to around 55 million years ago.[10]
Colymboides is widely known from earlyPriabonian – about 37million years ago (Ma) in theLate Eocene – toEarly Miocene (lateBurdigalian, less than 20 Ma)limnic andmarine rocks of westernEurasia north of theAlpide belt, between theAtlantic and the formerTurgai Sea. It is usually placed in the Gaviidae already, but usually[11] in asubfamilyColymboidinae, with the modern-type loons making up theGaviinae. But theColymboides material is generally quite distinct from modern loons, and may actually belong in a now-extinct family of primitive gaviiforms. Furthermore, the supposed genus could well be paraphyletic, so that for exampleDyspetornis – which is now contained therein – might have to be separated again. A leg of an undescribed small diver was found in theLate Oligocene deposits atEnspel (Germany); it too may or may not belong toColymboides. Of thecrown genusGavia, nearly ten prehistoric species have been named to date, and about as many undescribed ones await further study. The genus is known from the Early Miocene onwards, and the oldest members of them are rather small (some are smaller than thered-throated loon). Throughout the lateNeogene, the genus by and large followsCope's Rule.[12]
Some older fossils are sometimes assigned to the Gaviiformes. From theLate Cretaceous, the generaLonchodytes (Lance Formation, Wyoming) andNeogaeornis (Quinriquina Formation,Chile) have been described; both are usually allied with orders which are considered related to loons. In particular the latter is still sometimes explicitly proposed as a primitive loon as they both were initially, but other authors considerNeogaeornis ahesperornithiform; note however that neither Gaviiformes nor Hesperornithiformes are known from theSouthern Hemisphere or anywhere near it.Lonchodytes was more certainly quite close to loons, but probably closer still to some of the loons' relatives.Eupterornis from thePaleocene ofChâlons-sur-Vesle (France) has some features reminiscent of loons, but others seem more similar toCharadriiformes such as gulls (Laridae). A piece of acarpometacarpus supposedly fromOligocene rocks nearLusk, Wyoming was described asGaviella pusilla, but this handbone also shows some similarities to theplotopterids which were flightlesswing-propelled divers and if these areapomorphic would make an unconvincing member of the Gaviidae (though it still could be a small-winged gaviiform in a yet-undescribed family "Gaviellidae"[13]): while the carpometacarpus inGavia is somewhatconvergent to that of wing-propelled divers, enabling the wings to be used as rudders for quick underwater turns,Colymboides still had an unspecializedplesiomorphic hand.Parascaniornis, sometimes allied to the loons by early authors, was eventually determined to be ajunior synonym of the hesperornithiformBaptornis. A supposed mid-Eocene diver fossil formGeiseltal (Germany) was erroneously assigned toGavia.[14]
Media related toGaviiformes at Wikimedia Commons
Data related toGaviiformes at Wikispecies
