Podicipediformes
From EvoWiki
Podicipediformes (Sharpe 1899), an order within class Aves.
Podicipediformes constitutes a monotypic order of highly modified diving birds, the grebes (Podicipedidae). They are cosmopolitan in aquatic habitats, ranging from lakes to coastal bays and seasonally flooding scrubland and even ditches along roads. There are 22 species of grebes distributed across six genera of global distribution. A principal character of the group is the lobate webbing of their feet, observed elsewhere in the distantly related phalaropes (Charadriiformes: Phalaropidae), coots (Gruiformes: Rallidae) and sun-grebes (Gruiformes: Heliornithidae), thus presenting a classic case of convergence (Feduccia 1996).
The phylogenetic affinities of grebes remain intractable. Though it was a consensus following the work of Stolpe (1935) and Storer (1960, 1971) that loons and grebes were not, as classically thought, sister groups, recent cladistic analyses starting with those of Joel Cracraft (1982a, 1986) resurrected the discredited notion of a grebe/loon nexus. Though consistently retrieved (e.g., Mayr & Clarke 2003), this topology has not a shred of evidence to support it and a sister group relationship between loons and grebes exists only in the minds of some systematists(see Gaviiformes, and Parsimony and its role in Phylogenetic Reconstruction). Olson (1985a) observed similarities between the neck musculature of grebes and Gruiformes as well as similarities in the skulls of the two orders. Sibley & Ahlquist (1990) were unable to clarify grebe relationships in their DNA-DNA hybridization studies, merely observing that grebes apparently have no close living relatives, having diverged at a very early date in the neornithine adaptive radiation. Sibley & Ahlquist (1990) did suggest possible shared ancestry with frigatebirds, sulids, and members of the traditionally defined order Ciconiiformes. Recent molecular and morphological analyses have posited a sister group relationship between grebes and flamingos (Phoenicopteriformes), a novel topology that has hitherto been entirely overlooked.
The fossil record of grebes confirms the age of this amazing group of diving birds, and the oldest known morphologically well defined grebes date from the early Miocene, with the modern genus Podiceps present in 25 million year old deposits from Oregon. Another Miocene grebe, Thiornis sociata (Olson 1995) is known from Spain and from the Lower Miocene of former Czechoslovakia there is another grebe, Miobaptus walteri. Unfortunately these fossils shed little light on the origin of grebes, as they are morphologically indistinguishable from contemporary species.
References:
- Cracraft, J. 1982a. Phylogenetic relationships and monophyly of loons, grebes, and hesperornithiform birds, with comments on the early history of birds. Systematic Zoology, 31: 35-56.
- Cracraft, J. 1986. The origin and early diversification of birds. Paleobiology, 12(4): 383-399.
- Feduccia, A. 1996. The Origin and Evolution of Birds, First Edition. Yale University Press, New Haven.
- Mayr, G. & Clarke, J. 2003. The deep divergences of neornithine birds: a phylogenetic analysis of morphological characters. Cladistics 19: 527-553.
- Olson, S. L. 1985a. The fossil record of birds. In: Farner, D. S., King, J. R., and Parkes, K. C. (eds.), Avian Biology, Volume 8, 79-252.
- Olson, S. L. 1995. Redescription of Thiornis sociata Navas, a nearly complete grebe from Spain (Aves: Podicipedidae). Courier Forschungsinstitut Senckenberg 181: 131-140.
- Sibley, C. G. & Ahlquist, J. E. 1990. Phylogeny and Classification of Birds: A Study in Molecular Evolution. Yale University Press, New Haven.
- Stolpe, M. 1935. Colymbus, Hesperornis, Podiceps, ein Verglich ihrer hinteren Extremitat. Journal fur Ornithologie, 80: 161-247.
- Storer, R. 1960. Evolution in the diving birds. Proceedings of the Twelfth International Ornithological Congress, 694-707.
- Storer, R. 1971. Adaptive radiation of birds. In: D. S. Farner, and J. R. King (eds.), Avian Biology, 149-188
JGK
