Pelecaniformes

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Pelecaniformes (Sharpe 1891), an order of class Aves. Pelecaniformes represents one of the most spectacular orders of living birds, bracketing a vast array of aquatic piscivorous forms united by the shared presence of a totipalmate foot and a distensible gular pouch between the mandibular rami (albeit reduced in the tropicbirds). Pelecaniformes includes pelicans (Pelecanidae), boobies and gannets (Sulidae), cormorants (Phalacrocoracidae), anhingas (Anhingidae), frigatebirds (Fregatidae) and tropicbirds, (Phaethontidae). Beddard (1898a) wrote of the totipalmate birds:

"Though this group shows much divergency of structure, its naturalness can hardly be doubted."

Thus is reflected the long-held belief that Pelecaniformes are tightly holophyletic. Indeed, Linnaeus recognized the grouping as natural as early as 1758. Nevertheless, there has been considerable debate regarding the phylogeny of these forms and various researchers have argued for the polyphyly of this assemblage (e.g., Sibley & Ahlquist 1990).

Huxley (1867), in his major classification of the class Aves corroborated the reigning taxonomic theory regarding these birds (e.g., Nitzsch 1840) and argued that the totipalmate swimmers constituted a holophyletic assemblage. Sundevall (1872) placed the "Cohort Totipalmatae" within his "Natatores" and concurred with Huxley on the "naturalness" of this grouping. Garrod (1873d) catalogued differences in the pelvic myology of the various pelecaniforms and noted similarities to the ciconiiforms and falconiforms. Mivart (1878) in an analysis of postcranial characters upheld the holophyly of the Pelecaniformes, as did Shufeldt upon the basis of osteology (1883a, 1888b, 1894a, 1902a). Pycraft (1898b) added further skeletal evidence to support the hypothesis of pelecaniform holophyly.

The first major study to question these conclusions was that of Chandler (1916) on pterylosis and feather structure. Chandler argued that Phaethon was an "aberrant larid" unrelated to other taxa classically placed within Pelecaniformes. Mathews and Iredale (1921) concurred and erected a new suborder within Lari, the Phaethontiformes, to reflect their conclusion that Phaethon shares more recent common ancestry with the larids than with any other pelecaniforms.

Lowe (1926) strenuously disagreed with these interpretations of pelecaniform phylogeny and upon the basis of quadrate morphology argued that Phaethon is more closely allied with the other pelecaniform birds than with Laridae, thus defending pelecaniform holophyly. Lanham (1947) in a major analysis of morphological characters argued definitively for pelecaniform holophyly. Sibley (1960), from the electrophoretic patterns of egg-white proteins argued, albeit cautiously, for pelecaniform polyphyly. Simonetta (1963) in a study of cranial kinetics and morphology reached similar conclusions. Sibley & Ahlquist (1972) vacillated on pelecaniform phylogeny and concluded that though morphologically diverse, there was no convincing evidence for pelecaniform polyphyly. Olson (1977b) defended pelecaniform phylogeny in his analysis of Limnofregata azygosternon and argued that the morphology of this amazing Eocene form upheld conclusions reached some 30 years before by Lanham (1947). Olson particularly emphasized the presence of a salt-gland within the orbit itself and not within a supraorbital groove.

Cracraft (1985) presented a numerical cladistic analysis of 44 morphological and 11 behavioral characters and isolated at least 12 (possibly 14) unambiguous synapomorphies of Pelecaniformes, robustly corroborating the holophyly of the totipalmate assemblage. Cracraft particularly emphasized the following characters as synapomorphic of Pelecaniformes:

1. Loss of supraorbital depressions
2. Reduced or absent lip on medial condyle of quadrate
3. Relatively large ilioischiatic fenestra
4. Deep ligamental furrow of humerus
5. Totipalmate foot
6. Gular pouch
7. Prelanding call

Feduccia (1996) concurred with this assessment. Sibley & Ahlquist (1990) argued that the morphological disparity amongst included members of the order Pelecaniformes constituted evidence for polyphyly of this taxon and presented DNA-DNA hybridization evidence suggesting that the Pelecanidae and Fregatidae were more closely related to Balaeniceps and Procellariidae, respectively, than either is to other members of the traditional Pelecaniformes. The molecular data is entirely inconsistent with over a century of morphological analysis (cladistic and otherwise) conclusively demonstrating that Pelecaniformes is holophyletic and thus must be regarded with great skepticism, as Sibley & Ahlquist (1990) themselves noted. The totipalmate foot, presence of a gular pouch, lack of an incubation patch and intraorbital salt gland are characters that unambiguously support this conclusion and it most unlikely that they represent either convergence or plesiomorphic traits as suggested by some authors.

The intraordinal relationships of the Pelecaniformes are fairly well resolved. Olson (1977b) argued that contrary to classical analyses (e.g., Wetmore 1960) Phaethontidae and Fregatidae were basal to all other pelecaniform lineages based on the morphology of the basal Eocene form Limnofregata azygosternon. Cracraft (1985) concurred with this assessment. Pelecaniformes cleaves into two suborders, Phaethontes consisting solely of the tropicbirds, and Steganopodes, with Steganopodes bifurcating into the Pelecanoidea and Suloidea. The Anhingidae and Phalacrocoracidae, sister families within the Suloidea, are the most highly derived pelecaniform birds.

Thus is left the question of to which group is the holophyletic Pelecaniformes most closely related? Garrod (1873d) observed similarities in myology of the pelvic girdle, as noted, with both Ciconiiformes and Falconiformes. Sclater (1880b) concurred with this assessment. Stejneger (1885) generally concurred, comparing these birds favorably to the herons. Gadow (1889, 1893) argued that Ciconiiformes were the closest allies of Pelecaniformes, though he also noted similarites to the Procellariiformes. Shufeldt (1883a, 1888b, 1894a, 1902a) on the other hand, was among the first to make extensive comparisons between the pelecaniform birds and both Charadriiformes and Procellariiformes. Shufeldt argued that Phaethon resembled Puffinus in morphology and further compared Phaethon and the larids.

Shufeldt favorably compared the skulls of Fregata and albatross. Shufeldt's work aside, both Furbringer (1888) and Seebhom (1889, 1890a, 1895) concurred with a ciconiiform relationship for Pelecaniformes, with Furbringer finding favorable comparison with the diurnal raptors as well as storks and herons, and Seebhom comparing the pelecaniforms most favorably to herons and storks. Sharpe (1891) completely disagreed with prior studies and suggested a novel hypothesis whereby Anseriformes represented the nearest order to Pelecaniformes. Beddard (1898a) reiterated the earlier arguments of Shufeldt (e.g., 1883a) that Procellariiformes were the closest allies of Pelecaniformes. Beddard found marked similarities in cranial morphology between Fregata and Diomedea though he also argued that herons may be related to the pelecaniforms as well, on the basis of an eclectic array of characters.

Lanham (1947) argued compellingly for the procellariiform affinities of the totipalmate birds. Olson (1977b) concurred with this assessment and it was strongly corroborated by Cracraft's 1985 cladistic analysis. Harrison & Walker (1976a) also noted similarities between basal pelecaniforms (e.g., Prophaethon) and Charadriiformes. Saiff (1978) and Sibley & Ahlquist (1990) argued that Balaeniceps rex was the closet relative of the Pelecanidae. Sibley & Ahlquist (1990) further placed Fregatide with the procellariids. The totality of morphological data strongly indicates a sister group relationship between Pelecaniformes and Procellariiformes with both groups not far removed from Charadriiformes. Alleged synapomorphies of Balaeniceps rex and Pelecanus spp. can be accounted for as homoplasy arising from similar biomechanical responses to shared feeding behaviors (Cracraft 1985, Feduccia 1996). Given this data, the DNA-DNA hybridization studies of Sibley & Ahlquist (1990) can be effectively rejected.

Pelecaniformes has among the most spectacular of all avian fossil records, with the first morphologically well defined (though mosaic) pelecaniform fossils hailing from the Eocene London Clay at Isle of Sheppy, England. Harrison & Walker (1976a) described this primitive tropicbird as a new genus, Prophaethon and placed it within a separate family, the Prophaethontidae, nested within Phaethontes. From the Eocene Green River Formation in Wyoming, Olson (1977b) described the amazing tropicbird/frigatbird mosaic Limnofregata azygosternon, placing it within subfamily Limnofregatinae of the Fregatidae.

The first fossil anhingas are represented by Protoplotus beauforti known from a nearly complete skeleton from the Upper Eocene of Sumatra (Lambrecht 1931a, Olson 1985a). Phalacrocoracidae are also first known from the Upper Eocene (Feduccia 1996). Pelecaniformes extensively radiated in the mid-Tertiary, and by the Lower Miocene the fossil record of these birds proliferates splendidly. From Miocene rocks of France, the oldest known pelican, Pelecanus gracilis has been recovered and in Germany another form, P. intermedius more abundant than its French counterpart, has also been described (Feduccia 1996). Pelicans are represented by abundant fossils from the Miocene of Australia (Vickers-Rich 1991). Anhingidae radiated extensively in the Miocene and thereafter, as indicated by the fabulous fossils of the giant Nebraskan species Anhinga grandis (Martin & Mengel 1975, Feduccia 1996), also reported from remains in the latest Miocene in Florida (Becker 1987a). Similarly gigantic forms are known from the Miocene of Chile (Alvarenga 1995). Sulids do not appear in the fossil record until the Oligocene and are represented by Sula ronzoni of France.

Perhaps the most extraordinary representatives of this Miocene adaptive radiation of the Pelecaniformes are the Plotopteridae and the Pelagornithidae, or pseudodontorns. The plotopterids are gigantic wing-propelled divers convergent upon Sphenisciformes (Olson & Hasegawa 1979), and they represent the largest diving birds known, in one species measuring 1.8 meters (roughly six feet) from bill to tail (Feduccia 1996). Olson & Hasegawa (1979) and Feduccia (1996) presented evidence to suggest that the plotopterids are derived from within the Suloidea.

The pseudodontorns, most readily homogenized within a single family, the Pelagornithidae (Feduccia 1996), are among the most spectacular of all fossil birds. Enormous marine gliders characterized by massive bills with tooth-like denticles, these birds are arguably the most thin-boned and light-weight of all gliding forms. Howard (1957) estimated that the wingspan of Osteodontornis orri at more than 4.8 meters. Olson (1985a) revised this estimate upon review of further material and presented a figure of 5.5-6.0 meters (18-20 feet). Zusi & Warheit (1992) argued that based upon their size and gliding modifications, pseudodontorns fed in the fashion of boobies or tropicbirds, or alternatively, in frigatebird style. Olson (1985a) argued that they primarily fed on soft-bodied prey such as squid based on the structure of the pseudo-teeth for which these birds are named. They were cosmopolitan birds of the Tertiary as evidence by the range of their fossils (Tonni 1980a, Olson 1985a).

[edit] References

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