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Galloanserae: A Critical Examination
Galloanserae: A Critical Examination
One of the major points of consensus in contemporary ornithology, triumphantly hailed as immune to any refutation, is the sister-group relationship of the orders Anseriformes and Galliformes. In fairness, there has been a great deal of molecular evidence brought to bear in defense of this conclusion, though morphological data have been less forthcoming (e.g., Ericson 1997). The question remains, how valid is this assertion?
Critical examination of galloanserine monophyly has several times led to its rejection (e.g., Olson & Feduccia 1980b, Ericson 1996, 1997), while other researchers have been stricken with indecisiveness (i.e., Ericson et al. 2001). Perhaps the most problematic aspect of this debate is that there largely appears to be a schism in the evidence refuting or supporting galloanserine monophyly, between morphological and molecular data. Though some morphological studies have retrieved a monophyletic Galloanserae (e.g., Livezey & Zusi 2001, Mayr & Clarke 2003), the majority of the evidentiary support for this hypothesis is derived from molecular systematic work (e.g., Chubb 2004).
Combined with the discrepancies between the fossil record of both gallinaceous and anserine birds vis-a-vis the sister-group hypothesis, and the lack of any clear adaptive scenario whereby highly derived filter feeding ducks might arise from land-fowl, the morphological data strongly indicate that the monophyly of Galloanserae is not yet beyond dispute.
Huxley (1867) was among the first to comment on osteological similarities between Galliformes and Anseriformes. In his analysis of palatal morphology, Huxley noted that the shared presence of large, rounded pterygoid-parasphenoid articulations and an upwardly curved retroarticular process might suggest evolutionary affinity for land and water fowl. Though noted within his highly eclectic classification by Garrod (1873d, 1874a), it was the work of Seebohm (1889) that largely established within systematic ornithology, the traditional association of Galliformes and Anseriformes. Via the enigmatic South American screamers (Anhimidae), Seebohm felt Anatidae was linked with Galliformes. Seebohm reached this conclusion on the basis of the cranial characters elucidated by Huxely in 1867.
Furbringer (1888) did not closely ally Galliformes and Anseriformes, the former placed within his order "Alectoronithes" with tinamous. Gadow (1893) followed a similar classification and disavowed any close relationship between gallinaceous and anserine birds. Shufeldt (1901) in his typically abstruse manner, commented on general similarities in the skull of ducks and land-fowl but declined any hypothesis which placed them as sister groups.
Some six decades later Simonetta (1963) supported the sister-group relationship of Anseriformes and Galliformes upon the basis of the pterygoid-parasphenoid articulation noted a century beforehand. Bock (1969, 1970) regarded the cranial osteology of Anseriformes and Galliformes as indicative of an unequivocal sister-group relationship between the two orders. Sibley & Ahlquist (1972) using starch gel electrophoretic patterns of egg white proteins, did not recover a galliform/anserifom nexus and favored ciconiiform affinities for ducks.
Olson & Feduccia (1980b) extensively reviewed the osteological characters advanced in defense of galloanserine monophyly and rejected them, concluding on the basis of paleontological data that Anseriformes are most closely related to Charadriiformes. Cracraft (1981) was equivocal in his support for a galliform/anseriform nexus, noting that: "Certainly one must keep an open mind regarding the relationship between anseriforms and galliforms, for I agree with Olson and Feduccia that such a relationship is not well documented." Yet in the same paper he dismisses proposed charadriiform affinities for Anseriformes and states that: "Nevertheless, there is some tentative evidence to warrant keeping them close together."
Later, Cracraft (1998b) recovered a sister-group relationship for Anseriformes and Galliformes. Sibley, Ahlquist & Monroe (1988) and Sibley & Ahlquist (1990) using DNA-DNA hybridization argued for the monophyly of Galloanserae. Ericson (1996, 1997) strongly disputed the monophyly of Galloanserae using morphological characters. Feduccia (1996) reiterated prior arguments (e.g., Olson & Feduccia 1980b) for the charadriiform ancestry of ducks and disavowed any close link to land-fowl. Olson (1999) concurred with this assessment in his revision of Anatalavis rex.
Analysis of nuclear DNA sequences (e.g, Groth & Barrowclough 1999, Chubb 2004) and combined analysis of mitochondrial and nuclear DNA sequences (e.g., van Tuinen et al. 2000, Garcia-Moreno et al. 2003) have all retrieved a monophyletic Galloanserae. Livezey & Zusi (2001) in a preliminary analysis of avian phylogeny using comparative anatomy, argued for galloanserine monophyly. Cracraft & Clarke (2001), reiterating prior work, also favored galloanserine monophyly.
Ericson et al. (2001) presented extensive morphological and molecular evidence contravening galloanserine monophyly, but then backpedaled in an addendum and recanted. Olson (2002) sharply criticized the monophyly of Galloanserae and regarded the issue unresolved. Mayr & Clarke (2003), predictably, found a monophyletic Galloanserae to lie at the base of the neornithine tree, though in this analysis Galloanserae received a low bootstrap value of only 57%.
Classically, morphological support for a galliform/anseriform nexus has centered on two cranial characters: the presence of large and rounded pterygoid-parasphenoid articulations, and an upwardly curved retroarticular process. Though additional morphological characters have been added as "synapomorphies" of Galloanserae, it is telling that these two characters are still retained in even the most recent cladistic analyses (e.g., Mayr & Clarke 2003). There are in fact significant morphological differences between Galliformes and Anseriformes and while classifications cannot be constructed solely on the basis of anatomical disparity, such discrepancies cannot be categorically dismissed, either.
Bock (1970) stated that "all apsects...of cranial morphology" in Galliformes and Anseriformes supported a sister-group relationship thereof, but this assertion in no way withstands close scrutiny. The palate is desmognathous in Anseriformes and schizognathous in Galliformes. While the palatal configuration is not an accurate means of classification, it is nonetheless an interesting discrepancy between the two orders. There are in all few if any derived similarities in the cranial osteology of ducks and the land-fowl. Rostral elements in Galliformes exhibit little if any fusion, unlike the condition in Anseriformes and indeed that of most other birds. The nasals, jugals, and palatines do not ossify and thus the premaxilla is free. The nasals exhibit a characteristic rostral bifurcation unknown in Anseriformes. In Galliformes the lacrimals are unfused, whereas in all Anseriformes with the exception of Anseranas, they are extensively fused.
Galliformes retain a zygomatic process which fuses distally with the postorbital process enclosing a foramen while in Anseriformes the zygomatic process is absent entirely. Occipital fontanelles, a uniquely derived trait linking Anseriformes to Charadriiformes are absent in all gallinaceous birds. The ventral quadrate in ducks and allies is inflated caudally, whereas no such condition is observed in Galliformes. The pterygoids, though both bearing distinct articular surfaces for communication with the parasphenoid, have little else in common. Indeed, in Galliformes the parasphenoid articular surface is located cranially and the cranial spine of Anseriformes is absent. In Anseriformes the parasphenoid articular surface is medial and elongate.
The much discussed pterygoid-parasphenoid articulation, long advanced as a synapomorphic trait of Galloanserae, is in reality only superficially similar and in both ducks and land-fowl, of contested homology (Olson & Feduccia 1980b). The articular facets on the parasphenoid for the pterygoids are usually referred to as basipterygoid processes (e.g., Mayr & Clarke 2003), and yet McDowell (1978) demonstrated that the avian "basipterygoid process" is not homologous with that of reptiles and concluded that in birds this structure is a neomorph, likely having arisen multiple times in disparate taxa. The topological relationships of the respective articular surfaces in adult Galliformes and Anseriformes are not suggestive of homology. In ducks and their allies the articular surfaces on the parasphenoid are distinct, pedicellate and ovoid structures while in Galliformes they are far more indistinct and are not as markedly set off from the parasphenoid rostrum.
Dissection of embryonic duck (Aythya spp.) and chick (Gallus gallus) to determine the morphogenesis of the parasphenoid articular surfaces in both Anseriformes and Galliformes, strongly corroborated the hypothesis that these structures are not homologous across these taxa (Olson & Feduccia 1980b). In Anseriformes the parasphenoid articular surfaces arise as de novo cartilaginous condensations that only late in ontogeny fuse to the parasphenoid while displaying no medial contact. In contrast, the parasphenoid articular surfaces in Galliformes exhibit medial fusion as cartilaginous condensations and are outgrowths of the parasphenoid rostrum itself. These data strongly indicate that in Galliformes and Anseriformes large and ovoid parasphenoid articular surfaces are analogous only, having arisen in both groups to facilitate the sliding of the pterygoid during cranial kinesis (Olson & Feduccia 1980b).
The retroarticular process stressed by some analyses (e.g., Mayr & Clarke 2003) is only superficially alike in ducks and land-fowl. In Anseriformes the retroarticular process are laterally compressed, bladelike hooks whereas in Galliformes they are slender and rounded and exhibit no compression in lateral aspect.
Given the serious discrepancies between ducks and land-fowl in terms of cranial anatomy, it is thus natural to enquire if there are any postcranial characters which unite the two orders. Mayr & Clarke (2003) proposed in their diagnosis of Galloanserae that the presence of an osseous bridge from the transverse process to the caudal articular process of the third cervical vertebra optimizes as a galloanserine synapomorphy. This may or may not be so, and if it is a valid synapomorphy it stands as the singular character in the entire postcranium of ducks and land-fowl which would suggest any close affinity between these taxa.
An Examination of Galloanserine Synapomorphies
Given the morphological data enumerated above, what are we to make of the morphological analyses that present "unambiguous synapomorphies" diagnosing a monophyletic Galloanserae? As it is among the more recent morphological studies, Mayr & Clarke's 2003 study will suffice as a test case. Mayr & Clarke performed a numerical cladistic analysis of 43 higher neornithine taxa using 146 skeletal characters and retrieved, as noted prior, a monophyletic galloanserine assemblage at the base of the neognathous radiation. They diagnosed this clade on the basis of the following characters:
- Lacrimal without well developed descending process which touches or nearly touches the jugal bar
- Caudal ends of vomers fused
- Basipterygoid processes with facet for articulation with pterygoid large and ovoid
- Cranium with basiparasphenoid plate inflated, rounded, broad and meeting the parasphenoid rostrum at a very acute angle; ostia canalis carotici et opthalmici externi situated in a well marked depression
- Otic process of quadrate with eminentia articularis
- Quadrate with lateral condyle large and with greatest extension in mediolateral direction
- Mandible with long and strongly mediolaterally compressed retroarticular process
- Mandible with long, narrow, and dorsally oriented medial process
- Third cervical centra with osseous bridge from transverse process to the caudal articular process
Character (1) overlooks the significant differences in the lacrimal between Galliformes and Anseriformes in that in one (Anseriformes) the lacrimal is completely ossified while in the other it is not. Character (3) is based upon characters that are not homologous across the two constituent orders of Galloanserae as outlined above, and is thus of no utility in phylogenetic analysis. The discrepancy in palatal development indicated by the lack of homology between the "basipterygoid processes" of Anseriformes and Galliformes suggests at the very least that we treat character (4) with skepticism. The lack of homology in the structure of the parasphenoid indicates possibly disparate developmental programs governing the formation of the palate in ducks and land-fowl, and thus any alleged synapomorphies thereof would be automatically rendered convergent. The otic processes of the quadrate in Anseriformes and Galliformes are not especially similar (Olson & Feduccia 1980b) and the quadrate morphology presents no justification for the amalgamation of these orders into a monophyletic taxon. Indeed, as noted earlier, the quadrate, at least in ventral aspect, is quite different in ducks and land-fowl. The retroarticular process stressed in character (7) presents yet another case of a superficially similar element in the skeleton being regarded as a uniquely shared derived character when in fact there is no evidence for such an assertion.
Of Mayr & Clarke's list of "synapomorphies" diagnosing a monophyletic Galloanserae only characters (8) and (9) can credibly be advanced in defense of a galliform/anseriform nexus.
Adaptive Scenarios: Or, How to Get a Duck from a Chicken
Morphology aside, perhaps the single most problematic issue for the galloanserine hypothesis is the utter lack of a credible adaptive scenario whereby one might derive an advanced filter feeding duck from a cursorial, ground-feeding galliform. While this seems of little trouble to many systematists in their zeal to establish "monophyletic" groupings, it is nonetheless a critical component of evolutionary research. Without a valid adaptive scenario, any given transition from one level of organization to another is at best based upon faith in its having occurred as opposed to hard evidence.
Unfortunately for proponents of galloanserine monophyly, it strains creduilty to imagine a situation in which the highly derived filter feeding apparatus of Anseriformes might have been developed from the feeding apparatus of Galliformes. Ducks possess among the most unique of all trophic adaptations and while a detailed analysis of anserine filter feeding is beyond the scope of this essay, some general comments can be made. Ducks house a double-piston tongue in the upper bill (thus explaining its enlargement at the expense of the mandible), which serves as a rapid action suction/filtration device, drawing in water rostrally and expelling it caudally while lamellae retain algae upon which anserines feed (Olson & Feduccia 1980a, b). There is quite simply no conceivable way in which any currently known morphogenetic process might lead to the formation of such a system from the cranial anatomy and trophic system of the gallinaceous assemblage and no pseudophylogeny documenting the transition from galliform to anseriform can be provided.
In contrast, the derivation of the anseriform filter feeding apparatus from the charadriiform ancestors hypothesized by Olson & Feduccia (1980a, b), Feduccia (1996), Olson (1999) and Benson (1999) is readily explicable through a fairly simple process of fusion of existing cranial elements and elaboration on rudimentary filtering adaptations present in some shorebirds (e.g., Phaloropus). By fusion and subsequent ossification of the maxillopalatine area and the internasal region lying between the dorsal and ventral premaxillary rami, an anseriform bill could readily be derived from a plesiomorphic charadriiform state similar to that observed in phalaropes. Indeed in Phalaropus fulicarius this very character state, albeit incipient, has already been acheived. Olson & Feduccia (1980a) extensively reviewed the evolution of filter feeding in birds and were able to project a coherent pseudophylogeny both for the derivation of filter feeding trophic specializations in flamingos (Phoenicopteridae) and Anseriformes from a hypothetical shorebird ancestor with cranial anatomy and kinetics similar to that of the extant phalaropes.
Presbyornis and the Fossil Record
Significant light was shed on the origin of the anserine assemblage, previously an intractable riddle, with the redescription of Presbyornis pervetus Wetmore 1926 following the work of Alan Feduccia, Paul McGrew and Storrs Olson. Alexander Wetmore, late Dean of American Ornithology, described on the basis of a partial tarsometatarsus from the Lower Eocene Green River Formation of Utah a new species of bird which he referred to Recurvirostridae (Charadriiformes), which was subsequently augmented by numerous additional finds such that Presbyornis is one of the most heavily collected and best known of all extinct birds.
That Wetmore recognized similarities to the Recurvirostridae is an elegant testimonial to his perspicacity, and it was this very diagnosis which would lead Feduccia (1976) to hypothesize upon further morphological analysis, a shorebird ancestry for ducks. Presbyornis was initially interpreted as a flamingo-like wader implicated in the origins of the Phoenicopteridae (e.g., Feduccia & McGrew 1974) though this hypothesis was revised upon the recovery of multiple presbyornithid crania, which are strikingly anserine in appearance, down to a typically duck-like bill. Feduccia (1976) concluded that Presbyornis was a long-legged wader with the postcranium of a shorebird and the head of a duck--a veritable icon of an evolutionary intermediate. Olson & Feduccia (1980a, b) concurred with this assessment, as did Olson (1985a), Feduccia (1996, 1999), Olson (1999) and Benson (1999).
The anatomy of Presbyornis is strikingly plesiomorphic and bears marked similarities to that of the derived charadriiform birds. The following postcranial characters suggesting that Presbyornis is a charadriiform/anseriform mosaic are taken from Feduccia (1976) and Olson & Feduccia (1980b):
- Sternum with well developed manubrial spine
- Coracoid with deep undercut ventral to furcular facet and with diagonal rise separating the regions
- Humerus non-pneumatic
- Bicipital crest large and broadly curved
- Pneumotricipital fossa strongly excavated
- Tibiotarsus with large and cranially projecting, often rectangular inner cnemial crest
- Tarsometatarsus long and slender with inner trochlea elevated and retracted, exposing middle trochlea in internal view
- Lack of fusion between the innominate bones and the sacrum
- Trochanter of femur significantly elevated above iliac crest
Even in the duck like cranium of Presbyornis we observe several plesiomorphic traits and others which resemble the condition in Charadriiformes. As is the case in all Anseriformes occipital fontanelles are also present in Presbyornis, and as noted earlier, this character is one of limited distribution found elsewhere amongst potential anserine ancestors only in Charadriiformes (Olson & Feduccia 1980b). In lacking the characteristic ventral inflation of the quadrate observed in all other ducks, Presbyornis most closely resembles the Charadriiformes. The presbyornithid skull also retains the plesiomorphic protrusion of the palate in lateral aspect vental to the jugal bar. These data are difficult to reconcile with the highly derived position as sister-group to the Anatidae postulated for Presbyornithidae by some authors (e.g., Ericson 1997).
Given that the morphological data overwhelmingly corroborates the identification of Presbyornis as a charadriiform/anseriform mosaic, what conclusions can be drawn vis-a-vis the monophyly of Galloanserae? If Presbyornis is a basal anseriform, or even a more derived form which has retained a number of plesiomorphic characters, then the presence of astonishing similarities to Charadriiformes is shockingly at odds with the proposed galliform affinities of the water-fowl. Presuming this model to be correct, one would imagine that basal forms within Anseriformes would most closely resemble the gallinaceous birds from which they are allegedly derived, and not some other grouping. This said, it is safe to conclude that the fossil record is without question in opposition to galloanserine monophyly (Olson 2002).
Anhimidae and the Search for Galliform Ancestors of Ducks
The enigmatic South American screamers, family Anhimidae, are often implicated as potential intermediaries bridging the organizational level in Galliformes with that in Anseriformes. Among the strangest of all birds, screamers are roughly turkey-sized denizens of marshes and tropical and sub-tropical lowlands. Screamers are readily distinguished from other birds, including other Anseriformes, by a number of structural oddities. Screamers possess a thick layer of subcutaneous air cells, lack apteria or display only vestigial apteria, and have developed apparently in response to the absence of uncinates an autapomorphic muscle, the M. costisternalis externus, which runs diagonally across the rib cage (Beddard & Mitchell 1894). This melange of peculiarities has traditionally confounded the proper taxonomic allocation of the Anhimidae.
Parker (1863) in a very vauge way, noted similarities between screamers and Galliformes. Garrod (1873d, 1874a) placed screamers with his order "Galliformes" while it was Seebohm (1889) who lent the idea credulity with his elaboration of a galliform/anseriform nexus via the Anhimidae, as noted earlier. There are significant morphological and behavioral similarities between screamers and the monotypic Anseranatidae, noted by Delacour (1954) and Olson & Feduccia (1980b). Olson & Feduccia (1980b) reviewed the history of screamer relationships and noted that at best the alleged similarities between these birds and galliforms are superficial, relating primarily to the highly aberrant morphology of the Anhimidae (e.g., its chicken-like bill). Ericson (1996, 1997) recovered Anhimidae as basal within Anseriformes, a conclusion with which Olson (1999) apparently concurred in his review of the Anatalavis rex material.
Despite this consensus, the phylogenetic position of the Anhimidae vis-a-vis other anserines is not entirely clear. Olson & Feduccia (1980b) reported in both Chauna and Anhima the presence of vestigial, cornified lamellae along the interior surface of the upper jaw from the level of the commissure rostrally to the midpoint of the external naris. The retention of these structures, other than unequivocally demonstrating the anseriform relationships of the screamers, also argue that these enigmatic birds are highly modified derivatives from equally derived aquatic, filter-feeding ancestors. These data strongly suggest that screamers do not represent a morphological intermediate between the organizational level observed in gallinaceous birds and water-fowl (Feduccia 1996). The retention of vestigial lamellae also suggests, though by no means proves, that screamers may in fact be among the most derived of anseriform birds, as opposed to the most basal.
Molecular Phylogeny vs. Morphology
As noted earlier, molecular analysis has consistently retrieved a monophyletic Galloanserae, using a variety of means (e.g., DNA-DNA hybridization, nuclear DNA sequencing). From a molecular standpoint, galloanserine monophyly is robustly corroborated by the available data, though Ericson et al. (2001) did bring molecular data to bear in their initial conclusion that Galloanserae is not monophyletic. As reviewed herein, while morphological data have been cited in defense of galloanserine monophyly, a number of the characters are highly suspect and the majority of the morphological data strongly contradict any close relationship between Galliformes and Anseriformes. The question thus becomes, which data set is correct? If the molecular phylogenies are accurate then they must explain why both the morphology of the anserine birds and galliform birds are widely disparate and in no way similar, and why the fossil record of Anseriformes fails to corroborate the monophyly of Galloanserae (Olson & Feduccia 1980b, Olson 2002). On the other hand, if the morphological and paleontological data are correct, the recovery of a monophyletic Galloanserae using molecular methods must also be explained.
The DNA-DNA hybridization technique used by Sibley, Ahlquist & Monroe (1988) and Sibley & Ahlquist (1990) has been extensively criticized on methodological grounds (e.g., Gill & Sheldon 1991) and several researchers have concluded that DNA-DNA hybridization studies are only sufficient for clarifying relationships below the ordinal level (Prager & Wilson 1980, Houde 1987b, Feduccia 1996). Given that Sibley and his colleagues work with DNA-DNA hybridization is a simple measure of general similarity--ergo, phenetic--it is at odds with the consensus viewpoint in systematic research that only uniquely derived characters, synapomorphies, convey phylogenetic information (Hennig 1966). Thus cladists often question the utility of DNA-DNA hybridization on matters of principle, and are in this case warranted in their skepticism. When we further consider that the fundamental basis of these studies is the uniform rate of molecular evolution--for which there is not a single scrap of evidence--the DNA-DNA hybridization data supporting galloanserine monophyly becomes yet more questionable. Last but certainly not least consider that this method has several times recovered highly discordant groupings that are at odds with all morphological analysis (cladistic and otherwise), as for example in Sibley & Ahlquist's (1990) clustering of Aramidae (limpkins) with Heliornithidae (sungrebes). In total, these data strongly suggest that we view DNA-DNA hybridization results supporting galloanserine monophyly with great skepticism.
Sequencing of nuclear DNA, howeover, has also supported galloanserine monophyly (e.g., Chubb 2004). The use of nuclear DNA sequences in determining higher level avian systematics has been widespread in the past several years and has met with tremendous success, though there are numerous discrepancies with morphological analyses (e.g., Mayr et al. 2003) which have yet to be reconciled. More importantly, most of the nuclear DNA sequencing work has confirmed the monophyly of multiple orders and other taxa and in this is congruent with morphological data gathered by other researchers (Chubb 2004). Such congruence between morphological and molecular data suggest that, though not without exception, nuclear DNA sequencing often is a reliable means of analyzing the phylogenetic relationships of birds. Given this conclusion, the discrepancy between the morphological data in the case of galloasnerine monophyly and that yielded by the sequencing of nuclear DNA, remains to be satisfactorily explained.
There are two conceivable methods by which one might deal with this anomaly, either dismissing the alleged disparity entirely, or demonstrating some methodological flaw in the nuclear DNA sequencing analyses. Neither of these courses seem to be permitted by the available data. In truth, there is no clear reason for this discrepancy. Given the likelihood of a very rapid Paleocene adaptive radiation of neornithine birds (e.g., Feduccia 2003) or neoavian (sensu Sibley & Ahlquist 1990) taxa (Chubb 2004), it is possible that the rapid divergence from their respective phyletic nodes of both Anseriformes and Galliformes has obscured their true phylogenetic relationships. A similar proposal appears to have been made by Ericson et al. (2001), but until conclusively demonstrated, such a suggestion remains speculative and the nuclear DNA sequencing data remain the strongest support for galloanserine monophyly.
Interestingly enough, however, even the molecular data are not fully concordant on the monophyly of Galloanserae. Ericson et al. (2001) sequenced the single copy nuclear gene c-myc and using this gene did not recover a monophyletic Galloanserae; rather Anseriformes fell out with Ciconiiformes, Phoenicopteriformes, and Charadriiformes. The data from c-myc sequencing strongly support a shorebird derivation of ducks and have not been properly considered in subsequent analyses. Sequencing analyses using mitochondrial DNA strands (e.g., Mindell et al. 1997, 1999), though heavily criticized, have also failed to retrieve a monophyletic galloanserine assemblage. Considering these data, it becomes apparent that though the majority of the molecular data support the monophyly of Galloanserae, by no means are all analyses concurrent in this assessment.
There are substantial morphological data which contradict the hypothesis that ducks and land-fowl are sister groups, and alleged synapormophies of this assemblage (i.e., Galloanserae) are in many cases a classic example of "junk-in, junk-out." Furthermore there are no coherent adaptive scenarios modelling the origin of the highly derived trophic specializations of Anseriformes from those of gallinaceous birds and until such time as a logical pseudophylogeny has been projected, the galloanserine hypothesis must be regarded with skepticism. The fossil record of the anseriform birds in no way corroborates the hypothesis that land-fowl are the closest relatives of ducks, and this fact has been all but overlooked by the more strident advocates of galloanserine monophyly. In many cases they have attempted to turn Presbyornis, an unequivocal charadriiform/anseriform mosaic, into a highly derived anatid with no similarity to shorebirds. Reality simply refuses to cooperate.
Molecular data, including that from DNA-DNA hybridization and nuclear DNA sequencing, continues to support galloanserine monophyly and thus the discrepancy between morphological and paleontological data and the molecular information, must be accounted for by either model for the origins of ducks or land-fowl. The DNA-DNA hybridization studies of Charles Sibley and his colleagues, as reviewed, are highly suspect on methodological grounds. Nuclear DNA sequencing is a far more reliable method that thus far does not appear to be significantly flawed methodologically, and thus its support for galloanserine monophyly remains paradoxical. Though the monophyly of Galloanserae cannot be conclusively dismissed at this time, there are important data suggesting alternative hypotheses (e.g., shorebird ancestry for Anseriformes) are at the very least, still credible and in need of further examination.
In a recent editorial, [The Phylogenetic Status of Presbyornis] my confrere GFA, from here at the wiki, has challenged one of the principal conclusions of this essay--that Presbyornis is a shorebird-duck mosaic. While the crux of his post--that two cladistic anaylses have defended the alternative viewpoint expounded therein is beyond dispute, his article fails to address the major concerns elucidated herein. Problems of character validity and homology are not addressed, nor is a pseudophylogeny projected. For replies to Cracraft (1981) I refer the reader to Olson's (1982) review of Cracraft's '81 article.
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