The case for palaeognath monophyly
From EvoWiki
The implication that belief in a polyphyletic Palaeognathae is widespread in ornithology seems a rather odd thing to argue in view of the technical literature on this area - and there is an awful lot of it, too much to summarise here. For the record, 'ratite' is not synonymous with "palaeognath". Palaeognathae as presently conceived = Tinamiformes/Tinamidae + Ratitae (plus some ostensible basal palaeognaths that I won't discuss here; see Kurochkin, 1999, Hope, 2002).
While it has been proposed various times that ratites may have descended from specific neognaths (e.g., Olson, Feduccia and others have proposed that ostriches were ergilornithid gruiform derivatives; Olson suggested that kiwis might be derived ibises; both Olson and Feduccia have at times intimated that moa may be derived from anseriforms), little has been proposed in defence of this view. All recent studies that have examined ratite affinities find them to be monophyletic. Among the more important of these works include; Bock (1963) on cranial anatomy, Stapel et al(1984) on amino acid sequences of eye lens proteins, Cracraft (1974), Bledsoe (1988) and Mayr and Clarke (2003) on morphology, Cooper and Penny (1997) and van Tuinen et al. (1998) on DNA, and Lee et al. (1997) on both molecular and morphological data. Notably, even certain authors who have hinted at a non-monophyletic Palaeognathae regard ratite monophyly as sound. Houde and Haubold (1987) for example strongly supported ratite monophyly in their study of Palaeotis weigelti:
- We do not concur with the idea that the paleognathous [sic] palate is homoplastic because palaeognathous [sic] birds are generally alike each other and differ from neognathous birds in a plethora of non-osteological characters: myology (Hofer 1950, McGowan 1982), rhamphothecal grooves (Parkes and Clark 1966), plumage as chicks (Pycraft 1900, Jehl 1971), pterylography (Parker 1864, Chandler 1916), spermatozoa (McFarlane 1971), gonads (Meier 1979), Bursa of Fabricius (Berens von Rautenfeld and Budras 1982), ocular pectin (Seemba and Mathers MS), cardiovascular system (Glenny 1965, Baumel 1968), osseous microvascularization (Amprino and Godina 1944, Zavatarri and Cellini 1956), cortical neuroanatomy (Craige 1935a, b, 1940a, b, Pearson 1972), Eustachian tubes (Hopkin 1906), coelomic cavities (Duncker 1979), proteins (Sibley 1960, Wilson et al. 1964, Miller and Feeney 1964, Kaplan 1965, Fitch and Margoliash 1967, Osuga and Feeney 1968, Feeney and Alison 1969, Gysels 1970, Sibley and Ahlquist 1972, Sibley and Frelin 1972, Krampitz et al. 1974, Ho et al. 1976, Prager et al. 1976, Stapel et al. 1984), DNA (Takagi et al. 1972, Takagi and Sasaki 1974, de Boer 1980, Sibley and Ahlquist 1981, 1985), behavior (Meise 1963), and parasites (Kellogg 1913). (p. 37).
While it's therefore been fun to suggest that different ratites arose from different assorted neognaths, the evidence is strongly against it. As for claims that ratites look so different from one another (especially in pelvic anatomy) that they simply must be polyphyletic, this isn't much of a valid criticism (compare the diversity in ratite pelvic anatomy with, e.g., piciform or passeriform cranial anatomy [the point being that ratites don't look so different from one another at all]), and it's countered by the twenty or so derived characters that unite ratites and support their monophyly (for which see, e.g., Lee et al., 1997).
Unfortunately the question of palaeognath monophyly is often confused with the issue of ratite paedomorphy. Ratites probably do exhibit heterochrony (some of their features can be interpreted as peramorphic: they don't just exhibit paedomorphosis - see Elzanowski, 1989, for a good response to the paedomorphic argument), but claims that this process horribly masks their morphology or muddies any interpretation of their affinities are naive given that they still exhibit plenty of good characters that can be evaluated empirically.
The idea that tinamous might be allied to galliforms, rather than to ratites, is old; Lesson in 1831 was one of the first authors to propose this and it was later favoured by Seebohm, Goodchild, Chandler, Dubinin and Verheyen. Even Huxley (1867) placed tinamous within his Carinatae (though he did admit that they were 'the most struthious of all carinate birds' (p. 426)). However, the view that tinamous are closer to ratites, and that any similarities with galliforms are either convergent or not indicative of a special affinity, is also old and, historically, equally as popular if not more so. Pycraft (1900) was the first to include tinamous within Palaeognathae, and he placed them closer to rheids, dinornithids and aepyornithids than to other ratites; Stresemann in 1927 attributed similarities between tinamous and galliforms to convergence; Oliver in the 1940s regarded tinamous as volant kin of ratites; McDowell in 1948 treated tinamous as close relatives of rheids within a palaeognath clade; Zavattari and Cellini in 1956 regarded tinamous as close to ratites; Tyler and Simkiss in 1959 found exclusive egg shell similarities between tinamous and ratites, and so on.
Recent studies on palaeognath affinities support an affiliation of tinamous with ratites, and not with any neognath group. Among the more significant of these include Prager et al. (1976), Lee et al. (1997) and Mayr and Clarke (2003). Groth and Barrowclough (1999) on the nuclear RAG-1 gene - the study that led Ericson et al. (2001) to reject their hypothesis of non-monophyly of Galloanseres - found support for a monophyletic Palaeognathae. This work is also significant in recovering the palaeognath-neognath split as the basal dichotomy within Neornithes, something not recovered in all molecular studies (e.g., Mindell et al., 1997, Harlid and Arnason, 1999); the latter (and I am talking about the studies in which passeriforms are found to be most basal neornithines) is widely thought due to inadequate taxon sampling and long- branch attraction. Bock (1963) and Starck (1995) concluded from skull anatomy that tinamous and ratites formed a clade, McGowan (1984) supported affiliation of tinamous with ratites based on tarsal morphology and Gussekloo and colleagues, while questioning the validity of some of the morphological evidence supporting palaeognath distinction (Gussekloo and Zweers, 1999, Gussekloo et al., 2001), still find that palaeognaths are distinct from neognaths, more basal than neognaths, and that 'the Tinamidae are the closest relatives of the ratites' (Gussekloo and Zweers, 1999, p. 39). Palaeognath monophyly has also been upheld and supported by Elzanowski (1995), Kurochkin (1995, 1999), van Tuinen et al. (2000), Dyke (2001, 2003), Livezey and Zusi (2001), Cracraft (1981, 1988, 2001), Cracraft and Clarke (2001), Ericson et al. (2001), Bock (2002) and others. Despite his apparent championing of palaeognath non-monophyly, Feduccia has repeatedly published a phylogram in which palaeognath monophly is unambiguously supported (Feduccia, 1994, p. 180; Feduccia, 1995, p. 637; Feduccia, 1996, p. 172; Feduccia 2003, p. 173). I therefore conclude that even he supports palaeognath monophyly on at least some days of the week.
As for lithornithids, Houde's work was revisited by Dyke during his tenure at the AMNH under Cracraft. Dyke (2003) included tinamous, ratites and lithornithids in an analysis based mostly on the characters used by Lee et al. (1997), but failed to use the new characters introduced by Houde (1986, 1988). The results concurred exactly with Houde's: lithornithids are sister to some or all ratites, closer to ratites than are the tinamous.
Contrary to the idea that palaeognath polyphyly is popular, well supported, or thriving then, palaeognath monophyly - specifically, the existence of a neornithine clade that includes tinamous and ratites to the exclusion of other neornithines - is well corroborated and, in fact, the consensus view.
References
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External Links
The Evolution of the Palaeognathous Birds: Functional Morphology and Evolutionary Patterns - 182 page disseration from S.W.S. Gussekloo.
Acknowledgments
Darren Naish. Originally posted to the USC Dinosaur Listserve, and reprinted with permission by the author.
