Palaeognathae

Palaeognathae, or paleognaths is one of the two living clades of birds – the other being Neognathae. Together, these two clades form the clade Neornithes. Palaeognathae contains five extant branches of flightless lineages (plus two extinct clades), termed ratites, and one flying lineage, the Neotropic tinamous. There are 47 species of tinamous, 5 of kiwis (Apteryx), 3 of cassowaries (Casuarius), 1 of emus (Dromaius) (another became extinct in historic times), 2 of rheas (Rhea) and 2 of ostrich (Struthio). Recent research has indicated that paleognaths are monophyletic but the traditional taxonomic split between flightless and flighted forms is incorrect; tinamous are within the ratite radiation, meaning flightlessness arose independently multiple times via parallel evolution. There are three extinct groups, the Lithornithiformes, the Dinornithiformes (moas) and the Aepyornithiformes (elephant birds), that are undisputed members of Palaeognathae. There are other extinct birds which have been allied with the Palaeognathae by at least one author, but their affinities are a matter of dispute. The word Paleognath is derived from the ancient Greek for 'old jaws' in reference to the skeletal anatomy of the palate, which is described as more primitive and reptilian than that in other birds. Paleognathous birds retain some basal morphological characters but are by no means living fossils as their genomes continued to evolve at the DNA level under selective pressure at rates comparable to the Neognathae branch of living birds, though there is some controversy about the precise relationship between them and the other birds. There are also several other scientific controversies about their evolution (see below). No unambiguously paleognathous fossil birds are known until the Cenozoic (though birds occasionally interpreted as lithornithids occur in Albian appalachian sites ), but there have been many reports of putative paleognathes, and it has long been inferred that they may have evolved in the Cretaceous. Given the northern hemisphere location of the morphologically most basal fossil forms (such as Lithornis, Pseudocrypturus, Paracathartes and Palaeotis), a Laurasian origin for the group can be inferred. The present almost entirely Gondwanan distribution would then have resulted from multiple colonisations of the southern landmasses by flying forms that subsequently evolved flightlessness, and in many cases, gigantism. One study of molecular and paleontological data found that modern bird orders, including the paleognathous ones, began diverging from one another in the Early Cretaceous. Benton (2005) summarized this and other molecular studies as implying that paleognaths should have arisen 110 to 120 million years ago in the Early Cretaceous. He points out, however, that there is no fossil record until 70 million years ago, leaving a 45 million year gap. He asks whether the paleognath fossils will be found one day, or whether the estimated rates of molecular evolution are too slow, and that bird evolution actually accelerated during an adaptive radiation after the Cretaceous–Paleogene boundary (K–Pg boundary). Other authors questioned the monophyly of the Palaeognathae on various grounds, suggesting that they could be a hodgepodge of unrelated birds that have come to be grouped together because they are coincidentally flightless. Unrelated birds might have developed ratite-like anatomies multiple times around the world through convergent evolution. McDowell (1948) asserted that the similarities in the palate anatomy of paleognathes might actually be neoteny, or retained embryonic features. He noted that there were other feature of the skull, such as the retention of sutures into adulthood, that were like those of juvenile birds. Thus, perhaps the characteristic palate was actually a frozen stage that many carinate bird embryos passed through during development. The retention of early developmental stages, then, may have been a mechanism by which various birds became flightless and came to look similar to one another. Hope (2002) reviewed all known bird fossils from the Mesozoic looking for evidence of the origin of the evolutionary radiation of the Neornithes. That radiation would also signal that the paleognaths had already diverged. She notes five Early Cretaceous taxa that have been assigned to the Palaeognathae. She finds that none of them can be clearly assigned as such. However, she does find evidence that the Neognathae and, therefore, also the Palaeognathae had diverged no later than the Early Campanian age of the Cretaceous period. Vegavis is a fossil bird from the Albian period of Early Cretaceous Antarctica. Vegavis is most closely related to true ducks. Because virtually all phylogenetic analyses predict that ducks diverged after paleognathes, this is evidence that paleognathes had already arisen well before that time.