Paleognathae

The Palaeognathae or paleognaths are one of the two living superorders of birds. The other living superorder is Neognathae. Together these two clades form the subclass Neornithes.

There are extinct orders: the Lithornithiformes, the Dinornithiformes (moas), and the Aepyornithiformes (elephant birds). There are other extinct orders which have been allied with the Palaeognathae by at least one author, but their affinities are a matter of dispute: the Ambiornithiformes, the Gansuiformes, the Paleocursornithiformes, the Gobipterygidae.

Biogeography
In the Cretaceous, the southern continents were connected, forming a single continent known as Gondwana. Gondwana is the crucial territory in a major scientific question about the evolution of Palaeognathae, and thus about the evolution of all of the Neornithes.

Did the paleognathes evolve once, from one ancestor, on Gondwana during the Cretaceous, and then ride on the daughter landmasses that became today's southern continents, or did they evolve after the Cretaceous-Tertiary extinction event from multiple flying ancestors on multiple continents around the world? The former is often called the Gondwana vicariance hypothesis. It is supported most strongly by molecular clock studies, but it is weakened by the lack of any Cretaceous or southern fossil paleognaths. The latter is called the Tertiary radiation hypothesis. This hypothesis is supported by molecular phylogeny studies and matches the fossil record, but it is weakened by morphological phylogenetic studies. Both hypotheses have been supported and challenged by many studies by many authors.

Gondwana vicariance hypothesis
Cracraft (2001) gave a comprehensive review to the data and strongly supported the Gondwana vicariance hypothesis with phylogenetic evidence and historical biogeography. He cites molecular clock studies that show a basal divergence date for neornithes being around 100 million years ago. He credits the authors of the molecular clock studies with the observation that the lack of southern paleognath fossils may correspond to the relatively scarce southern Cretaceous deposits, and the relative lack of paleontological field work in the southern hemisphere. Moreover, Cracraft synthesiszes the morphological and molecular studies, noting conflicts between the two, and finds that the bulk of the evidence favors paleognath monophyly. He also notes that not only the ratites, but other basal groups of neognathous birds, show trans-Antarctic distribution, as we would expect if the paleognaths and neognaths had diverged in Gondwana.

Tertiary radiation hypothesis
Feduccia (1995) emphasized the K-T event as the probable engine of diversification in the Neornithes, picturing only one or very few lineages of birds surviving the end of the Cretaceous. He also noted that birds around the world had developed ratite-like anatomies when they became flightless, and saw the affinities of modern ratites, especially kiwis, as ambiguous. In this emphasis on the Tertiary, rather than Cretaceous period, as the time of basal divergences between neornithines, he follows Olson.

Houde demonstrated that the Lithornithiformes, a group of flying birds that were common in the Tertiary of the northern hemisphere, were also paleognaths. He argues that the lithornithiform bird Paleotis, known from fossils in Denmark (northern hemisphere), shared unique anatomical features of the skull that make it a member of the same order as the ostriches. He also argued that the kiwis should not have reached New Zealand, which moved away from the mainland in the Early Cretaceous, if their ancestor was flightless. He therefore deduced that Lithornithiform ancestors could have reached the southern continents some 30 to 40 million years ago, and evolved flightless forms which are today's ratites. This hypothesis is contradicted by some later molecular studies (Cooper 1997), but supported by others (Harshman et al. 2008) (see below).

Evolution
No unambiguously paleognathous fossil birds are known until the Cenozoic, but there have been many reports of putative paleognathes, and it has long been inferred that they may have evolved in the Cretaceous.

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 KT Boundary.

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 Maastrichtian period of Late 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 then.

An exceptionally preserved specimen of the extinct flying paleognathe Lithornis was published by Leonard et al. in 2005. It is an articulated and nearly complete fossil from the early Eoceneof Denmark, and thought to have the best preserved lithornithiform skull ever found. The authors concluded that Lithornis was a close sister taxon to tinamous, rather than ostriches, and that the lithorniforms + tinamous were the most basal paleognaths. They concluded that all ratites, therefore, were monophyletic, descending from one common ancestor that became flightless. They also interpret Limenavis, from Late Cretaceous Patagonia, as evidence of a Cretaceous and monophyletic origin for paleognathes.

An ambitious genomic analysis of the living birds was performed in 2007, and it contradicted Leonard et al. (2005). It found that tinamous are not primitive within the paleoganthes, but among the most advanced. This requires multiple events of flightlessness within the paleognathes and partially refutes the Gondwana Vicariance Hypothesis ( see above ). The study looked at DNA sequences from 19 loci in 169 species. It recovered evidence that the Paleognathes are one natural group (monophyletic), and that their divergence from other birds is the oldest divergence of any extant bird groups. It also placed the tinamous within the ratites, more derived than ostriches, or rheas and as a sister group to emus and kiwis, and this makes ratites paraphyletic.

A related study addressed the issue of paleognath phylogeny exclusively. It used molecular analysis and looked at twenty unlinked nuclear genes. It study concluded that there were at least three events of flightlessness that produced the different ratite orders, that the similarities between the ratite orders are partly due to convergent evolution, and that the Palaeognathae are polyphyletic.

Other authors have 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. One point is that unrelated birds have developed somewhat ratite - like anatomies multiple times around the world through convergent evolution. Mc Dowell (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 the mechanism by which various birds became flightless and came to look similar to one another.