Palaeognathae
Palaeognathae is an infraclass of birds, called paleognaths or palaeognaths, within the class Aves of the clade Archosauria. It is one of the two infraclasses of birds, the other being Neognathae, both of which form Neornithes. Palaeognathae contains five extant orders consisting of four flightless lineages, termed ratites, and one flying lineage, the Neotropic tinamous. There are 47 species of tinamous, five of kiwis, three of cassowaries, one of emus , two of rheas and two of ostriches. 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 that are undisputed members of Palaeognathae: the Lithornithiformes, the Dinornithiformes and the Aepyornithiformes, the latter two of which became extinct in the last 1250 years. 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.
Origin and evolution
No unambiguously paleognathous fossil birds are known until the Cenozoic, but there have been many reports of putative paleognaths, 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, 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.image:Pseudocrypturus.jpg|thumb|left|250px|Pseudocrypturus cercanaxius fossil cast, Copenhagen Zoological Museum
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 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.
Before the advent of genetic analysis, some 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 asserted that the similarities in the palate anatomy of paleognaths might actually be neoteny, or retained embryonic features. He noted that there were other features 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 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 stage of Late Cretaceous Antarctica. Vegavis is most closely related to true ducks. Because virtually all phylogenetic analyses predict that ducks diverged after paleognaths, this is evidence that paleognaths had already arisen well before that time.
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 Eocene of 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 lithornithiforms + tinamous were sister to the other paleognaths. They concluded that all ratites, therefore, were monophyletic, descending from one common ancestor that became flightless. They also interpret the paleognath-like Limenavis, from late Cretaceous Patagonia, as possible evidence of a Cretaceous and monophyletic origin for paleognaths.
Mysterious large eggs from the Pliocene of Lanzarote in the Canary Islands have been attributed to ratites.
An ambitious genomic analysis of the living birds was performed in 2007, and it contradicted Leonard et al.. It found that tinamous are not primitive within the paleognaths, but among the most advanced. This requires multiple events of flightlessness within the paleognaths and partially refutes the Gondwana vicariance hypothesis. The study looked at DNA sequences from 19 loci in 169 species. It recovered evidence that the paleognaths are one natural group, 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. This 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 monophyletic, but the ratites are not.
Beginning in 2010, DNA analysis studies have shown that tinamous are the sister group to extinct moa of New Zealand.
A 2020 molecular study of all bird orders found paleognaths and neognaths to have diverged in the Late Cretaceous or earlier, before 70 million years ago. However, all modern paleognath orders only originated in the latest Paleocene and afterwards, with ostriches diverging in the latest Paleocene, rheas in the early Eocene, kiwis very shortly after in the early Eocene, and finally Casuariiformes and tinamous diverging from one another in the mid-Eocene.
History of classifications
In the history of biology there have been many competing taxonomies of the birds now included in the Palaeognathae. The topic has been studied by Dubois, Sharpe, Shufeldt, Sibley and Ahlquist and Cracraft.Merrem is often credited with classifying the paleognaths together, and he coined the taxon "Ratitae". However, Linnaeus placed cassowaries, emus, ostriches, and rheas together in Struthio. Lesson added the kiwis to the Ratitae. Parker reported the similarities of the palates of the tinamous and ratites, but Huxley is more widely credited with this insight. Huxley still placed the tinamous with the Carinatae of Merrem because of their keeled sterna, and thought that they were most closely related to the Galliformes.
Pycraft presented a major advance when he coined the term Palaeognathae. He rejected the Ratitae-Carinatae classification that separated tinamous and ratites. He reasoned that a keelless, or "ratite", sternum could easily evolve in unrelated birds that independently became flightless. He also recognized that the ratites were secondarily flightless. His subdivisions were based on the characters of the palatal skeleton and other organ systems. He established seven roughly modern orders of living and fossil paleognaths.
The Palaeognathae are usually considered a superorder, but authors have treated them as a taxon as high as subclass or as low as an order. Palaeognathae was defined in the PhyloCode by George Sangster and colleagues in 2022 as "the least inclusive crown clade containing Tinamus major and Struthio camelus".
Cladistics
based on Mitchell with some clade names after Yuri et al. Yuri et al. named the clades Notopalaeognathae and Novaeratitae, the former defined in the PhyloCode by Sangster et al. as "the least inclusive crown clade containing Rhea americana, Tinamus major, and Apteryx australis", while the latter also defined in the PhyloCode by Sangster et al. as "the least inclusive crown clade containing Apteryx australis and Casuarius casuarius". Notopalaeognathae represents the grouping containing the majority of ratites with the exception of ostriches, and the clade Novaeratitae was named to support the relationship between kiwis, cassowaries, emus, and the extinct elephant birds.Cloutier, A. et al. in their molecular study places ostriches as sister to other palaeognaths, with the rhea as the sister lineage to the remaining, non-ostrich palaeognaths.
An alternative phylogeny was found by Kuhl, H. et al.. In this treatment, all members of Palaeognathae are classified in Struthioniformes, but they are still shown as distinct orders here.
Other studies have suggested that the relationships between the four main groups of non-ostrich palaeognaths are an effective polytomy, with only slightly more support for Novaeratitae over the alternative hypotheses of Apterygiformes+Aepyornithformes being more closely related to Rheiformes or to Tinamiformes+Dinornithformes. This lineage containing the sister relationship between tinamous and moas was given the clade name Dinocrypturi, being named and defined in the PhyloCode by Sangster et al. as "the smallest clade containing Tinamus major and Dinornis novaezealandiae".