Palaeognathae (/ˌpæliˈɒɡnəθi/; from Ancient Greek παλαιός (palaiós) 'old' and γνάθος (gnáthos) 'jaw') is an infraclass[1] of birds, called paleognaths or palaeognaths, within the class Aves of the clade Archosauria.

[citation needed] 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.

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.

[10] 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.

[11] 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.

McDowell (1948) asserted that the similarities in the palate anatomy of paleognaths might actually be neoteny, or retained embryonic features.

[13] Hope (2002) reviewed all known bird fossils from the Mesozoic looking for evidence of the origin of the evolutionary radiation of the Neornithes.

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.

[3][5][19][20] 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.

He established seven roughly modern orders of living and fossil paleognaths (Casuarii, Struthiones, Rheae, Dinornithes, Aepyornithes, Apteryges, and Crypturi – the latter his term for tinamous, after the Tinamou genus Crypturellus).

The Palaeognathae are usually considered a superorder, but authors have treated them as a taxon as high as subclass (Stresemann 1927–1934) or as low as an order (Cracraft 1981 and the IUCN, which includes all paleognaths in an expanded Struthioniformes[22]).

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".

[23] 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.

This bone, the sternum, has a central blade (the Carina sterni), with two long, slender lateral trabeculae, which curve to either side and nearly touch the keel posteriorly.

These trabeculae may also be thought of as the rims of two large foramina that incise the posterior edge of the sternum, and extend almost its whole length.

Kiwis are exceptional, however, and have large brains comparable to those of parrots and songbirds, though evidence for similar levels of behaviour complexity is currently lacking.

[citation needed] Today, the ratites are largely restricted to the Southern Hemisphere, though across the Cenozoic they were also present in Europe, North America and Asia.

[10] Cracraft (2001) gave a comprehensive review to the data and strongly supported the Gondwana vicariance hypothesis with phylogenetic evidence and historical biogeography.

Moreover, Cracraft synthesizes the morphological and molecular studies, noting conflicts between the two, and finds that the bulk of the evidence favors paleognath monophyly.

[35] Geological analyses have suggested that New Zealand may have been entirely under water as recently as 28 Mya, making it impossible for flightless birds to have survived.

[citation needed] However, the discovery of a Sphenodon fossil dating to the Early Miocene 19–16 Mya raises question as to whether the island mass was completely submerged.

This finding offers further evidence that ancient Sphenodon species lived on some portion of the land mass since it separated from Gondwana approximately 82 Mya.

[37][38] Feduccia (1995) emphasized the extinction event at the Cretaceous-Paleogene boundary 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.

[40] Houde demonstrated that the Lithornithiformes, a group of flying birds that were common in the Cenozoic 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; this claim at least has been vindicated by the discovery of the possibly volant Proapteryx.

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.

After Homo appeared and left Africa for other continents, they continued to encounter ostriches in Arabia and much of southern and central Asia.