The superorder Palaeognathae contains only the order Struthioniformes with the following families:
Timetree of Palaeognathae based on Prum et al. (2015), and Kuhl et al. (2021), with the distribution of each family being indicated by the colour-code used throughout this website (see Distribution code). The divergence times follow Kuhl et al. (2021), and differ significantly from Prum et al. (2015). The intrinsic phylogeny of Tinamidae represents the molecular tree in Bertelli & Porzecanski (2003) and partly Wang et al. (2019) .
Tinamidae (tinamous) are restricted to Central and South American, with 46 species in nine genera. Two subfamilies are distinguished: Rhynchotinae (Field Tinamous) and Tinaminae (Field Tinamous). Tinamous are remarkable for being the only volant birds in a group of otherwise flightless birds. However, the flying ability is very poor.
Casuariidae comprise two genera, Dromaius (emus) with only a single species Dromaius novaehollandiae (Emu), and Casuarius (cassowaries) with three species.
Struthionidae (Ostriches) are represented by a single genus, Struthio, restricted to Africa:
Traditionally regarded as a single species, Struthio camelus (Ostrich), mitochondrial DNA data seem to justify the distinction of two species, Struthio camelus (Common Ostrich) and Struthio molybdophanes (Somali Ostrich). Common and Somali ostriches are thought to have been sparated by the formation of the East African Rift Valley some 4 mya. Reching a top spead of about 70 km/h, or 45 mph, ostriches are the fastest running birds in the world.
Palaeognaths are the most intriguing and enigmatic clade of birds. Unfortunately, it turned out to be extremely difficult to reconstruct their phylogenetic interrelationships. The only consistent result obtained by molecular data is that Struthionidae branched off first. The intrinsic relationships among the ramining families, the Notopalaeognathae, are still obscure.
Within palaeognaths, only Tinamidae are capable of flight whereas other palaeognaths are strictly cursorial. Therefore, the ability to fly either must have been lost several times, or must have been gained independently from neognathous birds by tinamous. It should be noted, however, that tinamous are extremely poor and reluctant flyers, unable to fly for extended periods. Thus, it remains unclear whether ancestral Palaeognathae were volant.
Extant ostriches are restricted to Africa, while notopalaeognaths occur in South America, Australia, and New Zealand. The simplest explanation for this disjunct distribution is to assume that the ancestral notopalaeognath once happened to enter South America, either via Africa or via Central America.
All palaeognaths, with the exception of Struthionidae, show paternal care, with incubation of the eggs relying entirely on the male (Birchard et al., 2013, fig.2). Paternal care is a fairly unusual reproductive trait that occurs in species where remating opportunities are rare for both sexes and particularly scarce for males (Owens, 2002). In ostriches, on the other hand, the hen incubates during the day and the cock during the night.
Palaeognaths comprise a number of interesting fossil taxa:
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