The superorder Palaeognathae is restricted to the order Struthioniformes comprising the following families:
Genus-level timetree of extant 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). Divergence times follow Wang et al. (2019), and Kuhl et al. (2021), which differ significantly from Prum et al. (2015). The intrinsic phylogeny of Tinamidae follows Almeida et al. (2021). It should be noted, however, that while protein-coding nuclear gene sequences indeed pointed to tinamous and casuariids as sister clades, phylogenetic reconstructions based on non-coding nuclear DNA and mitogenomes, respectively, recovered Tinamidae and Rheidae as sister clades (Wang et al., 2019). Consequently, Simmons et al. (2021) consider palaeognath phylogeny still unsettled.
Tinamidae (tinamous) are remarkable for being the only volant birds in a group of otherwise cursorial birds. The flying ability, however, is very poor. To date, the phylogenetic position of tinamous within the order couldn’t be resolved with certainty.
Struthionidae (ostriches) are traditionally regarded as a single species, Struthio camelys. However, mitochondrial DNA seems to justify the distinction of two species, Struthio camelus (Common Ostrich) and Struthio molybdanes (Somali Ostrich). The two species are thought to have separated from each other by the formation of the East African Rift Valley some 4 Ma. Reaching a top speed of 70 km/h (45 mph), ostriches are the fastest running birds in the world.
Extant ostriches are restricted to Africa, while the remaining palaeognaths (Notopalaeognathae) occur in South America, Australia, and New Zealand.
All palaeognaths, except the Struthionidae, exhibit paternal care, with incubation of the eggs relying entirely on the male (Birchard et al., 2013). In ostriches the hen incubates during the day, and the clock during the night. Paternal care is a fairly unusual reproductive trait that is considered to be restricted to species where relating opportunities are rare for both species and particularly scarce for males (Owens, 2002).
Palaeognaths comprise a number of interesting fossil taxa:
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