Ostriches first appeared during the Miocene epoch, though various Paleocene, Eocene, and Oligocene fossils may also belong to the family. Ostriches are classified in the ratite group of birds, all extant species of which are flightless, including the kiwis, emus, and rheas. Traditionally, the order Struthioniformes contained all the ratites. However, recent genetic analysis has found that the group is not monophyletic, as it is paraphyletic with respect to the tinamous, so the ostriches are classified as the only members of the order.
The earliest fossils of ostrich-like birds are Paleocene taxa from Europe.Palaeotis and Remiornis from the Middle Eocene and unspecified ratite remains are known from the Eocene and Oligocene of Europe and Africa. These may have been early relatives of the ostriches, but their status is questionable, and they may in fact represent multiple lineages of flightless paleognaths. The African Eremopezus, when not considered a basal secretarybird or shoebill, is sometimes considered an ostrich relative or an "aepyornithid-like" taxon. Apart from these enigmatic birds, the fossil record of the ostriches continues with several species of the modern genus Struthio, which are known from the Early Miocene onwards. Several of these fossil forms are ichnotaxa (that is, classified according to the organism's footprints or other trace rather than its body) and their association with those described from distinctive bones is contentious and in need of revision pending more good material. While the relationship of the African fossil species is comparatively straightforward, many Asian species of ostriches have been described from fragmentary remains, and their interrelationships and how they relate to the African ostriches are confusing. In China, ostriches are known to have become extinct only around or even after the end of the last ice age; images of ostriches have been found there on prehistoric pottery and petroglyphs.
Ostriches have co-existed with another lineage of flightless didactyl birds, the eogruids. Though Olson 1985 classified these birds as stem-ostriches, they are otherwise universally considered to be related to cranes, any similarities being the result of convergent evolution. Competition from ostriches has been suggested to have caused the extinction of the eogruids, though this has never been tested and both groups do co-exist in some sites.
As with most other ratites, ostriches are believed to have developed flightlessness in the wake of the Cretaceous-Paleogene extinction event of about 66 Ma. The sudden absence of non-avian dinosaurs opened ecological niche space for large herbivorous animals and limited predatory threats, prompting volant ancestral paleognaths dispersed across the southern hemisphere to convergently lose the capability for flight.
In 2019, the species S. pannonicus, S. dmanisensis (the giant ostrich), and S. transcaucasicus were transferred to the genus Pachystruthio.
Distribution and habitat
A male Somali ostrich in a Kenyan savanna, showing its blueish neck
Today ostriches are only found natively in the wild in Africa, where they occur in a range of open arid and semi-arid habitats such as savannas and the Sahel, both north and south of the equatorial forest zone. The Somali ostrich occurs in the Horn of Africa, having evolved isolated from the common ostrich by the geographic barrier of the East African Rift. In some areas, the common ostrich's Masai subspecies occurs alongside the Somali ostrich, but they are kept from interbreeding by behavioral and ecological differences. The Arabian ostriches in Asia Minor and Arabia were hunted to extinction by the middle of the 20th century, and in Israel attempts to introduce North African ostriches to fill their ecological role have failed. Escaped common ostriches in Australia have established feral populations.
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