Ostrich
Struthio is a genus of birds in the order Struthioniformes, whose members are the ostriches. It is part of the infra-class Palaeognathae, a diverse group of flightless birds also known as ratites that includes the emus, rheas, and kiwis. There are two living species of ostrich: the common ostrich and the Somali ostrich. They are large flightless birds of Africa who lay the largest eggs of any living land animal. With the ability to run at 70 km/h (43.5 mph), they are the fastest birds on land. They are farmed worldwide, particularly for their feathers as they  are used as decoration and feather dusters. Their skin is also used for leather products. Ostriches are notable for being the heaviest living birds. Taxonomic history. The genus "Struthio" was first described by Carl Linnaeus in 1758. The genus was used by Linnaeus and other early taxonomists to include the emu, rhea, and cassowary, until they each were placed in their own genera. The Somali ostrich ("Struthio molybdophanes") has recently become recognized as a separate species by most authorities, while others are still reviewing the evidence. Evolution. 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 earliest fossils from this genus are from the early Miocene (20–25 mya [million years ago]) and are from Africa, so it is proposed that they originated there. Then by the middle to late Miocene (5–13 mya) they had spread to Eurasia. By about 12 mya they had evolved into the larger size of  which we are familiar. By this time they had spread to Mongolia and later southern Africa. While the relationship of the African fossil species is comparatively straightforward, many Asian species of ostrich 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. "Struthio" ostriches once co-existed with another lineage of flightless didactyl birds, the eogruids. Though Olson  1985 classified these birds as stem-ostriches, they until recently were universally considered to be related to cranes, any similarities being the result of convergent evolution. More recent analysis, however, has once again linked the ostriches to the eogruids. 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. Distribution and habitat. 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. Species. In 2008, "S. linxiaensis" was transferred to the genus "Orientornis." Three additional species, "S. pannonicus", "S. dmanisensis", and "S. transcaucasicus", were transferred to the genus "Pachystruthio" in 2019. Several additional fossil forms are ichnotaxa (that is, classified according to the organism's trace fossils such as footprints rather than its body) and their association with those described from distinctive bones is contentious and in need of revision pending more good material. The species are: