Struthioniformes is an order of birds with only a single extant family,

Struthionidae Struthionidae (; ) is a family of flightless birds, containing the extant ostriches and their extinct relatives. The two extant species of ostrich are the common ostrich and Somali ostrich, both in the genus ''Struthio'', which also contains seve ...

, containing the

ostriches Ostriches are large flightless birds of the genus ''Struthio'' in the order Struthioniformes, 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 ...

. Several other extinct families are known, spanning across the Northern Hemisphere, from the Early

Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...

to the early

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58flightless Flightless birds are birds that through evolution lost the ability to fly. There are over 60 extant species, including the well known ratites (ostriches, emu, cassowaries, rheas, and kiwi) and penguins. The smallest flightless bird is the ...

forms like the Paleotididae,

Geranoididae Geranoididae is a clade of extinct birds from the early to late Eocene and possibly early Oligocene of North America and Europe. These were mid-sized, long-legged flightless birds.Gerald Mayr (2009). Paleogene Fossil Birds Recent research shows ...

Eogruidae Eogruidae (also spelled Eogruiidae in some publications) is a family of large, flightless birds that inhabited Asia from the Eocene to Pliocene epochs. Related to modern ostriches, it was formerly thought to be related to cranes, limpkins and trum ...

and

Ergilornithidae Eogruidae (also spelled Eogruiidae in some publications) is a family of large, flightless birds that inhabited Asia from the Eocene to Pliocene epochs. Related to modern ostriches, it was formerly thought to be related to cranes, limpkins and tr ...

, the latter two thought to be closely related to Struthionidae.

Evolutionary history

According to Mayr and Zelenkov (2021), all Struthioniformes are united by the following characters: "a very long and narrow

tarsometatarsus The tarsometatarsus is a bone that is only found in the lower leg of birds and some non-avian dinosaurs. It is formed from the fusion of several bones found in other types of animals, and homologous to the mammalian tarsus (ankle bones) and meta ...

with short trochleae for the second and fourth toes, a tubercle next to the pons supratendineus on the distal end of the

tibiotarsus The tibiotarsus is the large bone between the femur and the tarsometatarsus in the leg of a bird. It is the fusion of the proximal part of the tarsus with the tibia. A similar structure also occurred in the Mesozoic Heterodontosauridae. These sm ...

, as well as a shortening of all non-ungual

phalanges The phalanges (singular: ''phalanx'' ) are digital bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones. ...

of the fourth toe except for the proximal one" All known members of the group are thought to have been flightless. Struthioniformes were widely distributed in the Northern Hemisphere during the

, including Paleotididae from Europe, and

from North America, and

and

in Asia. Ergilornthids would persist in Asia into the Early

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Struthionidae Struthionidae (; ) is a family of flightless birds, containing the extant ostriches and their extinct relatives. The two extant species of ostrich are the common ostrich and Somali ostrich, both in the genus ''Struthio'', which also contains seve ...

, the

sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and t ...

of Ergilornithidae, likely originated in Asia. Ostriches first appeared in Africa during the early Miocene, around 21 million years ago, before dispering into Eurasia during the late Miocene, beginning around 12 million years ago.

Taxonomy

After Mayr, and Zelenkov (2021) * Palaeotididae (Early mid-Eocene, Europe) ** ''

Palaeotis ''Palaeotis'' is a genus of paleognath birds from the middle Eocene epoch of central Europe. One species is known, ''Palaeotis weigelti''. The holotype specimen is a fossil tarsometatarsus and phalanx. Lambrect (1928) described it as an extinct ...

'' ** '' Galligeranoides'' *

(Early-mid Eocene, North America) * Unnamed clade **

(monotypic) *** '' Eogrus'' (mid-Late Eocene, Asia) ** '' Sonogrus'' (late Eocene, Asia) ** Unnamed clade *** '' Proergilornis'' (late Eocene, Mongolia) *** Unnamed clade ****

(Late Eocene-Early Pliocene, Asia) ****

(Early Miocene-Recent, Afro-Eurasia)

Gallery

References

{{reflist Bird orders Taxa named by John Latham (ornithologist) Ratites