Paleognathae
Encyclopedia
The Palaeognathae or paleognaths are one of the two living superorders of bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s. The other living superorder is Neognathae
Neognathae
Neognaths are birds within the subclass Neornithes of the class Aves. The Neognathae include virtually all living birds; their sister taxon Palaeognathae contains the tinamous and the flightless ratites....

. Together these two clades form the subclass
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

 Neornithes.

Paleognath is a word derived from the ancient Greek for "old jaws" in reference to the skeletal anatomy of the palate
Palate
The palate is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but, in most other tetrapods, the oral and nasal cavities are not truly separate. The palate is divided into two parts, the anterior...

, which is described as more primitive and reptilian than that in other birds. Paleognathous birds are uncontroversially the most primitive, or basal, living birds, though there is some controversy about the precise relationship between them and the other birds. There are also several other scientific controversies about their evolution (see below).

This superorder contains four extant order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

s of flightless ratite
Ratite
A ratite is any of a diverse group of large, flightless birds of Gondwanan origin, most of them now extinct. Unlike other flightless birds, the ratites have no keel on their sternum—hence the name from the Latin ratis...

s and one order of flying tinamou
Tinamou
The tinamous are a family comprising 47 species of birds found in Central and South America. One of the most ancient living groups of bird, they are related to the ratites. Generally ground dwelling, they are found in a range of habitats....

s;

The order Tinamiformes (tinamous) includes nine living genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 and forty-seven species.

The Apterygiformes (kiwi
Kiwi
Kiwi are flightless birds endemic to New Zealand, in the genus Apteryx and family Apterygidae.At around the size of a domestic chicken, kiwi are by far the smallest living ratites and lay the largest egg in relation to their body size of any species of bird in the world...

s) include five species in one genus (Apteryx).

The Casuariiformes have two genera. The cassowaries
Cassowary
The cassowaries are ratites, very large flightless birds in the genus Casuarius native to the tropical forests of New Guinea, nearby islands and northeastern Australia. There are three extant species recognized today...

 (Casuarius) have three species and the emu
Emu
The Emu Dromaius novaehollandiae) is the largest bird native to Australia and the only extant member of the genus Dromaius. It is the second-largest extant bird in the world by height, after its ratite relative, the ostrich. There are three subspecies of Emus in Australia...

s (Dromaius
Dromaius
Dromaius is a genus of ratite present in Australia. There is one extant species, Dromaius novaehollandiae commonly known as the Emu.In his original 1816 description of the emu, Louis Jean Pierre Vieillot used two generic names; first Dromiceius, then Dromaius a few pages later...

) have one living and one recently extinct species.

The Rheiformes (rhea
Rhea (bird)
The rheas are ratites in the genus Rhea, native to South America. There are two existing species: the Greater or American Rhea and the Lesser or Darwin's Rhea. The genus name was given in 1752 by Paul Möhring and adopted as the English common name. Möhring's reason for choosing this name, from the...

s) have two genera with one species each.

The Struthioniformes (ostriches) have only one living species.

There are extinct orders: the Lithornithiformes
Lithornithiformes
Lithornithiformes is an extinct order of early paleognath birds. Lithornithiform birds are known from fossils dating to the Upper Paleocene through the Middle Eocene of North America and Europe. All are extinct today....

, the Dinornithiformes (moa
Moa
The moa were eleven species of flightless birds endemic to New Zealand. The two largest species, Dinornis robustus and Dinornis novaezelandiae, reached about in height with neck outstretched, and weighed about ....

s), and the Aepyornithiformes (elephant bird
Elephant bird
Elephant birds are an extinct family of flightless birds found only on the island of Madagascar and comprising the genera Aepyornis and Mullerornis.-Description:...

s). There are other extinct orders which have been allied with the Palaeognathae by at least one author, but their affinities are a matter of dispute: the Ambiornithiformes, the Gansuiformes, the Paleocursornithiformes, the Gobipterygidae.

Paleognath anatomy and characteristics

Paleognathes are named for a characteristic, complex architecture of the bones in the bony palate. Cracraft (1974) defined it with five characters.
  1. The vomer is large and articulates with the premaxillae and maxillopalatines anteriorly. Posteriorly the vomer fuses to the ventral surface of the pterygoid, and the palatines fuse to the ventral surface of this pterygovomer articulation.
  2. The pterygoid prevents the palatine from articulating medially with the basisphenoid.
  3. The palatine and pterygoid fuse into a rigid joint.
  4. The articulation on the pterygoid for the basipterygoid process of the basicranium is located near the articulation between the pterygoid and quadrate.
  5. The pterygoid–quadrate articulation is complex and includes the orbital process of the quadrate.


Paleognathes share similar pelvis anatomy. There is a large, open ilio–ischiatic fenestra in the pelvis. The pubis and ischium are likely to be longer than the ilium
Ilium (bone)
The ilium is the uppermost and largest bone of the pelvis, and appears in most vertebrates including mammals and birds, but not bony fish. All reptiles have an ilium except snakes, although some snake species have a tiny bone which is considered to be an ilium.The name comes from the Latin ,...

, protruding out beneath the tail. The postacetabular
Acetabulum
The acetabulum is a concave surface of the pelvis. The head of the femur meets with the pelvis at the acetabulum, forming the hip joint.-Structure:...

 portion of the pelvis is longer than the preacetabular portion.

Paleognathes share a pattern of grooves in the horny covering of the bill. This covering is called the rhamphotheca. The paleognath pattern has one central strip of horn, with long, triangular, strips to either side.

In Paleognathes the male incubates the eggs. The male may include in his nest the eggs of one female or more than one. He may also have eggs deposited in his nest by females that did not breed with him, in cases of nest parasitism. Only in Ostriches does the female also assist in incubating the eggs.

Tinamou anatomy and characteristics

The tinamou
Tinamou
The tinamous are a family comprising 47 species of birds found in Central and South America. One of the most ancient living groups of bird, they are related to the ratites. Generally ground dwelling, they are found in a range of habitats....

s of Central and South America are primarily terrestrial, though they fly weakly. Tinamous have very short tail feathers, giving them an almost tailess aspect. In general they resemble galliform birds like quails and grouse.
Tinamous have a very long, keeled, breastbone with an unusual three-pronged shape. This bone, the sternum
Sternum
The sternum or breastbone is a long flat bony plate shaped like a capital "T" located anteriorly to the heart in the center of the thorax...

, has a central blade (the Carina sterni), with two long, slender lateral trabeculae which curve to either side and nearly touch the keel posteriorly. These trabeculae may also be thought of as the rims of two large foramina that incise the posterior edge of the sternum, and extend almost its whole length. Tinamous have a proper semicircular furcula
Furcula
The ' is a forked bone found in birds, formed by the fusion of the two clavicles. In birds, its function is the strengthening of the thoracic skeleton to withstand the rigors of flight....

, with no trace of a hypocleidium. There is an acute angle between the scapula
Scapula
In anatomy, the scapula , omo, or shoulder blade, is the bone that connects the humerus with the clavicle ....

 and coracoid, as in all flying birds.
The pelvis has an open ilio–ischiadic fenestra that incises the posterior edge between the ilium
Ilium (bone)
The ilium is the uppermost and largest bone of the pelvis, and appears in most vertebrates including mammals and birds, but not bony fish. All reptiles have an ilium except snakes, although some snake species have a tiny bone which is considered to be an ilium.The name comes from the Latin ,...

 and ischium, as in all paleognathes. Tinamous have no true pygostyle
Pygostyle
Pygostyle refers to a number of the final few caudal vertebrae fused into a single ossification, supporting the tail feathers and musculature. In modern birds, the rectrices attach to these....

, their caudal vertebrae remain unfused, as in ratites.
Tinamou feathers look like those of volant birds in that they have a rachis and two vanes. The structure of tinamou feathers is unique, however, in that they have barbs that remain joined at their tips. Thus the parallel barbs are separated only by slits between them.
Tinamous have uropygial gland
Uropygial gland
The uropygial gland, informally known as the preen gland, is a gland found in the large majority of birds that secretes an oil that birds use for preening. The chief compounds of preen oil are diester waxes called uropygiols....

s.
Tinamous range in size from 8 to 21 in (20.3 to 53.3 cm) and weigh 1.4 to 80 oz (39.7 to 2,268 g).

Ratite anatomy and characteristics

Ratite birds are strictly flightless and their anatomy reflects specializations for terrestrial life. The term "ratite
Ratite
A ratite is any of a diverse group of large, flightless birds of Gondwanan origin, most of them now extinct. Unlike other flightless birds, the ratites have no keel on their sternum—hence the name from the Latin ratis...

" is from the Latin word for raft, ratis, because they possess a flat breastbone, or sternum
Sternum
The sternum or breastbone is a long flat bony plate shaped like a capital "T" located anteriorly to the heart in the center of the thorax...

, shaped like a raft. This characteristic sternum differs from that in flighted birds, where the pectoral musculature is disproportionately large to provide the power for wingbeats and the sternum develops a prominent keel, or carina sterni to anchor these muscles. The clavicle
Clavicle
In human anatomy, the clavicle or collar bone is a long bone of short length that serves as a strut between the scapula and the sternum. It is the only long bone in body that lies horizontally...

s do not fuse into a furcula. Instead, if present at all, each is splint-like and lies along the medial border of the coracoid, attached there by a coraco–clavicular ligament. There is an obtuse angle between the scapula
Scapula
In anatomy, the scapula , omo, or shoulder blade, is the bone that connects the humerus with the clavicle ....

 and coracoid, and the two bones fuse together to form a scapulocoracoid
Scapulocoracoid
The Scapulocoracoid is the unit of the pectoral girdle that contains the coracoid and scapula.The coracoid itself is a beak-shaped bone that is commonly found in most vertebrates with a few exceptions.The scapula is commonly known as the shoulder blade...

. Ratites have reduced and simplified wing structures and strong legs. Except in some Rhea wing feathers, the barb filaments that make up the vane
Pennaceous feather
Pennaceous feathers are also known as contour feathers. This type of feather is present in most modern birds, and has been shown in some species of maniraptoran dinosaurs....

s of the feathers do not lock tightly together, giving the plumage a shaggier look and making it unnecessary to oil their feathers. Adult ratites have no preen gland (uropygial gland
Uropygial gland
The uropygial gland, informally known as the preen gland, is a gland found in the large majority of birds that secretes an oil that birds use for preening. The chief compounds of preen oil are diester waxes called uropygiols....

) that contains preening oil.

Ratite sizes range from 10 inches (25.4 cm) to 9 feet (2.7 m) and weight can be from 2.86 to 345 lb (1.3 to 156.5 kg).

Ostriches are the largest struthioniforms (members of the Struthioniformes order), with long legs and neck. They range in height from 5.7 to 9 ft (1.7 to 2.7 m) and weigh from 139 to 345 lb (63 to 156.5 kg). They have loose-feathered wings. Males have black and white feathers while the female has grayish brown feathers. They are unique among birds in that they retain only the third and fourth toe on each foot. Ostriches' wings have claws,or ungual
Ungual
An ungual is a highly modified distal toe bone which ends in a hoof, claw, or nail. Elephants and other ungulates have ungual phalanges, as did the sauropods and horned dinosaurs. A claw is a highly modified ungual phalange.As an adjective, ungual means related to nail, as in periungual .-External...

s, on the first and second fingers, and on the third in some individuals. Ostriches differ from other paleognathes in that they have a reduced vomer bone of the skull.

Emus are about 6.5 feet (2 m) in height and weigh 51 to 120 lb (23.1 to 54.4 kg). They have long, strong legs and can run up to 30 mile per hour. They have short wings and the adults have brown feathers.

Rheas are 4.5 to 5.6 ft (1.4 to 1.7 m) and weigh 55 to 88 lb (24.9 to 39.9 kg). Their feathers are gray or spotted brown and white. They have large wings but no tail feathers. They have no clavicles.

Cassowaries are 3.3 to 5.6 ft (1 to 1.7 m) in height and weigh 30 to 130 lb (13.6 to 59 kg). They have rudimentary wings with black feathers and six stiff, porcupine-like, quills in the place of their primary and secondary feathers.

Kiwis are the smallest of ratites, ranging in height from 14 to 22 in (35.6 to 55.9 cm) and weight 2.6 to 8.6 lb (1.2 to 3.9 kg). They have shaggy brown feathers.

Biogeography

Today, the ratites are largely restricted to the Southern Hemisphere. In the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

, these southern continents were connected, forming a single continent called Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...

. Gondwana is the crucial territory in a major scientific question about the evolution of Palaeognathae, and thus about the evolution of all of the Neornithes.

Did the paleognathes evolve once, from one ancestor, on Gondwana during the Cretaceous, and then ride on the daughter landmasses that became today's southern continents, or did they evolve after the Cretaceous-Tertiary extinction event
Cretaceous-Tertiary extinction event
The Cretaceous–Paleogene extinction event, formerly named and still commonly referred to as the Cretaceous-Tertiary extinction event, occurred approximately 65.5 million years ago at the end of the Maastrichtian age of the Cretaceous period. It was a large-scale mass extinction of animal and plant...

 from multiple flying ancestors on multiple continents around the world? The former is often called the Gondwana vicariance hypothesis. It is supported most strongly by molecular clock studies, but it is weakened by the lack of any Cretaceous or southern fossil paleognaths. The latter is called the Tertiary radiation hypothesis. This hypothesis is supported by molecular phylogeny studies and matches the fossil record, but it is weakened by morphological phylogenetic studies. Both hypotheses have been supported and challenged by many studies by many authors.

Gondwana vicariance hypothesis

Cracraft (2001) gave a comprehensive review to the data and strongly supported the Gondwana vicariance hypothesis with phylogenetic evidence and historical biogeography. He cites molecular clock studies that show a basal divergence date for neornithes being around 100 million years ago. He credits the authors of the molecular clock studies with the observation that the lack of southern paleognath fossils may correspond to the relatively scarce southern Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 deposits, and the relative lack of paleontological field work in the southern hemisphere. Moreover, Cracraft synthesiszes the morphological and molecular studies, noting conflicts between the two, and finds that the bulk of the evidence favors paleognath monophyly
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...

. He also notes that not only the ratites, but other basal groups of neognathous birds, show trans-Antarctic distribution, as we would expect if the paleognaths and neognaths had diverged in Gondwana.

Tertiary radiation hypothesis

Feduccia (1995) emphasized the K-T event as the probable engine of diversification in the Neornithes, picturing only one or very few lineages of birds surviving the end of the Cretaceous. He also noted that birds around the world had developed ratite-like anatomies when they became flightless, and saw the affinities of modern ratites, especially kiwis, as ambiguous. In this emphasis on the Tertiary
Tertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...

, rather than Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 period, as the time of basal divergences between neornithines, he follows Olson.

Houde demonstrated that the Lithornithiformes
Lithornithiformes
Lithornithiformes is an extinct order of early paleognath birds. Lithornithiform birds are known from fossils dating to the Upper Paleocene through the Middle Eocene of North America and Europe. All are extinct today....

, a group of flying birds that were common in the Tertiary
Tertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...

 of the northern hemisphere, were also paleognaths. He argues that the lithornithiform bird Paleotis, known from fossils in Denmark (northern hemisphere), shared unique anatomical features of the skull that make it a member of the same order as the ostriches. He also argued that the kiwis should not have reached New Zealand, which moved away from the mainland in the Early Cretaceous
Early Cretaceous
The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...

, if their ancestor was flightless. He therefore deduced that Lithornithiform ancestors could have reached the southern continents some 30 to 40 million years ago, and evolved flightless forms which are today's ratites. This hypothesis is contradicted by some later molecular studies (Cooper 1997), but supported by others (Harshman et al. 2008) (see below).

Evolution

No unambiguously paleognathous fossil birds are known until the Cenozoic
Cenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...

, but there have been many reports of putative paleognathes, and it has long been inferred that they may have evolved in the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

.

One study of molecular and paleontological data found that modern bird orders, including the paleognathous ones, began diverging from one another in the Early Cretaceous
Early Cretaceous
The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...

. Benton (2005) summarized this and other molecular studies as implying that paleognaths should have arisen 110 to 120 million years ago in the Early Cretaceous
Early Cretaceous
The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...

. He points out, however, that there is no fossil record until 70 million years ago, leaving a 45 million year gap. He asks whether the paleognath fossils will be found one day, or whether the estimated rates of molecular evolution are too slow, and that bird evolution actually accelerated during an adaptive radiation after the K–T boundary
K–T boundary
The K–T boundary is a geological signature, usually a thin band, dated to 65.5 ± 0.3 Ma ago. K is the traditional abbreviation for the Cretaceous period, and T is the abbreviation for the Tertiary period...

.

Hope (2002) reviewed all known bird fossils from the Mesozoic looking for evidence of the origin of the evolutionary radiation of the Neornithes. That radiation would also signal that the paleognaths had already diverged. She notes five Early Cretaceous
Early Cretaceous
The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...

 taxa that have been assigned to the Palaeognathae. She finds that none of them can be clearly assigned as such. However, she does find evidence that the Neognathae and, therefore, also the Palaeognathae had diverged no later than the Early Campanian
Campanian
The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...

 age of the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 period.

Vegavis
Vegavis
Vegavis is a genus of extinct bird that lived during the Late Cretaceous of Antarctica, some 65 mya. It belonged to the clade Anseriformes...

is a fossil bird from the Maastrichtian
Maastrichtian
The Maastrichtian is, in the ICS' geologic timescale, the latest age or upper stage of the Late Cretaceous epoch or Upper Cretaceous series, the Cretaceous period or system, and of the Mesozoic era or erathem. It spanned from 70.6 ± 0.6 Ma to 65.5 ± 0.3 Ma...

 period of Late Cretaceous
Late Cretaceous
The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series...

 Antarctica. Vegavis is most closely related to true ducks. Because virtually all phylogenetic analyses predict that ducks diverged after paleognathes, this is evidence that paleognathes had already arisen well before then.

An exceptionally preserved specimen of the extinct flying paleognathe Lithornis
Lithornis
Lithornis is a genus of extinct paleognathous birds. Lithornis were able to fly well, but are closely related to today's tinamous and ratites ....

was published by Leonard et al. in 2005. It is an articulated and nearly complete fossil from the early Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

 of Denmark, and thought to have the best preserved lithornithiform skull ever found. The authors concluded that Lithornis was a close sister taxon to tinamous, rather than ostriches, and that the lithorniforms + tinamous were the most basal paleognaths. They concluded that all ratites, therefore, were monophyletic, descending from one common ancestor that became flightless. They also interpret the paleognath-like Limenavis
Limenavis
Limenavis is a prehistoric bird genus from the Late Cretaceous. It lived about 70 million years ago, around the Campanian-Maastrichtian boundary...

, from Late Cretaceous
Late Cretaceous
The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series...

 Patagonia
Patagonia
Patagonia is a region located in Argentina and Chile, integrating the southernmost section of the Andes mountains to the southwest towards the Pacific ocean and from the east of the cordillera to the valleys it follows south through Colorado River towards Carmen de Patagones in the Atlantic Ocean...

, as possible evidence of a Cretaceous and monophyletic origin for paleognathes.

An ambitious genomic analysis of the living birds was performed in 2007, and it contradicted Leonard et al. (2005). It found that tinamous are not primitive within the paleoganthes, but among the most advanced. This requires multiple events of flightlessness within the paleognathes and partially refutes the Gondwana Vicariance Hypothesis (see above). The study looked at DNA sequences from 19 loci in 169 species. It recovered evidence that the paleognathes are one natural group (monophyletic), and that their divergence from other birds is the oldest divergence of any extant bird groups. It also placed the tinamous within the ratites, more derived than ostriches, or rheas and as a sister group to emus and kiwis, and this makes ratites paraphyletic.

A related study addressed the issue of paleognath phylogeny exclusively. It used molecular analysis and looked at twenty unlinked nuclear genes. It study concluded that there were at least three events of flightlessness that produced the different ratite orders, that the similarities between the ratite orders are partly due to convergent evolution
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...

, and that the Palaeognathae are monophyletic, but the ratites are not.

Other authors have questioned the monophyly
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...

 of the Palaeognathae on various grounds, suggesting that they could be a hodgepodge of unrelated birds that have come to be grouped together because they are coincidentally flightless. One point is that unrelated birds have developed somewhat ratite-like anatomies multiple times around the world through convergent evolution
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...

. McDowell (1948)) asserted that the similarities in the palate anatomy of paleognathes might actually be neoteny
Neoteny
Neoteny , also called juvenilization , is one of the two ways by which paedomorphism can arise. Paedomorphism is the retention by adults of traits previously seen only in juveniles, and is a subject studied in the field of developmental biology. In neoteny, the physiological development of an...

, or retained embryonic features. He noted that there were other feature of the skull, such as the retention of sutures into adulthood, that were like those of juvenile birds. Thus, perhaps the characteristic palate was actually a frozen stage that many carinate bird embryos passed through during development. The retention of early developmental stages, then, may have been the mechanism by which various birds became flightless and came to look similar to one another.

History of classifications

In the history of biology
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...

 there have been many competing taxonomies of the birds now included in the Palaeognathae. The topic has been studied by Dubois (1891), Sharpe (1891), Shufeldt (1904), Sibley and Ahlquist (1972, 1981) and Cracraft (1981).

Merrem (1813) is often credited with classifying the paleognathes together, and he coined the taxon "Ratitae" (see above). However, Linnaeus (1758) placed cassowaries, emus, ostriches, and rheas together in Struthio. Lesson (1831) added the kiwis to the Ratitae. Parker (1864) reported the similarities of the palates of the tinamous and ratites, but Huxley (1867) is more widely credited with this insight. Huxley still placed the tinamous with the Carinatae of Merrem because of their keeled sterna
Sternum
The sternum or breastbone is a long flat bony plate shaped like a capital "T" located anteriorly to the heart in the center of the thorax...

, and thought that they were most closely related to the Galliformes
Galliformes
Galliformes are an order of heavy-bodied ground-feeding domestic or game bird, containing turkey, grouse, chicken, New and Old World Quail, ptarmigan, partridge, pheasant, and the Cracidae. Common names are gamefowl or gamebirds, landfowl, gallinaceous birds or galliforms...

.

Pycraft (1900) presented a major advance when he coined the term Palaeognathae. He rejected the Ratitae-Carinatae classification that separated tinamous and ratites. He reasoned that a keelless, or "ratite", sternum could easily evolve in unrelated birds that independently became flightless. He also recognized that the ratites were secondarily flightless. His subdivisions were based on the characters of the palatal skeleton and other organ systems. He established seven roughly modern orders of living and fossil paleognaths (Casuarii, Struthiones, Rheae, Dinornithes, Aepyornithes, Apteryges, and Crypturi – the latter his term for tinamous, after the Tinamou genus Crypturellus
Crypturellus
Crypturellus is a genus of tinamous.-Etymology:Crypturellus is formed from the Greek words κρυπτός , "covered" or "hidden", οὐρά , "tail", and -ellus, a Latin diminutive suffix...

).

The Palaeognathae are usually considered a superorder, but authors have treated them as a taxon as high as subclass
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

 (Stresemann 1927-1934) or as low as an order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 (Cracraft 1981).

Evolutionary cladogram

├──────┐ (Lithornithiformes and other fossil paleognaths)

└──────┐ (Living ratites)
├──────┐ (Struthioniformes)
│ └───┐ (Palaeotis)
│ └─── (Struthio)

└──────┬──┐ (Rheiformes)
│ ├───┐ (Opisthodactylidae)
│ │ ├─── (Diogenornis)
│ │ └─── (Opisthodactylus)
│ │
│ └───┐ (Rheidae)
│ ├─── (Heterorhea)
│ ├─── (Hinasuri)
│ │
│ └───┬─── (Rhea americana)
│ │
│ └─── (Pterocnemia pennata)

├───┬───┐ (Aepyornithiformes)
│ │ └─── (Aepyornithidae)
│ │
│ └───┬───┐ (Dinornithiformes)
│ │ ├───┐ (Dinornithidae)
│ │ │ └─── (Dinornis)
│ │ │
│ │ └───┐ (Emeidae)
│ │ ├─── (Emeinae)
│ │ └─── (Anomalopteryginae)
│ │
│ └───┐ (Apterygiformes)
│ └───┐ (Apterygidae)
│ ├─── (Megapteryx)
│ └─── (Apteryx)

└───┐ (Casuariformes)
└───┬───┐ ("Emuwaries")
│ │
│ └─── (Dromaius)

└─── (Casuarius)

Classification

  • Order Lithornithiformes
    Lithornithiformes
    Lithornithiformes is an extinct order of early paleognath birds. Lithornithiform birds are known from fossils dating to the Upper Paleocene through the Middle Eocene of North America and Europe. All are extinct today....

     (fossil
    Fossil
    Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

    )
    • Family Lithornithidae
      Lithornithidae
      Lithornithidae are a family of palaeognathous birds from the early Tertiary of the northern hemisphere.-Species:The family was erected by Peter Houde in 1988. It includes the genera Lithornis, Paracathartes, and Pseudocrypturus...

      • Genus Lithornis
        Lithornis
        Lithornis is a genus of extinct paleognathous birds. Lithornis were able to fly well, but are closely related to today's tinamous and ratites ....

        (Paleocene - Early Eocene)

      • Genus Promusophaga (Early Eocene)

      • Genus Paracathartes
        Paracathartes
        Paracathartes is a genus of extinct bird from the Wasachtian horizon of lower Eocene Wyoming, USA. One species, Paracathartes howardae has been described....

        (Early Eocene of WC USA) - tentatively placed here

      • Genus Pseudocrypturus
        Pseudocrypturus
        Pseudocrypturus is a genus of extinct paleognathous bird. One species is known, Pseudocrypturus cercanaxius. It is a relative of such modern birds as ostriches. It lived in the early Eocene....

        - tentatively placed here

  • Order Dinornithiformes - moa (prehistoric
    Late Quaternary prehistoric birds
    Prehistoric birds are various taxa of birds that became extinct before recorded history, or more precisely, before they could be studied alive by bird scientists...

    /extinct)
    • Family Dinornithidae
      • Genus Dinornis - giant moa (2-4 species)

    • Family Anomalopterygidae  - lesser moas
      • Subfamily Emeinae
        • Genus Emeus - Eastern Moa

        • Genus Euryapteryx - broad-billed or turkey moas (2 species)

        • Genus Zelornis

      • Subfamily Anomalopteryginae
        • Genus Anomalopteryx
          Anomalopteryx
          Anomalopteryx is an extinct bird genus known colloquially as the Lesser moa, Little bush moa. or Bush Moa. It stood more than tall and weighed . It inhabited much of the North Island and small sections of the South Island of New Zealand. Its habitat was lowland conifer, broad-leafed, and beech...

          - Bush Moas

        • Genus Megalapteryx - upland moas (2 species)

        • Genus Pachyornis
          Pachyornis
          Pachyornis is an extinct genus of ratites from New Zealand which belonged to the moa family. Like all ratites it was a member of the order Struthioniformes. The Struthioniformes are flightless birds with a sternum without a keel. They also have a distinctive palate. This genus contains 2 species,...

          - stout moas (3-5 species)

  • Order Aepyornithiformes  - elephant birds (prehistoric
    Late Quaternary prehistoric birds
    Prehistoric birds are various taxa of birds that became extinct before recorded history, or more precisely, before they could be studied alive by bird scientists...

    /extinct)
    • Family Aepyornithidae
      • Genus Aepyornis
        Aepyornis
        Aepyornis is a genus of aepyornithid, one of two genera of ratite birds endemic to Madagascar known as elephant birds. This animal was the world's largest bird until its extinction, about 1000 years ago.-Description:...

        (4 species)

      • Genus Mullerornis
        Mullerornis
        Mullerornis is the smaller of the two genera of extinct elephant birds of Madagascar . Species include Mullerornis agilis Milne-Edwards and A. Grandidier, 1894, and M. grandis Lamberton, 1934...

        (4 species)


  • Order Struthioniformes - ostriches
    • Family Struthionidae
      • Genus Palaeotis
        Palaeotis
        Palaeotis is a genus of paleognath bird from the middle Eocene epoch of central Europe. One species is known, Paleotis weigelti. The holotype specimen is a fossil tarsometatarsus and phalanx. Lambrect described it as an extinct bustard , and gave it its consequent name . After a suggestion by...

        (fossil
        Fossil
        Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

        : Middle Eocene) - includes Palaeogrus geiseltalensis
        • Palaeotis wiegelti
      • Genus Struthio
        Struthio
        Struthio is a genus of bird in the order Struthioniformes.-Species:There are ten known species from this genus, of which eight are extinct. There are five more possible species of which trace fossils have been found...

        (1 living species)

  • Order Rheiformes
    • Family Opistodactylidae (fossil
      Fossil
      Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

      )
      • Genus Opisthodactylus
        Opisthodactylus
        Opisthodactylus is an extinct genus of rhea from the Miocene....

        (Miocene of Argentina) - rheid?

      • Genus Diogenornis
        Diogenornis
        Diogenornis is an extinct genus of ratite which lived during the Paleocene. It was described in 1983 by Brazilian scientist Herculano Marcos Ferraz de Alvarenga. The type species is D. fragilis.-References:...

        - tentatively placed here

    • Family Rheidae - rheas
      • Genus Heterorhea
        Heterorhea
        Heterorhea is an extinct genus of ratite in the rhea family....

        (fossil
        Fossil
        Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

        : Pliocene of Argentina)

      • Genus Hinasuri
        Hinasuri
        Hinasuri is an extinct genus of rhea from the Pliocene. Its fossils have been found in Argentina....

        (fossil
        Fossil
        Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

        )

      • Genus Rhea
        Rhea (bird)
        The rheas are ratites in the genus Rhea, native to South America. There are two existing species: the Greater or American Rhea and the Lesser or Darwin's Rhea. The genus name was given in 1752 by Paul Möhring and adopted as the English common name. Möhring's reason for choosing this name, from the...

        (2 species, includes Pterocnemia)
        • Rhea americana - common rhea

        • Rhea pennata - Darwin's rhea
          Darwin's Rhea
          Darwin's Rhea, Rhea pennata, also known as the Lesser Rhea, is a large flightless bird, but the smaller of the two extant species of rheas. It is found in the Altiplano and Patagonia in South America.-Description:...


  • Order Casuariiformes
    • Family Casuariidae
      Casuariidae
      The bird family Casuariidae has four surviving members: the three species of cassowary, and the only remaining species of Emu. The emus were formerly classified in their own family, Dromaiidae, but are regarded as sufficiently closely related to the cassowaries to be part of the same family.All...

       - cassowaries
      • Genus Casuarius
    • Family Dromaiidae
      • Genus Emuarius
        Emuarius
        Emuarius is an extinct genus of flightless bird from Australia that lived during the early Miocene and late Oligocene. It is one of two known genera of emu. There are two known species in the genus, Emuarius gidju and Emuarius guljaruba. The birds in this genus are known as emuwaries...

        - "emuwaries" (fossil
        Fossil
        Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

        : Late Oligocene - Late Miocene; formerly in Dromaius)
      • Genus Dromaius
        Dromaius
        Dromaius is a genus of ratite present in Australia. There is one extant species, Dromaius novaehollandiae commonly known as the Emu.In his original 1816 description of the emu, Louis Jean Pierre Vieillot used two generic names; first Dromiceius, then Dromaius a few pages later...

        - emus (1 living species, 2 recently extinct)
  • Order Apterygiformes - kiwis
    • Family Apterygidae
      • Genus Apteryx (about 6 living species, possibly 1 recently extinct)
  • Order Tinamiformes - tinamous
    • Family Tinamidae
      • Subfamily Tinaminae
        Tinaminae
        Tinaminae, the forest tinamous, is a subfamily of the family Tinamidae. It contains three genera, which are:* Crypturellus* Nothocercus* Tinamus-Description:...

        • Genus Crypturellus
          Crypturellus
          Crypturellus is a genus of tinamous.-Etymology:Crypturellus is formed from the Greek words κρυπτός , "covered" or "hidden", οὐρά , "tail", and -ellus, a Latin diminutive suffix...

        • Genus Tinamus
          Tinamus
          Tinamus is a genus of birds in the Tinamou family. This genus comprises some of the larger members of this South American family.The species in taxonomic order are:...

        • Genus Nothocercus
          Nothocercus
          Nothocercus is a genus of birds in the Tinamou family. Most of this family are flightless birds, with nothocerus being the exception, as they can fly, albeit, not too well...

        • Genus Taoniscus
        • Genus Tinamotis
          Tinamotis
          Tinamotis is a genus of birds in the tinamou family.-Taxonomy:All Tinamou are from the family Tinamidae, and in the larger scheme are also Ratites. Unlike other Ratites, Tinamous can fly, although in general, they are not strong fliers...

      • Subfamily Rhynchotinae
        • Genus Eudromia
          Eudromia
          Eudromia is a genus of birds in the tinamou family. This genus comprises two crested members of this South American family.-Etymology:...

          - crested tinamous
        • Genus Rhynchotus
          Rhynchotus
          Rhynchotus is a genus of birds in the Tinamou family. This genus comprises two members of this South American family.-Taxonomy:Tinamous have evolved from Ratites and are the only extant Ratites that fly, and are the closest to the ancestral flying Ratites.-Species:The species are:* The Red-winged...

        • Genus Nothoprocta
          Nothoprocta
          Nothoprocta is a genus of birds belonging to the tinamou family Tinamidae. They inhabit scrubland, grassland and open woodland in western South America, particularly in the Andes. They are poor fliers and spend most of their time on the ground. Their diet includes seeds and insects. They nest on...

        • Genus Nothura
          Nothura
          Nothura is a genus of birds in the tinamou family. This genus comprises five members of this South American family.Tinamous are paleognaths related to the flightless ratites...

          - nothura
          Nothura
          Nothura is a genus of birds in the tinamou family. This genus comprises five members of this South American family.Tinamous are paleognaths related to the flightless ratites...

          s

Alternate classification

  • Superorder Palaeognathae
    • Order Struthioniformes
    • Order Lithioniformes
    • Order Tinamiformes

Sometimes placed here

  • Ambiortus
    Ambiortus
    Ambiortus is a prehistoric bird genus. The only known species, Ambiortus dementjevi, lived about 130 million years ago during the Early Cretaceous in today's Mongolia....

  • Eremopezus
    Eremopezus
    Eremopezus is a prehistoric bird genus. It is known only from the fossil remains of a single species,the huge and presumably flightless Eremopezus eocaenus. This was found in Upper Eocene Jebel Qatrani Formation deposits around the Qasr el-Sagha escarpment, north of the Birket Qarun lake near...

    - includes Stromeria
  • Gansus
    Gansus
    Gansus is a genus of aquatic birds that lived during the Aptian or Albian age of the Early Cretaceous period in what is now Gansu province, western China...

  • Limenavis
    Limenavis
    Limenavis is a prehistoric bird genus from the Late Cretaceous. It lived about 70 million years ago, around the Campanian-Maastrichtian boundary...

  • Palaeocursornis
    Palaeocursornis
    Palaeocursornis is a monotypic genus of prehistoric bird. The species P. corneti, described in 1984, was initially assumed to be a flightless paleognathe, possibly a ratite, but it may actually be more primitive and not even a neornithine but an ornithuromorph bird or indeed not a bird at all...

  • Wyleyia
    Wyleyia
    Wyleyia is a prehistoric bird genus with a single species, Wyleyia valdensis, known from the Early Cretaceous of England. Even this is only known from a single damaged right humerus. It has been named to honor J. F. Wyley, who found the specimen in the Weald Clay deposits of Henfield in Sussex...



Paraphysornis
Paraphysornis
Paraphysornis is an extinct genus of giant flightless predatory birds of the family Phorusrhacidae or "terror birds" and the subfamily brontornithinae that lived in Brazil. Its length was about 2 meter and the skull had a length of 60 centimeter. The only known species is Paraphysornis...

is a phorusrhacid.

Ootaxa
Ichnotaxon
An ichnotaxon is defined by the International Code of Zoological Nomenclature as "a taxon based on the fossilized work of an organism", that is, the non-human equivalent of an artifact. Ichnotaxa are names used to identify and distinguish morphologically distinctive ichnofossils, more commonly...

  • Gobioolithus (Late Cretaceous) - paleognath?
  • Incognitoolithus (Eocene of North America) - ratite?
  • Type A ("aepyornithoid") eggs (Tsondab Early Miocene of Namibia - Pliocene of Asia)
  • Namornis (Middle Miocene of Namibia - Late Miocene of Kenya) - ratite?
  • Diamantornis (Middle Miocene of Namibia - Late Miocene of UAE and Kenya) - ratite?
  • Psammornis - may be from Eremopezus

Locomotion

Many of the larger ratite birds have extremely long legs and the largest living bird, the ostrich
Ostrich
The Ostrich is one or two species of large flightless birds native to Africa, the only living member of the genus Struthio. Some analyses indicate that the Somali Ostrich may be better considered a full species apart from the Common Ostrich, but most taxonomists consider it to be a...

, can run at speeds over 35 mph (60 km/h). Cassowaries, emus, and rheas show a similar likeness in agility and some extinct forms may have reached speeds of 45 mph (75 km/h). Moas, the largest birds, had legs over 3 feet high and may have been the fastest land animals to live outrunning even the cheetah
Cheetah
The cheetah is a large-sized feline inhabiting most of Africa and parts of the Middle East. The cheetah is the only extant member of the genus Acinonyx, most notable for modifications in the species' paws...

.

Paleognaths and humans

Paleognaths probably first interacted with Australopithecines about 3 million or so years ago in the middle Pliocene
Pliocene
The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...

 in the form of an ancient ostrich
Ostrich
The Ostrich is one or two species of large flightless birds native to Africa, the only living member of the genus Struthio. Some analyses indicate that the Somali Ostrich may be better considered a full species apart from the Common Ostrich, but most taxonomists consider it to be a...

 or elephant bird
Elephant bird
Elephant birds are an extinct family of flightless birds found only on the island of Madagascar and comprising the genera Aepyornis and Mullerornis.-Description:...

. As Homo erectus
Homo erectus
Homo erectus is an extinct species of hominid that lived from the end of the Pliocene epoch to the later Pleistocene, about . The species originated in Africa and spread as far as India, China and Java. There is still disagreement on the subject of the classification, ancestry, and progeny of H...

evolved and left Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...

 for other continents not much contact was made with ratites until the Maori and Aborigines arrived in New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

 and Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

. Like many other native species, they were not well-adapted to environments containing humans, and many ratites (and other Oceanic species) were hunted to extinction during this period. Worldwide, most giant birds became extinct by the end of the 18th century and most surviving species are now endangered
Endangered species
An endangered species is a population of organisms which is at risk of becoming extinct because it is either few in numbers, or threatened by changing environmental or predation parameters...

.

Today, ratites such as the Ostrich
Ostrich
The Ostrich is one or two species of large flightless birds native to Africa, the only living member of the genus Struthio. Some analyses indicate that the Somali Ostrich may be better considered a full species apart from the Common Ostrich, but most taxonomists consider it to be a...

 are farmed and sometimes even kept as pets. Ratites play a large role in human culture- they are farmed, eaten, raced, protected, and kept in zoos.

See also

  • Fossil birds
    Fossil birds
    Birds are generally believed to have evolved from certain feathered theropod dinosaurs, and there is no real dividing line between birds and dinosaurs, except of course that some of the former survived the Cretaceous-Tertiary extinction event while the latter did not. For the purposes of this...

  • Late Quaternary prehistoric birds
    Late Quaternary prehistoric birds
    Prehistoric birds are various taxa of birds that became extinct before recorded history, or more precisely, before they could be studied alive by bird scientists...

  • Extinct birds
  • Flightless birds

External links

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