The order contains about 290 species, inhabiting every continent except Antarctica, and divided into five families: Phasianidae (including chicken, quail, partridges, pheasants, turkeys, peafowl (peacocks) and grouse), Odontophoridae (New World quail), Numididae (guinea fowl), Cracidae (including chachalacas and curassows), and Megapodiidae (incubator birds like malleefowl and brush-turkeys).

Males of most species are more colorful than the females, with often elaborate courtship behaviors that include strutting, fluffing of tail or head feathers, and vocal sounds.

Despite their distinctive appearance, grouse and turkeys probably do not warrant separation as families due to their recent origin from partridge- or pheasant-like birds.

However, the Galliformes as they were traditionally delimited are called Gallomorphae in the Sibley-Ahlquist taxonomy, which splits the Cracidae and Megapodiidae as an order "Craciformes".

When the asteroid impact killed off all non-avian dinosaurs, and the dominant birds, it destroyed all creatures that lived in trees and on open ground.

The enantiornithes were wiped out, but the ancestors of galliformes were small and lived in the ground (unlike water for Anseriformes) which protected them from the blast and destruction.

Its partial left tarsometatarsus was found in the Austin Chalk near Fort McKinney, Texas, dating to about 85 million years ago (Mya).

The ichnotaxon Tristraguloolithus cracioides is based on fossil eggshell fragments from the Late Cretaceous Oldman Formation of southern Alberta, Canada, which are similar to chachalaca eggs,[12] but in the absence of bone material, their relationships cannot be determined except that they are apparently avian in origin.

A number of Paleogene to mid-Neogene fossils are quite certainly Galliformes, but their exact relationships in the order cannot be determined: For a long time, the pheasants, partridges, and relatives were indiscriminately lumped in the Phasianidae, variously including or excluding turkeys, grouse, New World quail, and guineafowl, and divided into two subfamilies – the Phasianinae (pheasant-like forms) and the Perdicinae (partridge-like forms).

This crude arrangement was long considered to be in serious need of revision, but even with modern DNA sequence analyses and cladistic methods, the phylogeny of the Phasianidae has resisted complete resolution.

[18] In 2021, Kimball et al. found the family to comprise three distinct subfamilies, with two containing multiple genera; these results were followed by the International Ornithological Congress.

[15][19][20] The partridge of Europe is not closely related to other partridge-like Galliformes, as already indicated by its sexually dimorphic coloration and possession of more than 14 rectrices, traits it shares with the other advanced phasianids.

[22] Talegalla Leipoa Alectura Aepypodius Macrocephalon Eulipoa Megapodius Chamaepetes Penelopina Aburria Penelope Ortalis Oreophasis Nothocrax Pauxi Mitu Crax Guttera Numida Acryllium Agelastes Ptilopachus Rhynchortyx Cyrtonyx Dactylortyx Odontophorus Oreortyx Dendrortyx Philortyx Colinus Callipepla ?Melanoperdix ?Rhizothera Xenoperdix Arborophila Rollulus Caloperdix Tropicoperdix Ammoperdix Synoicus Excalfactoria Anurophasis Margaroperdix Coturnix Tetraogallus Alectoris ?Pternistis †Ophrysia Perdicula Bambusicola Gallus Scleroptila Peliperdix Francolinus Rheinardia Argusianus Afropavo Pavo ?Galloperdix Haematortyx Polyplectron Ithaginis Tragopan ?Lerwa Tetraophasis Lophophorus ?Perdix Syrmaticus Phasianus Chrysolophus Lophura Catreus Crossoptilon Pucrasia Meleagris Bonasa Tetrastes Centrocercus Dendragapus Tympanuchus Lagopus Falcipennis Canachites Tetrao Lyrurus As their name suggests they are chicken-like in appearance, with rounded bodies and blunt wings, and range in size from small at 15 cm (6 inches) to large at 120 cm (4 feet).

Gallinaceous birds feed on a variety of plant and animal material, which may include fruits, seeds, leaves, shoots, flowers, tubers, roots, insects, snails, worms, lizards, snakes, small rodents, and eggs.

By contrast, the stem-galliform Scopelortyx appears to have been more aerial than modern fowl, with a flight style more suited for gliding and soaring.

Fairly unrelated species like the crested fireback (Lophura ignita), vulturine guineafowl (Acryllium vulturinum), and malleefowl (Leipoa ocellata) are outwardly similar in their body types (see also convergent evolution).

Most species that show only limited sexual dimorphism are notable for the great amount of locomotion required to find food throughout the majority of the year.

Herbivorous to slightly omnivorous galliforms, forming the majority of the group, are typically stoutly built and have short, thick bills primarily adapted for foraging on the ground for rootlets or the consumption of other plant material such as heather shoots.

The Himalayan monal (Lophophorus impejanus) has been observed digging in the rotting wood of deadfall in a similar manner to woodpeckers to extract invertebrates, even bracing itself with aid of its squared tail.

These galliform genera prefer instead to capture live invertebrates in leaf litter, in sand, or shallow pools or along stream banks.

These genera are also outwardly similar in that they each have exceptionally long, delicate legs and toes and the tendency to frequent seasonally wet habitats to forage, especially during chick-rearing.

The blue peafowl (Pavo cristatus) is famed in its native India for its appetite for snakes – even poisonous cobras – which it dispatches with its strong feet and sharp bill.

The Lady Amherst's pheasant (Chrysolophus amherstiae), green peafowl (Pavo muticus), Bulwer's pheasant and the crestless fireback (Lophura erythrophthalma) are notable for their aptitude to forage for crustaceans such as crayfish and other aquatic small animals in shallow streams and amongst rushes in much the same manner as some members of the rail family (Rallidae).

Similarly, although wild turkeys (Meleagris gallopavo) have a diet primarily of vegetation, they will eat insects, mice, lizards, and amphibians, wading in water to hunt for the latter.

Domestic hens (Gallus domesticus) share this opportunistic behaviour and will eat insects, mice, worms, and amphibians.

Many species of moderate altitudes—for example the long-tailed pheasants of the genus Syrmaticus—also find a great deal of their daily nutritional requirements in the tree canopies, especially during the snowy and rainy periods when foraging on the ground is dangerous and less than fruitful for a variety of reasons.

Although members of the genus Syrmaticus are capable of subsisting almost entirely on vegetarian materials for months at a time, this is not true for many of the subtropical genera.

The most extreme case are the Megapodiidae, where the adults do not brood, but leave incubation to mounds of rotting vegetation, volcanic ash, or hot sand.

The North American wild turkey – Meleagris gallopavo – has five distinct subspecies (Eastern, Rio Grande, Florida [Osceola], Merriam's, and Gould's).

Males have a "beard" of coarse black bristles hanging from the center of their upper breasts and tend to have more vibrantly colored plumage than do females.