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Crax is a genus of in the order Galliformes, a clade of large, heavy-bodied, ground-feeding birds. They are known from tropical South America with one species, the great curassow, ranging northwards through Central America as far as Mexico. The curassows in this genus are noted for their sexual dimorphism; males are more boldly coloured than females and have facial ornamentation such as knobs and wattles. They are also characterised by curly crests and contrastingly-coloured crissa (the area around the cloaca). Crax curassows probably originated as a distinct lineage during the Late Miocene. During the Messinian, the ancestral Crax split into two lineages separated by the Colombian Andes and the Cordillera de Mérida which uplifted at that time. The northern lineage radiated into the great, blue-billed, and yellow-knobbed curassows, while the four southern species evolved as they became separated by the uplifting of various mountain ranges.
This genus forms one of the two major lineages of curassows. It is distinguishable from its relatives by its pronounced sexual dimorphism (with the exception of the black curassow). In other genera sexual dimorphism is rarely exhibited or of minor appearance ( Nothocrax and Pauxi), or manifest by size only ( Mitu).
The northern lineage quite soon thereafter radiated into the ancestors of the great, blue-billed, and yellow-knobbed curassows, which were isolated from each other by the uplift of the northern Cordillera Occidental, and the Serranía del Perijá, respectively; it is fairly certain that these lineages were well distinct by the end of the Miocene. (Pereira & Baker 2004)
The evolution of the 4 southern species was somewhat more complex. In the Messinian, about 6–5.5 mya, the ancestors of the wattled curassow became isolated in the western Amazonas basin. With increasing of southeastern Brazil, the ancestors of the red-billed curassow found refuge in the mountain ranges between the Brazilian Highlands and the Atlantic during the mid-Zanclean, some 4.5-4 mya. The divergence between the bare-faced and black curassow lineages apparently took place around the Uquian–Ensenadan boundary, some 1.5 mya. This which coincides with one or several period(s) of elevated during which the lower Amazonas basin was a brackish lagoon which offered little curassow habitat. Their present ranges are consequently still separated by the Amazonas river. (Pereira & Baker 2004)
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