Rhea americana, the rhea, is a new world representative of the ratite birds, a group that includes the extant ostrich, emu, kiwis, and cassowaries as well as the extinct moas and elephant birds. None of the species allocated to Ratitae are able to fly. Some, such as the ostrich and emu, are cursorial (runners); however, the kiwis and cassowaries live in densely forested areas. Although morphologically diverse, the ratites are connected by a number of synapomorphies including a fused coracoid and scapula, the trochlea of metatarsi II of the tarsometatarsus is not plantarly deflected and the distal end does not reach farther distally than the distal end of the trochlea of metatarsi IV, the musculus flexor hallucis longus to the hallux is either weak or absent, and the oil gland is only minutely tufted or naked (Mayr and Clarke, 2003).
The ratites belong to the more inclusive clade, Palaeognathae, a taxon that also includes the tinamous, a group of birds that unlike ratites is able to fly. Historically, the palaeognaths were grouped together based on the morphology of their palate (‘dromaeognathous’ or, more commonly referred to as the ‘palaeognathous’ palate; Huxley, 1867; Pycraft, 1900). The features that comprise the palaeognathous palate are a broad vomer that unites rostrally with the broad maxillopalatine plates and caudally with both the palatines and the pterygiods, the lack of an articulation between the pterygoids and the parasphenoidal rostrum, basipterygoid processes that project from the body of the basisphenoid/parasphenoid complex rather than the parasphenoidal rostrum, the basisphenoidal processes articulate with the pterygoids near the distal ends of the pterygoids, and a single head to the quadrate (Huxley, 1867). In addition to these palatal characters, other morphological features unite Palaeognathae: a marked furrow just rostral of the nasal opening on the upper beak, rostral extension of the mesethmoid beyond the naso-frontal hinge, two strong grooves on the ventral surface of the mandibular symphysis, a flat dorsal surface of the mandibular symphysis, three to four costal processes on the sternum, and a greatly reduced or no hallux (Mayr and Clarke, 2003). Monophyly of Palaeognathae, although historically disputed, is supported in recent morphological, molecular, and total evidence studies (Cracraft, 1974; Bledsoe, 1988, Härlid et al., 1997, 1998, 1999; Cooper et al., 2000; Livezey and Zusi, 2001, Mayr and Clarke, 2003).
There are two living species allocated to Rheidae, whose fossil record extends back into the Eocene. The extant members include Rhea americana (greater rhea) and Pterocnemia pennata (lesser rhea), both of which are restricted to South America. The greater rhea ranges across the eastern expanse of the continent, and the lesser rhea is found isolated in southeastern-most South America and also along the western edge of the continent. Although not the largest bird (this distinction is given to another ratite the ostrich, Struthio camelus), the greater rhea is the largest amongst South American birds.
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Rhea americana on the Animal Diversity Web (The University of Michigan Museum of Zoology)
The lesser rhea, Rhea pennata, on the Animal Diversity Web (The University of Michigan Museum of Zoology)
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