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This paper describes a morphological study of the dodo Raphus cucullatus and solitaire Pezophaps solitaria extinct, flightless Columbiformes of the Mascarene Islands, Indian Ocean—based on mensural data from 387 skeletal elements, comparative data from four flighted species of Columbidae, ancillary mensural data from other Columbiformes, and the literature.

Raphus cucullatus and P. solitaria are characterized by great body size and substantially reduced pectoral limbs. Sexual size dimorphism is unusually great in both R. cucullatus and P. solitaria, and sexual dimorphism of P. solitaria may be the greatest of any carinate bird. Estimates of body mass (kg), based on femur lengths of flighted columbids and adjusted for flightlessness and seasonal deposition of fat, were: 21 and 17 for male and female R. cucullatus, and 28 and 17 for male and female P. solitaria, respectively. Pectoral reduction is greater in R. cucullatus than in P. solitaria, but both species are characterized by differential shortening of wing elements, changes in the sternum and scapulocoracoidal angle. R. cucullatus has disproportionately long femora, short tarsometatarsi and long digits, whereas P. solitaria has disproportionately long tibiotarsi, short tarsometatarsi and short digits; relative shaft widths of leg elements are substantially greater in both R. cucullatus and P. solitaria than in flighted columbids.

As is typical of other birds, giantism of R. cucullatus and P. solitaria probably was associated with physiological changes, increased longevity, enhanced thermodynamic efficiency and improved capacity for fasting. Evidently both R. cucullatus and P. solitaria were primarily frugivorous, and foraging-related morphological peculiarities include an enlarged crop and the retention of ‘gizzard stones’. Both species had clutch sizes of one, and egg masses of the flightless species approximated that predicted for a flighted columbid of equal size. Extreme sexual dimorphism and territoriality of P. solitaria suggest that the species may have been polygynous and perhaps lek-breeding. The description of ‘gigantic immaturity’ in R. cucullatus by Strickland & Melville (1848) probably represents the first recognition of paedomorphosis in any species of bird. Pectoral underdevelopment and (in at least R. cucullutus) comparatively ‘juvenile’ plumage in adults substantiate the role of paedomorphosis in the ontogeny of flightlessness in R. cucullatus and P. solitaria; both species also were characterized by peramorphic skulls, trunks and pelvic appendages. The common assumption of monophyly of the fligHtless species (it. Raphidae) lacks rigorous analytical support. In spite of the anthropogenous extinction of both species, both R. cucullarus and P. solitaria were evolutionarily innovative in ontogeny, morphological characters and life-history strategies.