M398_25b 330..334

330 NATURE | VOL 398 | 25 MARCH 1999 | www.nature.com characters, which allow a greater range of excursion of the shoulder joint in the locomotion of multituberculates and living therians, have evolved at least twice among the Mesozoic mammals. Alternatively, the mobile joints between the clavicle, interclavicle and scapula could be ancestral conditions shared by J. jenkinsi and the more derived mammals. If so, then the rigid clavicle±interclavicle articulation and relatively immobile scapula of monotremes (Fig. 3) would have to be regarded as atavistic reversals to the primitive conditions in the more distantly related non-mammalian cynodonts. For either evolutionary scenario, we must conclude that the pectoral girdles and forelimbs of early mammals underwent extensive convergent evolution, not only by comparison with the dental and cranial features, but also in relation to more conservative features of the pelvis and hindlimbs. M . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .