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Occurrence and Significance of the Os Transiliens in Gopher Tortoises

1974; American Society of Ichthyologists and Herpetologists; Volume: 1974; Issue: 1 Linguagem: Inglês

10.2307/1443010

1938-5110

Dennis M. Bramble,

Paleontology and Evolutionary Biology

The Os transiliens, a sesamoid bone in the central raphe of the external adductor muscle, has previously been described in three of the four living species of the tortoise genus Gopherus (G. polyphemus, flavomarginatus, agassizii). The Os transiliens is reported for the first time in Recent G. berlandieri and in the oldest recognized fossil Gopherus, G. laticunea from the Middle Oligocene of Colorado. The sesamoids from 16 G. berlandieri show marked variation in size and shape within and between individuals. There is a low degree of correlation (r = +.405) between sesamoid development and skull size. Evidence from Recent and fossil testudinids indicates that the Os transiliens is restricted to gopher tortoises. The presence of the bony sesamoid appears to be related to mechanical stresses arising from the specialized feeding mechanism of this group. The mechanism, which involves pronounced protraction and retraction of the mandible, permits more efficient utilization of the coarser vegetation types common to xerophytic floras. It is suggested that the specialized feeding mechanism and the associated Os transiliens originated as an adaptive response to North American climatic and vegetational changes during the Late Eocene-Early Oligocene period.