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ABSTRACT
Exostosis, terminating in a mushroom-like overgrowth of bone with a central distal depression, was found on the medial aspect of the distal radii of 61% (19 of 31) of the Oligocene Hesperocyon sp. examined. X-rays revealed that the trabecular and cortical regions of the neck of the exostosis are contiguous with those of the adjacent shaft. Lesions are bilaterally symmetrical in all individuals in which both radii were preserved. New findings after Romer's (1924) report of Daphoenus exostoses indicate that the tumors are not limited to the distal radius.
Presence of exostoses in 61% of Hesperocyon sp. and in the contemporary primitive caniform carnivorans Daphoenus and Cynodictis suggests an autosomal dominant disorder. Presence of sternal and radial shaft exostosis in Hesperocyon further supports the diagnosis of multiple hereditary osteochondroma. The distribution of osteochondroma has now been extended to the Oligocene canids.
