We report the presence of extremely dense, pervasive extrinsic fibers (fibers anchoring soft tissue to bone, sometimes called Sharpey’s fibers) in the dorsal ribs of an 8-meter-long specimen of the mosasaur Tylosaurus proriger from the Campanian of South Dakota, U.S.A. Such fibers are absent in surveyed dorsal ribs of extant and extinct semi-aquatic to marine reptiles, terrestrial reptiles the size of or larger than mosasaurs (e.g., dinosaurs), and mosasaurs’ closest extant relatives. Within Mosasauridae, ribs with extensive longitudinally ridged texture are saliently developed in Tylosaurinae. Longitudinal ridges are present but limited in density and distribution on the ribs of several other mosasaur genera (e.g., Clidastes, Mosasaurus), but are not as extensively developed as those of T. proriger. Ridged textures are absent or limited to the anterior and posterior edges of the bone in surveyed ribs of extant and extinct archosaurs as well as of other squamates. A similar ridged texture occurs on localized sites on other mosasaur bones (e.g., vertebrae, girdle and limb elements), but histological study of these elements in search of extrinsic fibers awaits investigation. In extinct diapsids, tendinous or aponeurotic muscle insertion sites are discernible histologically via the presence of dense extrinsic fibers, and there is some evidence that the density of extrinsic fibers in this type of attachment is related to the amount of stress experienced by the bone. The unusual osteohistological features are interpreted as evidence of tendinous or aponeurotic attachment of extensive and highly differentiated axial musculature, connecting the axial and appendicular skeleton to nearly the entire surface of the dorsal ribs. These osteohistological features perhaps indicate the production of powerful mediolateral movements throughout the trunk, or stabilization of the trunk relative to the tail during swimming.