HIERONYMUS, T.L.; Montana State University, Bozeman: Microanatomical correlates of muscle attachment and their implications for muscular reconstruction

Reconstructions of the musculature of Dinosauria based on osteological features are often used as a basis for paleobiological inference. Actualistic research on osteological correlates of muscle attachment has shown that most features interpreted as muscle attachment have little predictive reliability. The reliability of microanatomical features such as extrinsic fibers (Sharpey�s fibers) as correlates of muscle attachment was investigated. Jaw muscle attachments were examined in seven specimens representing Lepidosauria and Archosauria. Areas and types of muscle attachment were determined by dissection. Extrinsic fibers at 29 areas of attachment representing tendinous, aponeurotic, and direct attachments, as well as areas without muscle attachment, were characterised using SEM. The contralateral side of the same specimens were examined histologically to determine the microscopic relationships of the tissues present at the muscle/bone interface. Extrinsic fiber density per unit area of attachment (EFD) is significantly greater (p < 0.01 by Mann-Whitney U) for areas of tendinous and aponeurotic attachment than for the other types of attachment studied. Ligamentous and perimysial attachments also show high EFD, indicating that any attachment of dense regular connective tissue to membrane bone is accomplished by extrinsic fibers. EFD at direct attachments is not significantly different from EFD at areas of no attachment. EFD at tendinous and aponeurotic attachments shows a significant correlation (p < 0.05) with an index of stress exerted at the attachment ([muscle mass / muscle length] / attachment surface area), raising the possibility of using EFD as an indicator of attachment stress instead of an indicator of muscle attachment. A more accurate biomechanical model is needed to verify these results.