Tetrapod terrestrialisation: a weight-bearing potential already present in the humerus of the stem-tetrapod fish Eusthenopteron foordi

Abstract

Our study shows that the von Mises stress, induced by external load on the humerus of Eusthenopteron, dissipates through the cortex, trabeculae and the muscles of the pectoral appendage involved in elevation and protraction. As Eusthenopteron's microanatomy is similar to that of Devonian tetrapods, we expect them to share the same process of load dissipation and energy absorption through 1) cortical stress distribution; and 2) longitudinal trabecular conduction. Our FE simulations in hypothetical terrestrial conditions demonstrate that this type of microanatomical architecture could withstand the weight of Tiktaalik proportionally to the size of Eusthenopteron in standing posture. This tubular arrangement, including marrow processes originally involved in long-bone elongation, would have acquired a key secondary biomechanical function to increase the resistance and strength of the cancellous bone to external compressive load. As an exaptation, this specific trabecular architecture may have played a major role in the tetrapod land exploration about 400 million years ago.