The function(s) of the bone ornamentation in the crocodylomorph osteoderms: a biomechanical model based on a finite element analysis

This paper aims at assessing the influence of the bone ornamentation and, specifically, of the associated loss of bone mass on the mechanical response of the crocodylomorph osteoderms. To this aim, we have performed three dimensional modeling and finite element analyses on a sample which includes both extant dry bones and well-preserved fossils tracing back to the Late Triassic. We simulated an external attack under variable angles on the apical surface of each osteoderm and further repeated the simulation on an equivalent set of smoothed 3D-modeled osteoderms. The comparative results evidenced that the presence of an apical sculpture has no significant influence on the von Mises stress distribution in the osteoderm volume although it involves a slight increase in its numerical score. Moreover, performing parametric analyses, we evidenced that the Young’s modulus in the osteoderm which may vary depending on the bone porosity, the collagen fiber orientation or the calcification density has no impact on the von Mises stress distribution inside the osteoderm volume. As the crocodylomorph bone ornamentation is continuously remodeled by pit resorption and secondary bone deposit, we assume that the apical sculpture may be the outcome of a “trade-off” between the bone mechanical resistance and the implication in physiological functions. These physiological functions are indeed based on the set-up of a bone superficial vessel network and/or on the recurrent release of mineral elements into the plasma: heat transfers when basking and respiratory acidosis buffering during prolonged apnea in neosuchians and teleosaurids; compensatory homeostasis in response to general calcium deficiencies. On a general morphological aspect, the osteoderm geometrical variability within our sample lead us to assess that the global osteoderm geometry (whether square or rectangular) does not influence the von Mises stress whereas the presence of a dorsal keel would rather reduce the stress along the vertical axis.