Preuschoft, H. & Witzel, U. Senckenbergiana lethaea (2002) 82: 207. doi:10.1007/BF03043785
The skulls of reptiles and mammals can be loaded mechanically in three ways: the weight of the head acting downward, perhaps reinforced by a prey or bunch of food lifted from the ground or water surface; by forces acting in the plane of the tooth row, created by movements of the prey in relation to the head or by a movement of the head in relation to a fixed food object; and by the adduction of the mandible, which leads to reaction forces in the skull. While the former two evoke stress patterns comparable to that in a beam which is supported at its rear end (by the occipital condyle(s) and the neck muscles), the latter evoke stress patterns comparable to a beam supported at both ends. Its anterior bearing are the teeth which transmit a reaction force from the seized prey, the adductor muscles of the mandible move the intermediate part of the skull downward, and the posterior bearing is provided by the mandibular joint.
Three-dimensional FEM-analysis of the flow of stresses within solid, homogeneous bodies under loads like those described above have been made. As a result, the stress flows have been found to correspond closely to the arrangements of bony material in the akinetic skulls of Crocodiles, Lacertilia, Sphenodon. Except crocodiles and chelonians, reptilian skulls often show large gaps between the load-bearing plates and rods. These gaps correspond to little stressed areas between the stress-bearing parts. One of the stress-bearing rods is the small braincase. In long, slender jaws like those in crocodiles the stresses are concentrated on the periphery, with more or less stress-free areas in the center of the cross sections.
In many mammals (shrews, primates includingHomo), however, the very large bony nasal capsule and braincase lead to a distribution of the forces over large areas like in thin-walled shell structures, which are strong enough to sustain the existing forces, without reinforcing superstructures. Even the zygomatic arch can be dispensible.
The decisive a priori factors which determine the development of either a rod- or a shell-like structure in a FEM model are
1st the relative shape and length of the toothrow and its position in relation to the posterior part of the skull, especially the braincase, and 2nd the size of the nasal capsule and the braincase.
We conclude that the exact form of the skull in both classes of animals is determined by 1st the shape and length of the jaws and 2nd by the space requirements of the olfactory and the optical sense organs, and the braincase. The second factor is an expression of the overall evolutionary level. The literature contains plausible biological arguments to explain the high selective influence of lifestyle characteristics on the first factor. These arguments usually cover also the position of the eye openings, the nasal opening and the relative height and length of the whole skull. If these factors are given, the exact morphology of the bony structure turns out to correspond completely to the pattern of stresses, and no other reasons behind skull shape must be searched.
The arrangement of the muscles seems to follow in all cases the principle to distribute the force created at the origines on a large surface or on many individual bony elements.
reptilian skull mammalian skull three-dimensional FEM-analysis biostatics load-bearing structure bite force joint force occipital condyle