The skull of the lion marmoset Leontideus rosalia linneaus: A study in biomechanical adaptation

The lion marmoset (Leontideus rosalia Linneaus) is unique among primates in that it has a pair of deep fossae on either side of the midiine near the middle of the underside of its cranium. The entire cranium is elongated, having the appearance of being stretched in its middle third as compared with all other marmosets. The muscles that arise in this region (medial and lateral pterygoids) are a part of this complex. The medial pterygoid is especially elaborated. It is composed of three heads, one of which arises in a pocket bulging the lateral pterygoid plate inward, high above the floor of the orbit on its medial side. Insight as to the meaning of these structures emerges when a carefully composed “natural experiment” is set up. Parallel series of skulls arranged in order of increase in size in primates more and more remotely related to marmosets show parallel sets of changes in Callithricidae (marmosets), Cebidae (New World monkeys), Cercopithecidae (Old World monkeys), and Hominoidea (apes and man). Smaller skulls have orbits closely pressed together, separated only by a simple bony septum. As skulls increase in size the orbits move apart to be separated, first by slight hollows in the skull base, then progressing to increasingly complicated air or marrow spaces. The phenomenon thus seems to be a special problem of allometry. As the skull becomes larger viscerocranial increase outstrips neurocranial enlargement. These two elements are then bridged by stronger and more complex buttressing which seem clearly to be local biomechanical adaptations to the new spatial arrangement. The sinuses are simply the spaces between the braces.