Explanation for spheno-ethmoid contact on the organ-crowded hominoid anterior cranial floor
The anterior cranial floor is variably configured in primates. The sphenoid and ethmoid bones contact each other in strepsirrhines, orangutans, and modern humans, whereas a horizontal plate of frontal bone interrupts spheno-ethmoid contact in Old World monkeys, lesser apes, and, more variably, in New World monkeys and African great apes. Frontal contribution to the midline anterior cranial floor has been tied to differential growth of the brain and orbits, developmental competition for space, upper facial prognathism, and size of the supraorbital ridges. In anthropoids, the jugum sphenoidale and cribriform plate scale isometrically with brain size but total anterior cranial floor length scales with negative allometry, leaving insufficient room for frontal contribution to the midline at larger brain sizes. These allometric “spatial packing” limitations may explain secondarily-derived spheno-ethmoid contact in large-brained African apes, but the underlying scaling relationships need to be tested intraspecifically. Spheno-ethmoid contact in orangutans, on the other hand, may be the byproduct of crowding a large, projecting temporal lobe and dorsally-rotated cribriform plate onto the endocranial surface. These ideas are consistent with the hypothesis that spheno-ethmoid contact at the level of the anterior cranial floor is not developmentally homologous in small- and large- brained primates.