Ryan and colleages report in the Preceedings of the Royal Society B on a large study of fossilized inner ears of primates. In particular this study examined the size of the semicircular canals, part of the vestibular apparatus. This portion of the inner ear consists of three fluid filled tubes that are oriented at right angles to one another. The brain detects the movement of fluid through these canals, and this is a crucial part of how critters (ourselves included) detect angular rotation and maintain balance during locomotion.
The relative size of these canals is related to locomotor agility. Fast moving, agile critters tend to have larger semicircular canals. These bony tubes often get preserved as fossils, and with the help of CT scanning technology, they can be imaged and measured. This data helps illuminate the locomotor style of extinct primates, even those with no post-cranial fossils preserved. Ryan and colleagues offer locomotor agility reconstructions for 17 fossil anthropoids (the group that includes monkeys and apes - including humans humans). This study reveals three major aspects of primate locomotor agility. First, that the earliest anthropoids were slow and not very agile. Second, that New World monkeys from the Miocene are notable for being much more agile than earlier anthropoids. Finally, this study suggests that the earliest Old World monkeys were relatively agile, even though later Miocene descendants were relatively slow.
|Cast of semicircular canal of Aegypopithecus zeuxis. Ryan et al. ,2012 Figure 1.|