When many of us think of paleontology, we think of static reconstructions suspended by wire from cavernous ceilings in our favorite museums. Some of us think of little trilobites entombed in stone and sitting on our father’s desk as a paperweight. We might think of stately, dignified creatures striking one final pose for us to admire for an eternity, or perhaps of a dinosaur curling its head back in agony one last time. But the final in situ position of an animal can only tell us so much about its lifestyle- usually, how it died. Paleontologists are interested in that, of course, but we are even more interested in how an animal lived.
Luckily, the skeleton is a good place to look for clues. Fossil primates can pose some especially interesting questions to a paleoprimatologist. Because they live in trees, many different kinds of locomotion are possible. We can look at limb proportions to see if the little guys were clinging to vertical supports and then leaping from them, or perhaps walking on top of thick, horizontal branches, or maybe even swinging below these brances. We can look at the shape of the scapula to see whether the animal kept its arms underneath itself or used them to reach out to the side or above itself. We can look at the fingers to see if they were grasping branches or balancing above them. In species known only from cranial bones, we can also look at the ear bones to see how these guys positioned themselves while in the trees.
The Bony Labyrinth of the Inner Ear
The ear has three main parts. The outer ear is the part we can see. It directs sound energy into the middle ear, which is an air-filled cavity in which three tiny bones called the incus, malleus, and stapes vibrate against each other and the tympanic membrane to transmit sound. The inner ear includes the organ of hearing called the cochlea, and an organ of balance which includes the vestibule and a bony labyrinth of semicircular tubes. These tubes, called the semicircular canals, are the darlings of paleoprimatology because they can tell us so much about balance and locomotion.
In life, the vestibular system (including the canals) is filled with fluid called endolymph and lined with hair-like cilia which sense where the fluid is and when. The semicircular canals are positioned orthogonally to each other so that, when used together, they can sense both vertical and horizontal movements. We think that this is especially important in stabilizing gaze. If you draw a dot on a piece of paper and then shake your head back and forth and up and down, you can still focus on the dot because the cillia in your semicircular canals are sensing the movement of the endolymph and telling your brain what your head is doing. If you’re a small primate moving through the trees and trying to jump from one branch to another, your brain needs to know where your head is so it can tell your body how to land safely.
Mammals which move more quickly and with more agility tend to have larger semicicular canals with more cilia, which allows them to detect changes in direction, attitude, or speed more quickly. In living primates, we know that leapers like sifakas and tarsiers have much larger semicircular canals in terms of both radius and length than arboreal quadrupeds or slow-climbing animals like the loris. Mary Silcox and her colleagues used data from living primates to examine the locomotor patterns of extinct primates, many of whom are known only from cranial remains. She examined five families of the Plesiadapiforms (pre-primate animals resembling modern tree shrews) and several Euprimates as well, including omomyids (the tarsier-like guys with small snouts and large eyes) and adapoids (the lemur-like animals with long snouts).
From post-cranial bones, we have reconstructed the locomotor repertoires of a number of these species. Plesiadapiforms had a diverse locomotor repertoire. Some were living on or clinging to the underside of broad horizontal supports like modern tree shrews; some were “bounding and scampering” about like modern marmosets; others were slowly and deliberately moving on broad supports or the ground. Among the Euprimates, the adapoids displayed a clear divide between the North American group, called the Notharctids, and the European group, called the adapids. The North American notharctids were active arborealists, and most of them practiced leaping to at least some degree. The European adapids were less active. They most likely stayed on top of the branches. The omomyids almost certainly practiced leaping in all species, but some were much more specialized than the others.
From these post-cranial reconstructions, we expect the Plesiadapiforms and the European adapids to have rather small semicircular canals, and notharctids and omomyids to have larger canals. Using CT scans, Silcox and her colleagues measured the radius of the semicircular canals, scaled them for estimated body mass, and compared them with those of 91 extant species. Generally, the predictions were borne out. Plesiadapiforms had smaller canals that were consistent with slower, less agile movements than the omomyids, who had larger canals, and the adapoids fell in between. Silcox and her colleagues looked at Rooneyia, a fossil omomyid who is only known from cranial elements, and determined it to be an “occasional leaper,” similar to other omomyids with known post-crania. We can thus make the prediciton that, when a more complete Rooneyia skeleton is found, it will have a body consistent with a leaper.
Paleontologists love to take the static images of fossilized museum specimens and reconstruct them as dynamic creatures, quickly bounding from branch to branch or slowly scampering across a big tree branch. A good paleontologist will be able to do this not only in her imagination, but by knowing the primate body inside and out (pun intended, maybe) and being able to wield some statistical weapons to make her case.
Literature cited:
Silcox M, Bloch J, Boyer D, Godinot M, Ryan T, Spoor F, Walker A (2009). Semicircular canal system in early primates. Journal of Human Evolution. 56:315-327.
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