The rolling chew

Mammals chew their food. Most everything else seizes it and swallows it whole, or—at most—slices it up using batteries of more-or-less identical teeth, the cusps of which are arranged in line. A mammalian innovation is the so-called tribosphenic molar, in which the cusps are arranged in two dimensions to give a chewing surface, such that teeth in the upper jaw can interlock with those on the lower jaw. This allows for much more thorough processing of food before it gets to the gut. But chewing involves much more than teeth. In some mammals the jaw articulation is mobile, enabling the lower jaw to move from side-to-side, or back-and-forth, relative to the upper jaw. The two halves of the lower jaw join in front at the mandibular symphysis, and this might be relatively loose, allowing each hemi-mandible to ‘roll’ relative to the other. The question I’m sure is on the tip of your teeth is when such jaw movements evolved. Did they evolve alongside the tribosphenic molar, or later? Here Bhart-Anjan Bhullar and colleagues opt for the first view. They have used 3D X-ray cinematography to watch opossums chewing, and show that the jaws can roll. This is an advance because opossums represent a rather primitive grade of mammal, making it likely that mammalian chewing evolved early on, and that the current state of mandibular fusion is a feature of later mammals.