Meeting Abstract
This study surveys animals that use soft tissues rather than rigid links to build jaw joints. Rigid biting elements are useful; having hard surfaces to use on substrates or other organisms can directly impact survival and reproduction. Typically, biting surfaces are connected through rigid jaw links that transmit the bite reaction forces. As such, jaws must incorporate joints that resist compression resulting from bites. Most jaw joints are “sliding joints”, in which jaw links come into direct contact and the shape of the siding contact surfaces dictates possible link motion. There are, however, organisms that have biting elements on jaws that are made of flexible muscle and connective tissues. If arranged as a muscular hydrostat, in which multiple muscle fiber orientations may co-contract to provide turgid skeletal support, the multifunctional joint may a) provide the force to move the biting elements, b) create pivots and c) transmit bite reaction forces. Such flexible joints, termed “muscle articulations”, may be important to a number of “soft” invertebrates. In this survey, we review muscle articulation function of the joints found between inarticulate brachiopod valves, cephalopod beaks, kalyptorhynch flatworm hooks, and errant polychaete jaws. We also present a novel interpretation of the hagfish knotting/feeding behavior as a putative muscle articulation. Hagfish toothplates, and their flexible support structures, were analyzed using dissections, X-ray microCT using PTAH (contrast agent), and histology. Videos of hagfish feeding show that coordinated jaw plate/body movements provide the leverage needed for strong “bites”, even despite the lack of an element opposing the toothplate.
