Muscle Articulations: Flexible Jaw Joints Made of Soft Tissues.

This study surveys animals that use soft tissues rather than rigid links to build jaw joints. Hard biting elements are useful; they are used in piercing or shearing during feeding and interactive behaviors and can directly impact survival and reproduction. The best understood biting systems include biting elements that are mounted on rigid jaw links that form a joint capable of transmitting the bite reaction forces. As such, jaws must incorporate joints that resist compression. Many jaw joints are "sliding joints", in which jaw links come into direct contact and the shape of the sliding contact surfaces dictates possible motions. 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 orientations of the muscle fibers may co-contract to provide turgid skeletal support, the multifunctional joint not only (a) provides the force to move the biting elements, but also (b) creates repositionable pivots and (c) transmits bite reaction forces. Such flexible joints, termed "muscle articulations", may be important to a number of soft-bodied animals. In this survey, we review the function of previously described muscle articulations: the joints found between inarticulate brachiopods' valves, cephalopods' beaks, the hooks of kalyptorhynch flatworms, and errant polychaetes' jaws. We also review the morphology, physiology, and feeding behaviors of the hagfish as a putative muscle articulation in an effort to understand how this jawless craniate is capable of biting with surprising force, seemingly without the benefit of any obvious method of opposing the force of the dental plate that is used to remove portions of food. Initial analysis suggests that a muscle articulation may be a key feature in coordinating head and body movements to provide the leverage needed for strong "bites".