WILGA, Cheryl/D.; Univ. of Rhode Island: Evolution of the Jaw Suspension in Lamniform Sharks

Morphological characters of the jaw suspension and jaw muscles of lamniform sharks were mapped onto a cladogram to shed light on how changes in jaw suspension and protrusion may have evolved. Several evolutionary changes in the jaw suspension and protrusion mechanism has taken place among lamniform sharks. Galeomorph sharks have paired ethmopalatine ligaments connecting the orbital process of the upper jaw to the cranium. Basal lamniform sharks acquired a novel single palatonasal ligament attaching the upper jaw symphysis to the cranium. Lamnid sharks subsequently lost the original ligament while retaining the novel ligament. The preorbitalis muscle plays a major role in protruding the upper jaw in squaliform and carcharhiniform sharks. It originates from the quadratomandibularis muscle and inserts onto the nasal capsule in these sharks. Alopid sharks are the first lamniform group to evolve a partially subdivided preorbitalis with the new division inserting onto the orbital process and surrounding connective tissue. Lamnid sharks retained the two divisions but have modified insertions. The original division inserts onto the connective tissue surrounding the mid upper jaw, while the new division inserts onto the surrounding connective tissue and skin more posterior on the upper jaw. The retractor muscle of the jaws, levator hyomandibularis, has also been modified in the evolution of lamniform sharks. In most sharks, including basal lamniforms, it inserts onto the hyomandibula to retract the jaws after protrusion. In Alopid and lamnid sharks, the levator hyomandibularis primarily inserts onto the upper and lower jaws around the jaw joint. Thus, there has been at least one instance of character loss, acquisition, subdivision and modification in the jaw suspension and muscles associated with jaw protrusion and retraction in lamniform sharks.