Morphology and Evolution of the Jaw Suspension in Lamniform Sharks

The morphology of the jaw suspension and jaw protrusion mechanism in lamniform sharks is de- scribed and mapped onto a cladogram to investigate how changes in jaw suspension and protrusion have evolved. This has revealed that several evolutionary modifications in the musculoskeletal apparatus of the jaws have taken place among lamniform sharks. Galeomorph sharks (Car- charhiniformes, Lamniformes, Orectolobiformes, and Het- erodontiformes) have paired ethmopalatine ligaments connecting the ethmoid process of the upper jaw to the ethmoid region of the cranium. Basal lamniform sharks also acquired a novel single palatonasal ligament connect- ing the symphysis of the upper jaw to the cranium mid- ventral to the nasal capsule. Sharks in the family Lamni- dae subsequently lost the original paired ethmopalatine ligament while retaining the novel palatonasal ligament. Thus, basal lamniform taxa (Mitsukurina owstoni, Car- charius taurus, Alopias vulpinnis) have increased liga- mentous support of the lateral region of the upper jaw while derived species (Lamnidae) have lost this lateral support but gained anterior support. In previous studies the morphology of the jaw suspension has been shown to play a major role in the mechanism of upper jaw protru- sion in elasmobranchs. The preorbitalis is the primary muscle effecting upper jaw protrusion in squalean (sister group to galeomorphs) and carcharhiniform (sister group to lamniforms) sharks. The preorbitalis originates from the quadratomandibularis muscle and inserts onto the nasal capsule in squalean and carcharhiniform sharks. Carcharhiniform sharks have evolved a subdivided preor- bitalis muscle with the new division inserting near the ethmoid process of the palatoquadrate (upper jaw). Alopid sharks have also independently evolved a partially subdi- vided preorbitalis with the new division inserting at the base of the ethmoid process and surrounding connective tissue. Lamnid sharks have retained the two preorbitalis divisions but have modified both of the insertion points. The original ventral preorbitalis division now inserts onto the connective tissue surrounding the mid-region of the upper jaw, while the new dorsal preorbitalis division in- serts onto the surrounding connective tissue and skin at a more posterior position on the upper jaw. The retractor muscle of the jaws, the levator hyomandibularis, has also been modified during the evolution of lamniform sharks. In most sharks, including basal lamniforms, the levator hyomandibularis inserts onto the hyomandibula and func- tions to retract the jaws after protrusion. In alopid and lamnid sharks the levator hyomandibularis inserts pri- marily onto the upper and lower jaws around the jaw joint and is a more direct route for retracting the jaws. Thus, there has been at least one instance of character loss (ethmopalatine ligament), acquisition (palatonasal liga- ment), subdivision (preorbitalis), and modification (ven- tral preorbitalis, dorsal preorbitalis, and levator hyoman- dibularis) in the ligaments and muscles associated with the jaw suspension and jaw protrusion mechanism in lam- niform sharks. While derived lamniform sharks (Lamna nasus, Carcharodon carcharius, and Isurus oxyrinchus) lost the ancestral passive lateral support of the ethmoid articulation of the upper jaw, they simultaneously ac- quired muscular support by way of the levator hyoman- dibularis, which provides a dynamic mechanism for lat- eral support. The evolution of multiple divisions of preorbitalis insertions onto the palatoquadrate and mod- ification of the levator hyomandibularis insertion directly onto the jaws provides an active mechanism for multiple protractions and retractions of the upper jaw, which is advantageous in those sharks that gouge or saw pieces from large oversized prey items. J. Morphol. 265:102-119, 2005. © 2005 Wiley-Liss, Inc.