Figure 1.

Phylogeny of elasmobranchs and outgroups (from Shirai 1996). Differences in prey type (represented schematically to the right of the phylogeny) correlate with aspects of jaw mechanics. Increased direct control and mobility of the upper jaw (UJ) has evolved twice and led to increased processing capabilities. Batoids have derived lower jaw (LJ) mobility, allowing them to process complex prey internally, while basal sharks must spit and reingest prey in order to manipulate it. Carcharhiniform and lamniform sharks have evolved finer control of upper jaw protrusion mechanisms, allowing them to reduce large prey externally as compared with squalean sharks that swallow smaller prey whole. Jawless fishes (agnathans) and holocephalans exhibit less specialized musculature for control of the mouth and processing.