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GNATHOSTOMATA

Jawed Vertebrates

The Gnathostomata is an extraordinarily successful lineage of over 60,000 species characterized by upper and lower jaws that are derived from modified gill arches. Most gnathostomes also possess paired pectoral and pelvic limbs, vertebral centra, slit-like gill openings at some stage of development, and three semicircular canals (Forey, 1995; Nelson, 2006). These and a host of other features mark them as active, mobile predators with more powerful sensory abilities than agnathans (Gans, 1987; Gans and Northcutt, 1983; Shimeld and Holland, 2000). Living gnathostomes include the Chondrichthyes (sharks and rays), Sarcopterygii (lobe-finned fishes and tetrapods), and Actinopterygii (ray-finned fishes). Phylogenetic relationships of gnathostomes were summarized by Stiassny et al. (2004).