Himalayan lakes at elevations over to flatfishes (order Pleuronectiformes) in the Challenger Deep, the deepest ocean trench at about . Fishes of myriad varieties are the main predators in most of the world's water bodies, both freshwater and marine. The rest of the vertebrate species are tetrapods, a single lineage that includes amphibians (with roughly 7,000 species); mammals (with approximately 5,500 species); and reptiles and birds (with about 20,000 species divided evenly between the two classes). Tetrapods comprise the dominant megafauna of most terrestrial environments and also include many partially or fully aquatic groups (e.g., sea snakes, penguins, cetaceans). Classification There are several ways of classifying animals. Evolutionary systematics relies on anatomy, physiology and evolutionary history, which is determined through similarities in anatomy and, if possible, the genetics of organisms. Phylogenetic classification is based solely on phylogeny. Evolutionary systematics gives an overview; phylogenetic systematics gives detail. The two systems are thus complementary rather than opposed. Traditional classification Conventional classification has living vertebrates grouped into seven classes based on traditional interpretations of gross anatomical and physiological traits. This classification is the one most commonly encountered in school textbooks, overviews, non-specialist, and popular works. The extant vertebrates are: Subphylum Vertebrata Class Agnatha (jawless fishes) Class Chondrichthyes (cartilaginous fishes) Class Osteichthyes (bony fishes) Class Amphibia (amphibians) Class Reptilia (reptiles) Class Aves (birds) Class Mammalia (mammals) In addition to these, there are two classes of extinct armoured fishes, the Placodermi and the Acanthodii, both of which are considered paraphyletic. Other ways of classifying the vertebrates have been devised, particularly with emphasis on the phylogeny of early amphibians and reptiles. An example based on Janvier (1981, 1997), Shu et al. (2003), and Benton (2004) is given here († = extinct): Subphylum Vertebrata Palaeospondylus Infraphylum Agnatha or Cephalaspidomorphi (lampreys and other jawless fishes) Superclass Anaspidomorphi (anaspids and relatives) Infraphylum Gnathostomata (vertebrates with jaws) Class Placodermi (extinct armoured fishes) Class Chondrichthyes (cartilaginous fishes) Class Acanthodii (extinct spiny "sharks") Superclass Osteichthyes (bony vertebrates) Class Actinopterygii (ray-finned bony fishes) Class Sarcopterygii (lobe-finned fishes, including the tetrapods) Superclass Tetrapoda (four-limbed vertebrates) Class Amphibia (amphibians, some ancestral to the amniotes)—now a paraphyletic group Class Synapsida (mammals and the extinct mammal-like reptiles) Class Sauropsida (reptiles and birds) While this traditional classification is orderly, most of the groups are paraphyletic, i.e. do not contain all descendants of the class's common ancestor. For instance, descendants of the first reptiles include modern reptiles as well