Teleosts per se arose in the Middle Mesozoic (probably Late Triassic, c. 200 million years ago), from a neopterygian ancestor, possibly a pachycormiform (see Subclass Neopterygii).
The earliest teleosts were probably pholidophoroids or leptolepoids, groups that consisted of several families and that may have been ancestral to more than one of the main lineages of teleosts, including the osteoglossomorphs and elopomorphs. The important point to remember, reiterating the phylogenetic account, is that modern teleosts arose during four major radiations that produced the subdivisions
the latter being by far the largest.
A listing of teleostean families is unavoidable, in part to appreciate their tremendous diversity but also because most fishes encountered anywhere in the world will belong to one of the 40 orders and 448 families (and 4278 genera) of teleosts. Despite their amazing diversity, teleosts share a number of characters that indicate common ancestry, particularly in the more advanced subdivision of the euteleosts or “true teleosts” (see below). The primary shared derived (synapomorphic) characters that unite the teleosts involve numerous bones of the tail and skull (Fig. 14.1). Importantly, the ural neural arches of the tail are elongated intouroneural bones. This means that in the tail base region, the neural arches that sit dorsal to the vertebral column fuse into elongate bones termed uroneurals. These new bones serve as basal supports for the rays that form the upper lobe of the tail fin and thus help stiffen it; their number and shape change during teleostean phylogeny. In the skull, among other characters, teleosts have a mobile premaxillary bone rather than having the premaxilla fused to the braincase. A mobile premaxilla is essential for upper jaw protrusion and allows a fish to shoot its mouth forward during prey capture, creating suction pressures and also overtaking prey.
In sum, major changes that define the teleosts contributed to the advances in locomotion and feeding that apparently led to their success. Most of the characteristics described here are discussed in more detail in Wheeler (1975), Berra (1981, 2001), Carroll (1988), and Nelson (1994, 2006). The overall classification followed, many of the characteristics described, and the numbers of species provided for different orders are based on Nelson (2006) with exceptions as noted.
Phylogenetic relationships among living teleosts. The numbered characteristics defining the branching points (synapomorphies) are selected from a much larger list; groups after a branch point share the traits (although traits may be secondarily lost), groups before a branch do not share the trait. Italicized numbers are unique derived traits (autapomorphies) particular to a group and not shared by other taxa. Characteristics 1–7 largely repeat characters 11–18 in the cladogram of Fig. 11.23. Monophyly and definition of some groups are a matter of debate and no synapomorphies are given (i.e., the Protacanthopterygii (osmeriform smelts, salmoniform salmons) remain a problematic group that lacks well-defined, unifying characteristics). Additional details can be found in Lauder and Liem (1983), Nelson (1994), Pough et al. (2005), and papers cited in those publications. 1, mobile premaxilla, posterior neural arches (uroneurals) elongate, ventral pharyngeal toothplates unpaired; particular combination of skull bones present (basihyal, four pharyngobranchials, three hypobranchials); 2, toothplate on tongue bites against roof of mouth; intestine lies to the left of stomach; 3, two uroneural bones extend over the second tail centrum; epipleural intermuscular bones abundant in abdominal and caudal region; 4, ribbon-shaped (leptocephalus) larva; 5, neural arch of first tail vertebra reduced or missing; upper pharyngeal jaws fused to gill arch elements, jaw joint with unique articulation and ossification; 6, specialized ear-to-gas-bladder connection; 7, dorsal adipose fin and nuptial tubercles on head and body; first uroneural bones of tail have paired anterior membranous outgrowth;8, anterior vertebrae and ribs modified to connect gas bladder to inner ear (Weberian apparatus); epidermal cells produce alarm substance; 9, first vertebra articulates with three bones of the skull (basioccipital and the two exoccipitals), retractor dorsalis muscle connects vertebral column with upper pharyngeal jaws, hinged jaw teeth capable of depression posteriorly; 10, unique photophore histology and tooth attachment; 11, unique gill arch structure involving second and third pharyngobranchials; 12, fifth upper pharyngeal toothplate and associated internal levator muscle missing; 13, upper pharyngeal jaw dominated by third pharyngobranchial; 14, configuration of rostral cartilage and its ligamentous connection to premaxilla; lateral ethmoids joined to vomer;15, uniquely protrusible upper jaw; 16, ligament connecting palatine and premaxilla in a unique position; 17, expanded premaxillary processes; 18, dorsal (neural) spine attached to second preural vertebra; 19, branchial retractor muscle (retractor dorsalis) inserts only on third pharyngobranchial; well-developed ascending process of premaxilla allows increased jaw mobility; ligament supporting pectoral girdle (Baudelot’s ligament) originates on basioccipital of skull rather than on first vertebra; 20, no direct connection between pelvic girdle and cleithrum of pectoral girdle; 21, jaw protrusion occurs without ball-and-socket joint between palatine and maxilla; fourth pharyngobranchial lost; 22, pelvic girdle attached to pectoral girdle; anterior pelvic process displaced ventrally; pelvic fins have one spine and five soft rays. Fish drawings from Nelson (2006), used with permission.