Teleosts (“perfect bone”) far outnumber all other living fish groups, accounting for more than 26,000 species – more species than in all other vertebrate classes combined. For the present discussion, it is important to realize that teleostean evolution largely repeats and extends trends that originated with the ancestral palaeoniscoids and were continued in early neopterygians. Refinements in the structure and function of mouths and fins appear to explain much of the success of the group. Evidence of these trends is preserved both in the fossil record and in the ancestral traits retained by recognizably primitive teleostean taxa. These trends are detailed in Fig. 11.23 and summarized below.
Teleosts, despite their incredible diversity, form a definable group with a recognizable ancestry. On cladistic grounds, at least 27 anatomical synapomorphies support the contention that teleosts constitute a monophyletic group. Chief among these are ural neural arches elongated to form the uroneurals of the tail support, unpaired basibranchial toothplates, a distinctive urohyal, and the prevalence of a mobile premaxilla (Nelson 2006) (Fig. 11.23).
Teleosts arose in the Middle or Late Triassic (215 mybp), followed by major diversifi cation into modern groups in the Cretaceous. Teleostean evolution apparently involved four major radiations, three that each gave rise to distinct, primitive subdivisions, and a fourth that produced the major advanced groups alive today (between three and six other radiations died out during the Mesozoic). Multiple radiations imply that modern teleosts as a group could be polyphyletic, more a developmental grade than a single clade. Yet shared traits among the modern groups imply a monophyletic clade (Lauder & Liem 1983).
Separate ancestors are postulated for the different radiations, but all may have been derived from the pholidophoriforms, an early mainstem teleost group, now extinct. Five families of pholidophoriforms are recognized. Two other poorly understood teleostean Mesozoic orders are the leptolepidiforms and tselfatiiforms.
The first three major radiations of modern teleosts produced the osteoglossomorphs (bony tongues), elopomorphs (tarpons and true eels), and otocephalans or ostarioclupeomorphs (herrings and minnow relatives). These groups stand separately as subdivisions of the Teleostei, apart from the larger, more advanced, fourth radiation, the subdivision Euteleostei. Osteoglossomorphs include two living orders and possibly two extinct Jurassic and Cretaceous orders, the highly predatory ichthyodectiforms (with five families and including the 4 m Xiphactinus) and the lycopteriforms. Elopomorph eels and tarpons are contained in four orders, all extant. The subdivision Otocephala is divided into two superorders. The Clupeomorpha (herrings and anchovies) contain one living order, theClupeiformes, and one extinct order, the Cretaceous to Eocene Ellimmichthyiformes. The other otocephalan superorderOstariophysi contains five orders, all living. Euteleosts include the advanced, living, bony fishes, divided into seven (or nine) superorders, 28 (or 29) orders, 346 families, and more than 17,000 species. Most groups are well represented in early Cenozoic deposits, such as the famous Eocene sites in Green River, Wyoming, and Monte Bolca, northern Italy (see Frickhinger 1995; Long 1995; Maisey 1996).
Phylogenetic relationships among actinopterygian fishes. 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 the branch do not share the trait. Italicized numbers are unique derived traits (autapomorphies) particular to a group and not shared by other taxa. Pholidophoriforms are one of several possible groups ancestral to modern teleosts. Daggers indicate extinct groups. Additional details can be found in Lauder and Liem (1983), Pough et al. (1989, 2005), Nelson (1994, 2006), and papers cited in those publications. 1, single dorsal fin; ganoin in scales, which have an anterior peglike process; pectoral fin with enlarged basal elements (“propterygium”); 2, fully ossified, sutureless adult braincase; 3, dentinous tooth cap; basal elements of pelvic fin fused; modifications to jaw and gill arch muscles; 4, dorsal fin spines uniquely flaglike; pectoral fin base platelike; 5, modifications to dermal elements of skull, pectoral girdle, and fins; spiracle penetrates postorbital process of skull; fins preceded by specialized scales (“fulcra”); 6, upper jaw bones fused; 7, number of endoskeletal elements supporting rays of median fins reduced to a 1 : 1 correspondence; caudal fin more symmetrical, with reduction in upper lobe; dentition of upper pharyngeal consolidated into a tooth-bearing plate; clavicle reduced or lost; 8, vertebral centra convex anteriorly and concave posteriorly (“opisthocoelus” condition); elongate upper jaw largely constructed from infraorbital bones; 9, maxilla mobile; interopercle and median neural spines present; 10, jaw articulation involves quadrate and symplectic bones; gular plate present; 11, mobile premaxilla; posterior neural arches (uroneurals) elongate; ventral pharyngeal toothplates unpaired; 12, particular combination of skull bones present (basihyal, four pharyngobranchials, three hypobranchials); 13, toothplate on tongue bites against roof of mouth; intestine lies to the left of stomach; 14, two uroneural bones extend over the second tail centrum; epipleural intermuscular bones abundant in abdominal and caudal region; 15, ribbon-shaped (leptocephalus) larva; 16, neural arch of first tail vertebra reduced or missing; upper pharyngeal jaws fused to gill arch elements; jaw joint with unique articulation and ossification; 17, specialized ear to gas bladder connection; 18, dorsal adipose fin and nuptial tubercles on head and body; first uroneural bones of tail have paired anterior membranous outgrowth. Additional characteristics of modern teleosts are given.
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