Th e Osteichthyes include two major groups: the ray-fi nned ray-fi shes (the Actinopterygii, Greek actin = a ray and pter = a fi n or wing) and the lobe-fi nned fi shes (the Sarcopterygii, Greek sarco = fl esh). Th e Actinopterygii account for the largest number of fi shes in the
conven-Single dorsal fin Endochondral bone
Bony operculum Lung or swim bladder
Fin skeleton extended
down limbs Basals in fin skeleton reduced
Basals lost from fin skeleton Mobile maxilla
Stiffened tail fin (uroneural bones) Mobile premaxilla
Mobile pharyngeal jaws Teleosts
Bowfins Garpikes
Acipenseriforms
“Paleonisciforms”†
Polypterids SARCOPTER
YGIANS
(lobe-finned fishes) CHONDRICHTHYES
(cartilaginous fishes)
OSTEICHTHYES (bony fishes)
ACTINOPTERYGIANS (ray-finned fishes) ACTINOPTERI
“CHONDROSTEANS” NEOPTERYGIANS
Figure 6–2 Simplified cladogram of bony fishes. Quotation marks indicate paraphyletic groups.
and Acipenseriformes. Th e tendency in actinopte rygian evolution is to reduce the endoskeletal fi n elements. In teleosts (the most derived forms of actinopterygians) the basals have been lost, the radials attach directly to the limb girdles, and the visible external part of the fi n consists of only the fi n rays.
In contrast, all but the earliest sarcopterygians have a monobasal fi n in which only a single basal articulates with the limb girdles. Th e metapterygial bony axis extends from the most posterior of the basals down into the fi n. Bony elements called mesomeres form a straight line, with radials branching off to one or both sides, and the fi n muscles follow the bones to form the fl eshy lobe of the fi n.
In the tetrapodomorph sarcopterygian fi shes (the precursors of tetrapods), the fi n skeleton is asymmetri-cal; the radials branch from only the anterior (preaxial) interrelated, and Table 6–1 lists the modern groups of
bony fi shes. Some representative early osteichthyans are shown in Figure 6–3 .
Fins As their names suggest, actinopterygians and sar-copte rygians diff er in the form of their fi ns. Th e ances-tral condition for the ostheichthyan fi n ( Figure 6–4 on page 127) is rather like that of the chondrichthyans—
a row of basals that articulate with the limb girdles (scapulocoracoid for the pectoral girdle, ischiopubic plate for the pelvic girdle), then a row of radials, ele-ments that articulate with the basals, and fi nally fi n rays that branch out from the radials and support the web of the fi n. Th e basals and radials are endochondral bone, whereas the fi n rays (lepidotrichia) have a dermal origin.
Th is ancestral form of osteichthyan fi n is seen in some living actinopterygians, such as the polypterids
Epichordal lobe
Posterior dorsal fin
Anterior dorsal fin
Hypochordal fin
Anal fin
Pelvic fin
Pectoral fin 1 cm
Epichordal lobe
Posterior dorsal fin
Anterior dorsal fin
Hypochordal
lobe Anal fin Pelvic fin
Pectoral fin
1 cm Hypochordal
lobe Anal fin Pelvic fin
Pectoral fin 1 cm
1 cm
50 mm (a)
(c) (b)
(d) (e)
Basal scute Ridge scales
Dorsal fin Main lateral line Maxilla
Premaxilla
Dentary Operculum
Supracleithral series of dermal bone (part of pectoral girdle)
Figure 6–3 Basal osteichthyans. (a) Early sarcopterygian Guiyu (Late Silurian). Th e arrow shows where the water exits from the gills. (b) Relatively unspecialized sarcopterygian (dipnoan) Dipterus, Middle Devonian. (c) Long-snouted sarcopterygian (dipnoan) Griphognathus , Late Devonian. (d) Osteolepiform sarcopterygian (tetrapodomorph) Osteolepis , Middle Devonian.
(e) Typical early actinopterygian Moythomasia , Late Devonian. Note the diff erences in the scale bars.
Brains Many sarcopterygians (such as ourselves) may develop large brains, but they retain the ancestral form of brain development seen in other vertebrates, where the cerebral hemispheres fold inward on them-selves during growth. In contrast actinopterygians have brains that develop by folding the cerebral hemi-spheres outward.
Scales Although most living bony fishes have re-duced, thin scales (elasmoid scales), early forms had thick scales, which differed between the two groups: actinopterygian scales had a thick layer of an enamel-like material (ganoine), while sarcopteryg-ian scales had a thick layer of a dentinelike material (cosmine).
6.2 Evolution of the
Actinopterygii
Stem actinopterygians include a variety of taxa that were formerly placed in a group of extinct fi shes, the
“paleonisciformes,” that is no longer considered to be edge of the skeleton so that the mesomeres and radials
form a “one bone, two bones” pattern, foreshadowing the pattern of tetrapod limbs. It has long been assumed that the symmetrical arrangement seen in the extant lung-fi shes and coelacanths is the ancestral sarcopterygian condition, and that the alternating pattern in tetrapodo-morphs is derived. However, new skeletal material of a primitive Devonian coelacanth, Shoshonia arctopteryx , shows an asymmetrical pattern, suggesting that asym-metry may be the ancestral sarcopterygian condition.
Skulls Other ways in which actinopterygians and sarcop-terygians diff er include their feeding mechanisms and skull anatomy. Although early actinopterygians, such as polypterids, retain a complete dermal skull roof and have no mobility of their upper jaw, more derived actinopter-ygians have reduced the dermal bone in their skull and obtained greater mobility of the marginal mouth bones of the upper jaw, the maxilla and premaxilla. Th e sarcop-terygians have not reduced the dermal bone of the skull, and if they have jaw mobility, it is derived from move-ment within the skull itself, between the anterior and posterior portions of the chondrocranium.
Table 6.1 Classification and geographic distribution of Osteichthyes, the bony fishes Only the evolutionarily or numerically most important groups are
listed. The subdivision of the Neopterygii varies greatly from author to author. Groups in quotation marks are not monophyletic, but relationships are not yet understood. Names in square brackets are alternative names for the groups.
Sarcopterygii (fleshy-finned fishes and tetrapods)
Actinistia [Coelacanthiformes] : 2 species of coelacanths from the western Indian Ocean and central Indonesia, deep-water marine, 1 to 1.5 m
Dipnoi : 6 species of lungfishes from the Southern Hemisphere, freshwater, shorter than 1 to 1.8 m
Tetrapoda : More than 40,000 species of terrestrial and secondarily aquatic vertebrates
Actinopterygii (ray-finned fishes)
Polypteriformes [Cladistia] : At least 16 species of bichirs and the reedfishes, Africa, freshwater, shorter than 30 to 90 cm
Acipenseriformes [Chondrostei] : 27 species of sturgeons and paddlefishes, Northern Hemisphere, coastal and freshwater, about 2 m to at least 5 m
Neopterygii
Lepisosteiformes [Ginglymodi] : 7 species of gars, North and Central America, freshwater and brackish water, less than 1 m to about 3 m Amiiformes : 1 species, the bowfin, North America, freshwater, up to 90 cm
Teleostei
Osteoglossomorpha : At least 219 species of bonytongues, world-wide, mostly tropical freshwater, 10 cm to at least 2.5 m
Elopomorpha : At least 856 species of tarpons and eels, worldwide, mostly marine, 1 to 4 m
Clupeomorpha : About 364 species of herrings and anchovies, worldwide, especially marine, 6 cm to 1 m
Ostariophysi : More than 7931 species of catfishes and minnows, worldwide, mostly freshwater, 1 cm to 5 m
Euteleostei
“ Protacanthopterygii” : About 366 species of trout, salmon, and their relatives, temperate Northern and Southern Hemisphere, freshwater, from about 7 cm to at least 1.4 m
“Stem Neoteleosts” : About 916 species of lanternfishes and their relatives, worldwide, mostly mesopelagic or bathypelagic marine, 7 cm to 1.8 m
Paracanthopterygii : About 1340 species of cod and anglerfishes, Northern Hemisphere, marine and freshwater, 6 cm to 2 m Acanthopterygii
Atherinomorpha : About 1624 species of silversides, killifishes, and their relatives, worldwide, surface-dwelling, freshwater and marine, less than 4 cm to about 2 m
Perciformes : More than 13,173 species of perches and their relatives, worldwide, primarily marine, 8 mm to 3 m
the fi n membrane was greater than the number of sup-porting radials.