More About Ray-Fins: Actinopterygii
When we think of fish, we think of ray-fins
The ray-fin fishes (Actinopterygii) are the largest group of fishes living today. In fact, with about 24,000 identified species, the only credible vertebrate rival of living ray-fin diversity is that of all living tetrapods (amphibians, reptiles, birds and mammals) combined. In addition to their impressive diversity, ray-fins are the most abundant vertebrates in almost any aquatic habitat.
Ray-fins and lobe-fins (Sarcopterygii) are the two known lineages of bony fishes (Osteichthyes). They share a number of important features including the possession of a swim bladder (or lung) and paired pectoral (shoulder) and pelvic fins. One important difference between these groups is found in these paired fins. Those belonging to lobe-fin fishes articulate with the shoulder or pelvis via a single bone (humerus or femur) and contain extensive musculature. On the other hand, the paired fins of ray-fins are supported by lepidotrichia (thin, long rays of endoskeletal bone) and are powered primarily by muscles located inside the body trunk. Most ray-fins can also be distinguished by the presence of a single dorsal fin, acrodine teeth (hypermineralized dentine on the tooths tip), ganoine-coated scales and dermal bones (a distinctive layer of enamel overlying dentine) and the peg-and-socket articulations of their scales.
The earliest records of ray-fin fishes consist of isolated scales, teeth and dermal bones from the Upper Silurian. The record becomes more substantial in the Middle Devonian with the recovery of articulated fossils of Cheirolepis. Early diversification occurs in the Late Devonian and Early Carboniferous, but the variety of ray-fins remains relatively modest until the Mesozoic. Although Late Devonian ray-fins were typically much less diverse than their lobe-fin, shark and placoderm contemporaries, they were often among the most abundant fishes in their ecosystems. For example, Limnomis was one of the most numerous fishes at Red Hill and Mimia was numerous at the marine Gogo locality in Australia.
Determining the relationships among ray-fins has proved difficult. One problem is that many ray-fins are small animals in which the diagnostically useful endoskeleton rarely fossilizes. Another problem is their incredible taxonomic and morphological variety. Finally, most of the 19th and 20th century classification schemes are based primarily on living species. Consequently, most fossil taxa had to be shoehorned into living groups. The most popular scheme included three groups: the overwhelming majority (>99%) belonged to the teleosts; the bowfin (Amia), gars and many Mesozoic taxa belonged to the holosteans; the paddlefish (Polydon), sturgeons (Acipenser), reedfishes or bichirs (Polypterus and Calamoichthys), and a host of extinct "paleoniscoids" belonged to the chondrosteans.
Investigation over the last several decades have revealed that the teleosts are probably a natural group, but the holosteans and chondrosteans are not. A more phylogenetic scheme includes two major divisions: stem and crown ray-fins. Reedfishes are the only living representatives of the stem ray-fins. The crown group has two major clades or lineages containing living fishes: Chondrostei (sturgeons and paddlefish) and Neopterygii (gars, bowfin and teleosts).
Most fishes formerly placed within the "paleoniscoids" belong to the stem ray-fins. All of the ray-fins from the Devonian and Carboniferous fishes belong to this group as do many of those from Permian. The fossil record of reedfishes only extends back to the Late Cretaceous, while those of other stem ray-fins go extinct before or during the Jurassic. Stem ray-fins have thick rhomboid scales, a rigid skull, a long jaw that articulates with the head behind the braincase via a simple hinge joint, and a primitive epicercal tail fin (the notochordnal lobe extends upward while the fin rays extend back and down). Examples include Cheirolepis, Limnomis, Mimia, Moythomasia, Redfieldius, Platysomus and Cheirodus.
Crown ray-fins, which first appeared in the Permian, are distinguished from stem ray-fins primarily in details of skull anatomy. Ironically, Palaeoniscum, the fossil that prompted the erection of the taxon "paleoniscoids", is more advanced (or derived) than most of the other fishes traditionally assigned to the "paleoniscoids". In fact, some authorities consider Palaeoniscum to be an early member of the Neopterygii. In addition to Palaeoniscum and the Neopterygii, crown ray-fins include the Chondrostei (e.g., Polydon and Acipenser) and the related saurichthids or lizard fishes (e.g., Saurichthys).
Neopterygians (=new fin) account for most of the ray-fin diversity during the Triassic and Jurassic and nearly all of it thereafter. They exhibit a suite of evolutionary innovations that include the greater mobility of mouthparts, a reduction in the number and increases in the ossification of vertebrae, the simplification of fin structures, and the transformation of a shark-like tail to more compact, symmetrical and adjustable structure. These innovations, which enhance feeding mechanics and swimming efficiencies, were probably major contributors to their success.
Perleidus, a widely distribute fish from the Triassic, is an excellent example of an early neopterygian. It has a free maxilla (rear upper jaw bone). In later neoptergyians, the free maxilla could swing forward as the jaw was opened and increase the internal volume of the mouth. This in turn would create a suction that drew in food. Perleidus also has an internal shortening of the tail and dorsal and anal fins with the same number of rays are there are bony supports within the body.
The Mesozoic saw the radiation of several major groups of neopterygians. Some of these, including the pychodontids (e.g., Neoproscinetes and Proscinetes), semionotids (e.g., Dapedium and Lepidotes) and caturids (e.g., Caturus), were successful but limited to the Mesozoic. Three other major groups of neopterygians (lepisosteids, amiids, and teleosts) have living relatives. Lepisosteids, or gars (e.g., Lepisoseteus and Obaichthys), were widely distributed during from the Cretaceous through the Eocene, but are now restricted from Costa Rica to northeastern North America. Amiids (e.g., Amia and Calamopleurus) were also widely distributed but are now represented by a single species, the bowfin (Amia calva), which is restricted to eastern and mid-eastern North America.
Teleosts comprise the other major group of neopterygians. They first appear in the Middle Triassic, but diversification doesnt really take off until the late Jurassic. These phenomenally diverse fishes account for the majority of ray-fins by the Mid Cretaceous, but their greatest radiation doesnt occur until the Eocene. Today, teleosts account for more than 99% of all living ray-fins. The success of the teleosts lies in part on their elaborations of the evolutionary innovations found in other neopterygians. The mouths of advanced teleosts have become even more mobile. A free premaxilla (front upper jaw bone) allows the front of the mouth to protrude when the jaws are opened. Thus creating an even more powerful suction. Further refinements of the tail, including the development of uroneurals, a reduction in the number of vertebrate and the reduction of tail fin rays, led to substantially greater thrust and flexibility while at the same time reducing drag.
Relationships among the primitive teleosts, which include pholidophorids (e.g., Philidophorus), pachycormids (e.g., Pachycormus), aspidorhynchids (e.g., Aspidorhynchus), ichthyodectids (e.g., Xiphactinus), and leptolepids (e.g., Leptolepis), are poorly resolved. The major groups of more derived teleosts include clupeomorphs (Knightia, herring), salmoniforms (e.g., pike, trout) osteoglossomorphs (e.g., bony-tounges, mooneye), elopomorphs (e.g., bonefish and tarpon), ostariophysans (e.g., carp, catfish and characins), and acanthomorphs (e.g., Ctenothrissa, Priscarara, perch and surgeonfish). Two of these advanced groups, the ostariophysans and acanthomorphs, account for about 90% of living teleosts.
One ray-fin fish, Limnomis, has been found at Red Hill.
- FishBase, a global information system on fishes:
- Oceans of Kansas' web page on Xiphactinus audax:
- Palaeos.com's overview of ray-fin fishes (under Telesotomi):
- Tree of Life (www.tolweb.org) web page on Actinopterygii:
- U.C. Museum of Paleontology: Introduction to the Actinopterygii www.ucmp.berkeley.edu/vertebrates/actinopterygii/actinintro.html
- WhoZoo web page on bichirs (reedfishes), sturgeons and gars:
- FishBase, a global information system on fishes:
- Carroll, R. L. 1988. Vertebrate Paleontology and Evolution. New York: W. H. Freeman & Co.
- Janvier, P. 1996. Early Vertebrates. Oxford: Claredon Press.
- Long, J.A. 1995. The Rise of Fishes: 500 Million Years of Evolution. Baltimore and London: John Hopkins Univ. Press.
- Maisey, J.G. 1996. Discovering Fossil Fishes. New York: Henry Holt & Co.
- Scientific Literature:
- Lehman, J,-P, 1966. "Actinopterygii." In: J. Piveteau (ed.) Traité de Paléontologie, 4(1):1-242. Paris: Masson S.A.
- Patterson, C. 1964. "A review of Mesozoic acanthpterygian fishes, with special reference to those of the English chalk." Phil. Trans. Roy Soc. London 247(B):213-482.
- Woodward, A.S., 1891-1901. Catalogue of Fossil Fishes in the British Museum, Parts 1-4. London: British Museum.
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