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Science Olympiad
Class Osteichthyes
As a group, fish account for some important first appearances in the fossil record. Fish were the first vertebrates, the first vertebrates with jaws, the first vertebrates to evolve lungs, and the first vertebrates with bony skeletons. Their evolutionary success was not limited to aquatic environments as fish are the ancestors to all land vertebrates. To this day bony fishes (class Osteichthyes) are the most abundant and diverse vertebrates. At over 20,000 species, bony fish account for half of all known living vertebrates (Dixon, 1988, p. 36). Bony fish have skeletons made of bone and a gas-filled chamber in the form of a swim bladder or lung. Osteichthyans range from the Silurian to recent times. Bony fish are divided into two clades, the Actinopterygii and Sarcopterygii (Benton, 2005, p. 62).

Lobe-Finned Fish

Sarcopterygians (subclass Sarcopterygii "flesh wing") have fleshy, muscular paired pectoral and pelvic fins. Each lobe-like fin is supported by a centeral core of articulated bones, which are homologous with tetrapod arm and leg bones (Prothero, 1998, p. 357). These robust bones support fin rays. The scales have a rhomboid shape and possess peg-and-socket articulations. The scales (cosmoid scales) and teeth of sarcopterygians are covered with a porous bone material called cosmine. The teeth have a wrinkled enamel surface and have a labyrinth-like appearance when viewed in cross-section. The labyrinthodont teeth of sarcopterygians are also found in archaic tetrapods (Prothero, 1998, p. 358). Sarcopterygians underwent an adaptive radiation during the Devonain, reaching their greatest diversity. Since this time, lobe-finned fish have dwindled in importance. There are two living sarcopterygian groups, the lungfish (order Dipnoi) and coelacanths (order Actinistia) (Benton, 2005, p. 67). The structure of sarcopterygian lobed fins is of great evolutionary significance. Some member of the sarcopterygians evolved into the first amphbians during the Devonian period (Dixon, 1988, p. 44).

In 2004 Neil Shubin and his colleagues found a 375 million year old sarcopterygian in the Canadian Arctic, which represents an intermediate between fish and basal tetrapods (see our page on amphibians). Shubin used the term "fishapod" to describe this fossil that exhibits both fish and tetrapod characteristics. Tiktaalik roseae like other fish had gills, scales, and fins. However, the fins were weight bearing possessing both wrist bones and finger-like bones. Tiktaalik also possessed tetrapod characteristics. The head of Tiktaalik ("large freshwater fish"-taken from the Inuktitut language) was flat with eyes positioned on the top of the skull (Ridley, 2009, p. 70). The pectoral girdle was separate from the skull forming a neck and allowing the head to turn. The ribs were designed to support the body and allow breathing. The fact that Tiktaalik had spiracles on its skull and a more robust ribcage indicates this organism had lungs as well as gills.

Ray-Finned Fish

Ray-finned actinopterygians (subclass Actinopterygii) were the first bony fish to appear. Actinopterygians differ from sarcopterygians in the structure of their fins. Actinopterygians possess parallel bony rays supporting their fin. Ray-finned fish first appear in the Silurian and were relatively rare during the Devonian. Ray-finned bony fishes underwent three major phases of radiation: basal atinopterygian Carboniferous-Triassic, basal neopterygian Triassic-Jurassic, and teleost Jurassic-Present (Benton, 2005, pp. 169-170).

Basal Actinopterygians

Basal actinopterygians are sometimes referred to as chondrosteans, although this is a paraphyletic grouping. Chondrosteans underwent an adaptive radiation from the Carboniferous to the Triassic. These primitive ray-finned fish were once called palaeoniscids, also a paraphyletic grouping. The basal actinopterygians are characterized by thick bony articulated rhomboid shaped scales covered with a shiny layer of enamel called ganoin. Gaoid scales have been found in Silurian and Devonian deposits. Basal actinopterygians had heavy bony skulls with large eyes and a short snout. Their bony jaws were capable only of a snapping bite. The anal fins as well as the paired pelvic and pectoral fins were heavy and triangular in shape. The caudal fin was heterocercal. The first complete actinopterygian specimens are known from Devonian deposits. Basal actinopterygians became the dominant fish in the Permian and disappear in the Cretaceous.

Basal Neopterygians

The neopterygians clade (superdivision Neopterygii) arose in the Late Permian and includes gars, semionotids and other extinct forms, bowfins, and the teleosts. Basal neopterygians are sometimes referred to as holosteans, which is a paraphyletic group. Holosteans underwent an adaptive radiation from the Triassic through the Jurassic. The teleost radation started in the Triassic and continues to this day. Neopterygians are characterized by modifications in the jaw structure, braincase, tail, and scales. The upper jawbones were freed from the cheekbones, which permits the mouth to open wider and allows for the development of stronger muscles. This jaw structure also produced a sucking effect that helps to draw food into the mouth. The tail fin became more symmetrical, but not totally homocercal. Evolutionary trends in scales included a change from rhomboid to circular shape, thinning, increased flexibility, as well as the loss of enamel covering. The holostean radiation was followed by the radiation of a more advanced group of neopterygians, the teleosts.

Modern Fish: The Teleosts

The radiation of teleosts (subdivision Teleostei) began in the Triassic and accelerated during the Cretaceous. Teleosts make up 99% of all living fish and account for half of all living vertebrates (Prothero, 1998, p. 352). The flexibility of the jawbone increases with the teleosts allowing them to protrude their mouth in a circular shape, sucking up their food, rather than biting hard. The skull of the teleost is lightweight and flexible. The swim bladder evolved into a more efficient organ in the teleost making them neutrally buoyant. This allowed the pelvic and pectoral fins to become thin and flexible adapted for fine steering control and hovering. In more primitive ray-finned fish heavy fins were designed primarily for thrust. The vertebrae of teleosts become increasingly ossified. Teleosts have a symmetrical, fully homocercal caudal fin with distinctive radiating elements known as uroneurals modified from the spinal column. The bodies of advanced teleosts became covered with thin, flexible, rounded, overlapping scales with no enamel. Knightia is the state fossil for Wyoming.

Science Olympiad Fossil Event

The 2016 Science Olympiad Fossil List includes the superclass Osteichthyes (Bony Fish). The ray-finned fish are listed under the class Actinopterygii. The Coelacanth is listed under the class Sarcoptergii (lobed-finned) and order Coelacanthiformes. The genus Tiktaalik is also included under this order. To learn more about Tiktaalik see the amphibian section of our website.

Priscacara serrata
Green River Formation
Wyoming, USA
20 cm long

Diplomystus sp.
Green River Formation
Wyoming, USA

48 cm long

Notogoneus sp.
Green River Formation
Wyoming, USA

48 cm long

Priscacara liops
Green River Formation
Wyoming, USA
12 cm long

Phareodus sp.
Green River Formation
Wyoming, USA
16 cm long

Mioplosus sp.
Green River Formation
Wyoming, USA
20 cm long


Benton, M.J. (2005) Vertebrate Palaeontology [3rd Edition]. Blackwell Publishing: Main, USA.

Dixon D., Cox, B., Savage, R.J.G., & Gardiner, B. (1988). The Macmillan Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals: A Visual Who’s Who of Prehistoric Life. New York: Macmillan Publishing Company.

Paleos Actinopterygii Page

Paleos Neopterygii Page

Paleos Sarcopterygii Page

Paleos Teleostei

Prothero, D.R. (1998). Bringing Fossils to Life: An Introduction to Paleobiology. New York: McGraw-Hill.

Ridley, M. (2009). The Darwin Bicentennial Part II: Modern Darwins. National Geographic, February 2009, Vol. 215, No. 2.

Tiktaalik roseae (2008):

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