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
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
(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
the Silurian to recent times. Bony fish are divided into
two clades, the Actinopterygii and Sarcopterygii (Benton,
2005, p. 62).
Sarcopterygii "flesh wing")
have fleshy, muscular paired pectoral and pelvic fins.
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
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
of the sarcopterygians evolved into the first amphbians
during the Devonian period (Dixon, 1988, p. 44).
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
roseae like other fish
had gills, scales, and fins. However, the fins were weight
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
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 actinopterygians (subclass
Actinopterygii) were the first bony fish to appear. Actinopterygians
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).
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
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
The first complete actinopterygian
specimens are known from Devonian deposits. Basal actinopterygians
became the dominant fish in the Permian and disappear
in the Cretaceous.
(superdivision Neopterygii) arose in the Late Permian
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
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
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.
Fish: The Teleosts
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
flexible adapted for fine
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
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
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.
M.J. (2005) Vertebrate Palaeontology [3rd Edition].
Blackwell Publishing: Main, USA.
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
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.
roseae (2008): http://tiktaalik.uchicago.edu/