protruding jaws, lined with serrated teeth, of the Great White
Shark (Charcharodon carcharias) are familiar images
from movies and nature shows. Members of the class Chondrichthyes
skates, rays, and chimaeras) were among the first vertebrates
to evolve jaws and bony teeth and to this day populate the
seas in great abundance.
class name Chondrichthyes (“cartilaginous fish”)
refers to the cartilaginous skeleton of these organisms. In
fact, the skeleton of chondrichthyans is calcified, reinforced
with granules of calcium carbonate deposited in the outer layers
of cartilage. The calcite granules are arranged in a mosaic
pattern unique to Chondrichthyes. This mosaic pattern created
from small platelets or prisms is referred to as prismatic
calcified cartilage and distinguishes the chondrichthyan cartilaginous
skeleton from that of Agnathans and Placoderms (Benton, 2005,
pp. 58-59). A thin layer of bone (calcium phosphate) covers
the cartilage (Dixon, 1988, p. 28). Cartilaginous fish have
paired fins reinforced with horny rays of cartilage. Male chondrichthyans
possess claspers behind their pelvic fins, which are used for
internal fertilization. Most fish fertilize eggs externally
and produce large numbers of offspring. Chondrichthyans not
only have internal fertilization, they also produce small numbers
of offspring. Chondrichthyans lack a swim bladder and have
some problems with buoyancy; they must continuously swim to
avoid sinking. Contrary to popular belief they do not have
to swim to breathe (Prothero, 1998, p. 347). The skin of chondrichthyans
bristles with teeth-like placoid scales or denticles, which
give the skin a rough texture. Cabinet makers of the 19th century
used shark skin, called shagreen, as a sandpaper (Dixon, 1988,
p. 28). The teeth of Chondrichthyans are modified placoid scales.
The teeth and denticles are constantly replaced during the
fish’s lifetime. The cartilaginous skeleton of chondrichthyans
is not easily fossilized, so it is the teeth, horny rays, and
placoid scales that are most often found in the fossil record.
The First Chondrichthyans
for the first possible chondrichthyans comes from scales
and teeth found in Ordovician and Silurian deposits.
The first undisputed evidence of chondrichthyans comes from
remains containing prismatic calcified cartilage from the Early
Devonian. Chondrichthyans underwent an adaptive radiation during
the Devonian, becoming abundant and diverse during the Carboniferous
and Permian. At the end of the Permian many of these groups
became extinct, greatly reducing diversity. Chondrichthyans
underwent another adaptive radiation during the Jurassic and
modern forms appeared and diversified. Chondrichthyans evolved
along two lines, the elasmobranchs (subclass Elasmobranchii),
which includes today's sharks and rays and the holocephalans
(chimaeras) and their relatives (subclass Subterbranchialia).
Some of the basal sharks from the Late Devonian look similar
to some living sharks. Cladoselache (Family Cladoselachidae)
is the best-known shark from the Late Devonian. Cladoselache was a 2 meter long shark possessing two dorsal fins, paired
pectoral and pelvic fins. The skeleton was made of calcified
cartilage. Most sharks have heterocercal tails (an asymmetrical
tail with the major lobe pointed upward). Although Cladoselache had a symmetrical tail, the notochord bends upward into the
dorsal lobe only.
from the Carboniferous
Symmoridans (order Symmorida) include some unusual chondrichthyan
families. Mid-Carboniferous limestone deposits at Bear Gulch
in central Montana make up a conservation Lagerstatten that
contains one of the most diverse fossil fish assemblages in
the world (Hagadorn, 2002, p.167). Among the fish found in
this deposit are bizarre sharks that possess various shoulder
spines. Mature male symmoridans such as Falcatus and Stethacanthus were equipped with shoulder spines (modified dorsal fins) rooted
deeply in the muscles of the shoulder region. The first dorsal
fin of Stethacanthus males was somewhat anvil shaped, while
the shoulder spine of Falcatus may remind one of a bayonet.
These structures may have been used in courtship behavior.
Fossil beds of the same age in Glasgow, Scotland that represent
marine and freshwater deposits also contain well preserved
sharks similar to Bear Gultch.
Whorls of Teeth
or edestids (order Eugeneodontiformes) range
from the Carboniferous to the Permian and possess an unusual
tooth whorl. The spiral shaped tooth whorl fits between the
lower jaws and operates against sharp teeth in the upper jaw.
When older teeth are worn newer teeth rotated into place. Older,
smaller teeth were retained in the whorl. Sarcoprion is a well-known
Petalodonts (order Petalodontiformes) range from the Carboniferous
to the Permian. Janassa had the form of a ray; Belantsea had
a bulbous-shaped body. Belantsea had four powerful-ridged teeth
arranged as a pavement good for crushing molluscs and corals.
Spines Behind the Head
Xenacanths (order Xenacanthiformes) were freshwater forms
that range from the Devonian to the Triassic. Xenacanths had
elongated dorsal tails. Some had serrated spines protruding
from behind their head. Xenacanths had very distinctive teeth
with two hook-like cusps at the base of each tooth.
Ctenacanths (order Ctenacanthiformes) range from the Devonian
to the Triassic. Both dorsal fins had deep-rooted anterior
Hybodonts (order hybodontiforms) range from the Devonian to
the Cretaceous. Hybodonts had paired pectoral and pelvic fins
with a much more modern support structure. The paired fins
could be turned and flexed for steering and stabilization.
They had two dorsal fins with thick anterior spines. Hybodonts
had differentiated teeth, with sharp cutting teeth in front
and blunt crushing teeth in back of the jaw, indicating they
may have fed on both fish and crustaceans. Hybodonts had a
fully heterocercal caudal tail and were probably capable of
short bursts of speed. Hybodonts reached a length of 2 meters.
Hybodonts diversified during the Triassic and became the dominant
sharks during the Jurassic. They lived in both marine and freshwater
environments. Hybodus, for which the order is named, was one
of the most common and long-lived fossil sharks ranging from
the Late Permian to the Late Cretaceous. It looked like a small
version of the present day Blue shark (Dixon, 1988, p. 29).
Modern Day Sharks
Neoselachii) or modern sharks range from the Triassic to
recent times. Neoselachians underwent
adaptive radiations during the Jurassic and Cretaceous. Modern
sharks have greater mobility about their jaw (when the
its lower jaw and protrudes its upper jaw), serrated teeth,
larger brains and enhanced sensory areas (especially olfactory),
and calcified vertebrae that enclose the notochord. Primitive
sharks had a cartilaginous sheath that covered the notochord.
The cartilaginous vertebrae of modern sharks improve swimming
ability. Five clades make up the neoselachians. Galeomorphs
(Division Galeomorphii) include such familiar sharks as: bullheads,
whale sharks, great whites, and hammerheads. In general galeomorphs
live in shallow tropical and warm temperate seas feeding on
crustaceans, molluscs, and fish. The largest extinct sharks
were the giant great white sharks Carcharodon megalodon,
which reached almost 12 meters in length. Squaliforms (Division
live in deep cold waters and retain spines in front of the
dorsal fins. The spiny dogfish is a representative. Squatiniforms
(order Squatiniformes) have flattened bodies with broad pectoral
fins and a long slender tail. The angel shark and monkfish
are extant representatives. Batoids (superorder Batoidea) are
the skates and rays. These chondrichthyans have flattened bodies
with wing-like pectoral fins and whip-like tails. The eyes
are on top, while the mouth and gills are on the underside
providing a form evolved for living on the bottom of the sea.
Batoids have flattened pavements of teeth used for crushing
hard-shelled molluscs (Benton, 2005, pp. 164-169). The shark
tooth is the state fossil for Georgia.
The chondrichthyans developed along two lines. Groups
described thus far belong to the Elasmobranchs. The subterbranchialians
(subclass Subterbranchialia) evolved during the Carboniferous.
This subclass includes two extinct and one extant group (the
Holocephali) appear in their modern form during
They have large eyes, broad pectoral fins, and a long rat-like
tail. Their pavement-shaped tooth plates are used for crushing
molluscs. Their jaw is fused and not protrusible, which gives
it strength for crushing shells. Chimaeras are very spiny (Prothero,
1998, p. 349).
Olympiad Fossil Event
The 2011 Science Olympiad Fossil List includes Chondrichthyans--sharks
rays under the category Cartilagenous Fish.
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Fish of Bear Gulch: Janassa Page: Belantsea Page
Fish of Bear Gulch: Falcatus Page
Fossil Fish of Bear Gulch: Janassa Page
Fossil Fish of Bear Gulch: Stethacanthus:
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Walking With Dinosaurs: Hybodus Page