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| Class Chondrichthyes | |
| The
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
(Sharks,
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.
Characteristics The 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 Evidence
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. Strange Sharks 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. Spiral Whorls of Teeth Eugeneodontids 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 example. Petal Teeth 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. Dorsal Spines Ctenacanths (order Ctenacanthiformes) range from the Devonian
to the Triassic. Both dorsal fins had deep-rooted anterior
dorsal spines. 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 Neoselachians (cohort
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
shark gapes
it drops
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
Squalea)
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.
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Bibliography |
|
Benton,
M.J. (2005) Vertebrate Palaeontology [3rd Edition].
Blackwell Publishing: Main, USA. Fossil
Fish of Bear Gulch: Falcatus Page Martin, R.A. (2008). Biology of Sharks and Rays: Carcharodon
vs. Carcharocles: What's in a Name? http://www.elasmo-research.org/education/evolution/ |
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