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Science Olympiad
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).

Elasmobranchs

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.

Jurassic Sharks

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.

Holocephalans

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 chimaeras). Chimaeras (superorder Holocephali) appear in their modern form during the Jurassic. Ratfish or rabbit fish live in deep waters. 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).

Science Olympiad Fossil Event

The 2011 Science Olympiad Fossil List
includes Chondrichthyans--sharks (shark teeth) and rays under the category Cartilagenous Fish.



Shark Tooth
Carcharocles megalodon
Miocene
South Carolina
7 cm across root x 7.5 cm tall

 
 
 

Bibliography

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.

Fossil Fish of Bear Gulch: Janassa Page: Belantsea Page
http://www.sju.edu/research/bear_gulch/pages_fish_species
/Belantsea_montana.php

Fossil Fish of Bear Gulch: Falcatus Page
http://www.sju.edu/research/bear_gulch/pages_fish_species/
Falcatus_falcatus.php


Fossil Fish of Bear Gulch: Janassa Page
http://www.sju.edu/research/bear_gulch/pages_fish_species/
Janassa_clarki.php


Fossil Fish of Bear Gulch: Stethacanthus:
http://www.sju.edu/research/bear_gulch/pages_fish_species/
Stethacanthus_altonensis.php


Hagadorn, J.W. (2002). Bear Gulch: An Exceptional Upper Carboniferous Plattenkalk. In Bottjer, D.J., Etter, W., Hadadorn, J.W., & Tang, C.M. [Eds.] Exceptional Fossil Preservation: A Unique View on the Evolution of Marine Life (167-183). New York: Columbia University Press.

Martin, R.A. (2008). Biology of Sharks and Rays: Carcharodon vs. Carcharocles: What's in a Name? http://www.elasmo-research.org/education/evolution/
carcharodon_vs_carcharocles.htm


Paleos: Neoselachii Page http://www.palaeos.com/Vertebrates/Units
/080Neoselachii/ 080.100.html#Neoselachii


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

Purdy, R.W. (2008). The Orthodonty of Heliocoprion. Smithsonian: http://paleobiology.si.edu/helicoprion/index.html

Walking With Dinosaurs: Hybodus Page
http://www.abc.net.au/dinosaurs/fact_files/sea/hybodus.htm


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