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Science
Olympiad
| Class Acanthodii |
|
The first vertebrates to evolve jaws (gnathostomes) are the
acanthodians (class Acanthodii "spines") or spiny sharks.
Although these
fish have a shark-shaped body with paired fins and
an upturned tail (heterocercal),
they are not sharks. There is good reason
to believe that the jaws of acanthodians evolved
from
the first
gill arch
found in their ancestral jawless fish (Dixon, D., 1988, p
32). Acanthodians were typically small fish around 20 cm. Spiny sharks had a streamlined body with a relatively large head and eyes. Acanthodians had skeletons made of cartilage, close fitting scales in their skin made of bone, bone protecting their head and girdle, and fins with a broad bony base that extended along the anterior edge as a dentine spine. The close fitting scales grew by addition of bone with dentine at the margins forming concentric lines that recorded the animal's growth. All of the fins had a reinforcing spine except for the tail. Some early forms had six pairs of spines along their belly. Later forms, like Acanthodes, may have been able to erect their pectoral spines as a defense mechanism (Benton, 2005, p. 60). Some acanthodians developed a bony operculum covering their gill openings. Many acanthodians lacked teeth and instead used gill-rakers for suspension feeding. Acanthodians first appear in the Late Ordovician as marine fish, but evolve into many species including freshwater forms. The class Acanthodii goes extinct in the Permian. Science Olympiad Fossil Event The 2016 Science Olympiad Fossil List does not include the class Acanthodii. |
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. Paleos
Teleostomi Page: http://www.palaeos.com/Vertebrates/ |