Spiny Sharks ("Acanthodians")
are a group of extinct fish,
and are probably the earliest known jawed vertebrates. Their jaws
are believed to have evolved from the gill arches of earlier jawless
fishes, some of which used jointed cartilage as a "gill skeleton".
Their
fossils
first
appear during the early Silurian period, perhaps around 440 million
years ago, and they went extinct during the late Permian period, perhaps
around 250 million years ago. They evolved initially as marine species,
but later during the Devonian and Carbiniferous periods colonized freshwater
environments too - from the Devonian period onwards, freshwater species
predominate.
Despite their name, and an outward appearance
that somewhat resembled sharks, the Acanthodians were not sharks - rather
had features of both bony fish
(Osteichthyes) and cartilaginous fish (Chondrichthyes) - for example,
they had a cartilaginous skeleton, but their fins had a bony base.
Acanthodians are called "spiny", because all their fins, except
the tail, were supported by bony spines.
Another interesting feature of Spiny Sharks is that their epidermis was
covered with tiny rhomboid-shaped scales - although in some species particular
scales are greatly enlarged, covering the top of the head, the shoulder girdle,
or providing a bony flap over gill openings. Each order of Acanthodii
had their own distinctive arrangement of scales, and as a result it
is often possible to determine the relative age of sedimentary rocks
by examining Acanthodii fossils
and their scales.
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