Acanthodii
Fossil range: Latest Ordovician to
Early Permian |
|
Scientific classification |
|
Orders |
Climatiiformes
Ischnacanthiformes
Acanthodiformes |
Acanthodii (sometimes called spiny sharks)
is a class of extinct
fishes, having features of both bony fish (Osteichthyes)
and cartilaginous fish (Chondrichthyes).
They appeared in the early
Silurian (430 mya) and lasted until the late Permian (250
mya). The earliest ancanthodians were marine, but during the
Devonian, freshwater species became predominant. They are
distinguished in two respects: they were the first known
jawed vertebrates, and they had stout spines supporting
their fins, fixed in place and non-movable (like a shark's
dorsal fin).
There were three orders: Climatiiformes,
Ischnacanthiformes and Acanthodiformes. Climatiiforma had
shoulder armor and many small sharp spines,
Ischnacanthiforma with teeth fused to the jaw, and the
Acanthodiforma were filter feeders, with no teeth in the
jaw, but long gill rakers.
Almost all of them were small, slender fish with large
eyes, heterocercal tails, with the caudal vertebrae
supporting the top lobe of the tail fin, like a shark's tail
has today. All had pairs of bony spines along the ventral mid-body line, that often supported a web of
tissue between the spine and the body, creating a fin. Thus
the "spiny shark" nickname. These distinctive spines give
the class its name, from the Greek akanthos.
The
scales of Acanthodii are unique and used in determining
relative age of sedimentary rock. The scales are tiny, with a bulbous
base, a neck, and a flat or slightly curved diamond-shaped
crown.
References
Long, J.A. The Rise of Fishes: 500 Million Years of
Evolution. Johns Hopkins Univ. Press. Baltimore and London.
1995.
External links