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''Tristychius'' (from el, τρεις , 'three' and el, στῐ́χος 'row') is an extinct genus of 
shark
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimo ...
from the Carboniferous
The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
period ( Visean). Fossils of ''T. arcuatus'', the type and only species, including fin spines have been found in Scotland
Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to the ...
.
''Tristychius'' was a small shark, about long. It had a well-developed upturned caudal fin
Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...
, similar to that of many modern sharks. Physically it may have resembled a modern dogfish. ''Tristychius'' also had spikes attached to the bases of its dorsal fin
A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. Many species of animals possessing dorsal fins are not particularly closely related to each other, though through conv ...
s, probably for protection against predators.
References
Fossil taxa described in 1837 Carboniferous sharks Carboniferous fish of Europe Taxa named by Louis Agassiz {{paleo-shark-stub