''Tristychius'' (from , 'three' and 'row') is an extinct genus of euselachian

chondrichthyan Chondrichthyes (; ) is a class (biology), class of jawed fish that contains the cartilaginous fish or chondrichthyans, which all have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fish'', which ...

from the

Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...

period ( Visean). Fossils of ''T. arcuatus'', the type and only species, including fin spines have been found in

Scotland Scotland is a Countries of the United Kingdom, country that is part of the United Kingdom. It contains nearly one-third of the United Kingdom's land area, consisting of the northern part of the island of Great Britain and more than 790 adjac ...

. ''Tristychius'' was a small animal, up to about long. It had a

heterocercal Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported only ...

caudal fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported only ...

, and large spines on two dorsal fins. Unlike other chondrichthyans that have three basal plates on pectoral fin (propterygium, mesopterygium and metapterygium), its pectoral fin had only two basal plates and lacked mesopterygium. Even through it is one of the earliest known stem-elasmobranchs, it is estimated to be a specialized

suction feeding Aquatic feeding mechanisms face a special difficulty as compared to feeding on land, because the density of water is about the same as that of the prey, so the prey tends to be pushed away when the mouth is closed. This problem was first identifi ...

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...

predator, either ambushing or using stealth to approach its prey. While historically considered a

hybodont Hybodontiformes, commonly called hybodonts, are an extinct group of shark-like cartilaginous fish (chondrichthyans) which existed from the late Devonian to the Late Cretaceous. Hybodonts share a close common ancestry with modern sharks and rays ( ...

, a 2016 study considered it to be a basal euselachian, with hybodonts more closely related to Neoselachii (the group of modern sharks and rays) than to ''Tristychius.'' The study placed the genus in the family Tristychiidae along with '' Acronemus.''

References

Fossil taxa described in 1837 Carboniferous sharks Carboniferous fish of Europe Taxa named by Louis Agassiz {{paleo-shark-stub