Acanthodii

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, Ischnacathiforma 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 bulbus base, a neck, and a flat or slightly curved diamond-shaped crown.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Fish: : This page is about the animals. For other meanings, please see Fish (disambiguation)....

Osteichthyes: Class Osteichthyes are the bony fish, a group paraphyletic to the land vertebrates, which are sometimes included. Most belong to the Actinopterygii. The others are called lobe-finned fish, and include lungfish and coelacanths. They are traditionally treated as a class of vertebrates, with subclas...

Silurian: The Silurian is a major division of the geologic timescale that extends from the end of the Ordovician period, about 443.7 million years ago (mega years ago, mya), to the beginning of the Devonian period, about 416.0 mya (ICS 2004). As with other geologic periods, the rock beds that define the perio...