Complete fish are relatively rare as fossils; they soon decay and their skeletons disintegrate after death, especially with sharks and their relatives in which the skeleton is made of cartilage. Teeth are often the toughest part of any fish and commonly are the only fish remains found as fossils. Bony spines supporting the fins also may be found as fossils.

In Northern Ireland complete fossil fish are very rare and known from only a few sites. However, many different types of fish are known from isolated teeth. In the Carboniferous Limestone (c.340 million years old) of Co. Armagh fish teeth are among the most conspicuous of fossils; many hundreds were collected from small quarries in the 19th Century, and the Ulster Museum has a large collection of these.

Petalorhynchus	Chomatodus	Copodus	Ctenoptychius	Deltodus
Harpacodus	Helodus crassus	Helodus didymus	Labodus	Polyrhizodus
Psammodus	Psephodus	Rhizodont	Streblodus	Xystrodus