Illaenus crassicauda is a species of trilobites belonging to the family Illaenidae.

These trilobites lived in the middle Ordovician and in the Silurian age (443 - 418 million years ago).

Fossils of this species have been found in the sediments of Sweden and Russia.

[1][2] The Latin species name crassicauda means "fat-tailed", with reference to the shape of the pygidium.

Illaenus crassicauda can reach a length of about 14 centimetres (5.5 in).