This 5 cm. prehistoric fish of the Ordovician, Arandaspis was supposed to be the first true fish and it just happens to be a boney fish. It has no dorsal fin and cannot close it's mouth. Arandaspis only has two fins, one on each side of it's body and it's tail fin is in the shape of a diamond with a small notch on the end. It's head is covered in bone and here is a picture of one anyway (or how I think of one).

As I said in the first line Arandaspis lived in the Ordovician alongside the Nautiloid, Lingula and Sacabamaspis. Before it in the Cambrian lived the Trilobite, Conodond Animal and Pikaia. After it in the Silurian there lived the Pterygotus, Stylonurus, and the Climatius.