Fish do not have tusks, but some Paleozoic-age fish do have long, curved, pointed defensive and fin support spines, particularly ancathodians and certain types of sharks. These do look much like "tusks". Several of these fish were present in freshwater environments in the Carboniferous, and are known from coal-bearing strata in Europe, North America, and other areas. Some of the spines were up to a metre in length. These possibilities and others must be eliminated before an interpretation of "tusks" could be considered, let alone demonstrated.

Xenacatnhus, a type of shark with a large dorsal defensive spine (arrowed).

ancathodian These fish have several spines for defensive purposes and for supporting fins (the fins are not visible on this specimen).

Defensive and fin support spines are usually found as isolated specimens because the rest of the skeleton and scales of sharks and ancathodians are relatively fragile and have poor preservation potential. Here is a selection of isolated spines.

From Zittel, 1887:

Ctenacanthus -- Carboniferous p.117. Hybodus -- Jurassic p.68. Machaeracanthus -- Devonian p.116.

Gyracanthus magnificus Dawson, 1868, p.210. According to Dawson, this specimen is "22 inches long". It is from Carboniferous-age rocks on Cape Breton Island, Nova Scotia, Canada. The specimen is stored at the Nova Scotia Museum in Halifax.