Climatius is a genus of the order Climatiiformes ³⁾ which is the earliest group of Acanthodian (or Acanthodii) ¹⁾.

Acanthodian, its name derived from the Greek term "acanthos" meaning "spine" ¹⁾, lived from the Lower Silurian to the Upper Permian and flourished during the Devonian ^{1), 2), 5)}, but Climatiiformes became uncommon after the Middle Devonian ²⁾.

The type spieces, Climatius reticulatus ⁴⁾, is 7.5cm - 15cm in length ^{3), 4)}.

Acanthodians characteristically have strong spines in front of their fins, which distinguish them clearly from all other fishes ^{1), 2), 5)}. Especially, Climatius has fifteen spines, two of dorsal fins, a pair of pectral fins, a pair of pelvic fin, a anal fin, and four pairs of median spines between pectral fins and pelvic fins ^{3), 4), 5)}.

The caudal fin includes a prolonged and slender end of the body, and a small trianglar hypocaudal lobe ^{2), 5)}.

They have large eyes which are positioned nearly the anterior end of the head ^{2), 5)}.

Climatius reticulatus has been found in the Lower Old Red Sanstone of England ⁵⁾ (The Old Red Sandstone rocks were formed in the non-marine terrestrial environment). So, C. reticulatus would have lived in the freshwater environment.

Climatius has the large eyes, the developed caudal fin and the strong pectral fins, indicating a good swimmer and a predator ⁴⁾.

Since they were small, they may have swum in school though they had many "spine" and were predators.