Thelodus is an extinct genus of primitive jawless fish

Fish

Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

 that lived in the Silurian

Silurian

The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

 period. Fossils have been found worldwide.

Thelodus belonged to the Thelodonti

Thelodonti

Thelodonts are a group of small, extinct jawless fishes with distinctive scales instead of large plates of armor....

, a group of jawless fish

Agnatha

Agnatha is a superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. The group excludes all vertebrates with jaws, known as gnathostomes....

 lacking the armored plating characteristic of the related Heterostraci

Heterostraci

Heterostraci is an extinct class of jawless vertebrate that lived primarily in marine and estuary environments. They arose during the Ordovician, and all, save for the Psammosteids, became extinct during the late Devonian...

. The position of Thelodus`s mouth suggests that it fed on the ocean floor. Despite this lifestyle it was also a good swimmer, as indicated by its elongated lower caudal lobe and stabilizing dorsal fin

Dorsal fin

A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...

, anal fin and paired pectoral fins.