Tremataspis is an extinct genus of primitive jawless fish from the Silurian period. This early osteostracan, (also called the cephalaspids("head shields") appeared in the Late Silurian which was about 80 million years after the earliest heterostracan fishes. They evolved in the sea and then colonized freshwaters.) had the typical flattened body and ventral mouth of a bottom dweller.

Compared with its relatives, the shield was unusually elongated, covering the whole front of the body, and was more rounded in shape. Eyes and single nostril were there on the top of the head near the midline.

By using the gill muscles in the throat as a suction pump it sucks the tiny food particles from the seabed. The bony shield extended half way along the body. This shield consisted of one solid piece, it probably did not grow during the animal's life. Paleontologists think that osteostracans had an unarmored lava and that the bony shield developed only when the fish was full grown.