Tremataspis 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...

 from 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 of what is now Estonia

Estonia

Estonia , officially the Republic of Estonia , is a state in the Baltic region of Northern Europe. It is bordered to the north by the Gulf of Finland, to the west by the Baltic Sea, to the south by Latvia , and to the east by Lake Peipsi and the Russian Federation . Across the Baltic Sea lies...

.

An osteostraci

Osteostraci

The class Osteostraci was a group of bony-armored jawless fish, termed "ostracoderms", that lived in what is now North America, Europe and Russia from the Middle Silurian to Late Devonian....

d, Tremataspis was about 10 centimetres (3.9 in) in length, and had an armored shield covering its head. Compared with its relatives, the shield was unusually elongated, covering the whole front of the body, and was more rounded in shape. It is thought that Tremataspis used its rounder shield to burrow in the ocean floor, searching for food. Because the shield consisted of one solid piece, it probably did not grow during the animal's life; presumably, the larva

Larva

A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...

e lacked the shield, which only appeared later in life.