Holonema is an extinct genus of relatively large, barrel-shaped placoderms that were found in oceans throughout the world from the Mid to Late Devonian

Devonian

The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

, when the last species perished in the Frasnian-Fammian extinction event. Most species of the genus are known from fragments of the armor, but the Gogo Reef species, H. westolli, is known from whole, articulated specimens. According to these specimens, species of Holonema lived by grazing on stony, stromatolite

Stromatolite

Stromatolites or stromatoliths are layered accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria ....

-like algae called oncholite, apparently by snipping off the points with a specialized snout.