Thalassocystis was a species of algea that thrived during the Silurian period. Although fossilized plants from the Paleozoic are somewhat uncommon, they are well represented in the hard limestone of Northern Ontario. We recently worked with researchers on confirming that the northern Ontario species is indeed the same as the ones described in Michigan Silurian deposits.
 
Thalassocystis striata
Earlton fm, Stormbed Quarry
Temiskaming shores, Northern Ontario, Canada
Silurian (Llandovery, the jury is still out)