Here is an alga fossil called cf. Rhizomorphia sp. (Matthew, 1908). It existed in the Silurian Period (443-417 million years ago). The fossil was found in Herkimer County, New York USA.
The fossil was on display in the Evolving Planet section of The Field Museum of Natural History in Chicago Illinois, USA as of August 2020. Accession number is PP46694.
Reference:
1908, Matthew, On Some Species of Silurian and Devonian Plants, Trans. Act. Roy. Soc. of Canada