Here is an alga fossil called Buthotrephis newlinii (White, 1902). It existed in the Silurian Period (443-417 million years ago). The fossil was found in Kokomo, Indiana USA. It was probably found in the Wabash Formation. The genus was named by James Hall in 1847.
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 UC875.
Reference:
White, D. (1902). Two new species of algae of the genus Buthotrephis, from the Upper Silurian of Indiana. Proceedings of the United States National Museum, 24, 265–273. See this link. The original document can be found here: https://repository.si.edu/bitstream/handle/10088/13582/USNMP-24_1255_1901.pdf?sequence=1&isAllowed=y
Below is an image for David White's Buthotrephis newlinii published in 1902 as plate XVIII.
