Answer: stromatolite

AnswerCrossword Clue
stromatoliteA calcareous mound built up of layers of lime-secreting cyanobacteria and trapped sediment, found in Precambrian rocks as the earliest known fossils, and still being formed in lagoons in Australasia
stromatolitea rounded, multi-layered structure, found in rocks dating back at least 2800 million years, having present-day equivalents that result from the growth, under special conditions, of blue-green algae