Fossil Stromatolite Pair Precambrian, 600 million years B.P. A pair of well-detailed fossil stromatolites. 100 grams total, 42-52 mm (1 5/8 - 2 in.). Morocco, North Africa. Private Shropshire, UK collection. Stromatolites are layered biochemical accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains in biofilms (specifically microbial mats), especially cyanobacteria. They represent the oldest organisms on earth, some dating back 3.5 billion years. [2, No Reserve]