Such microbial mats are modern analogs to stromatolites : Precambrian laminated sedimentary rocks , which are the oldest dated microfossils ( known to be 3.5 billion years old ) .
Such microbial mats are modern analogs to stromatolites : Precambrian laminated sedimentary rocks , which are the oldest known microfossils ( dated to be 3.5 billion years old ) .