Final answer:
The fossilized skeletons of many phytoplankton turn into stromatolites, which are solid, layered rock formations containing fossils of photosynthetic bacteria from around 3.5 billion years ago.
Step-by-step explanation:
Fossilized skeletons of many phytoplankton turn into solid, layered rock formations known as stromatolites. These are thought to be the fossils of oxygen-producing photosynthetic bacteria. Stromatolites consist of laminated organo-sedimentary structures formed by the precipitation of minerals by prokaryotes and represent some of the earliest fossil records of life on Earth. These microbial mats are essential in understanding the development of life on our planet, dating back to nearly 3.5 billion years ago. In these rock formations, blue-green bacteria, which have the ability to photosynthesize, play a crucial role in trapping and binding the sedimentary grains.