One «must» for all four ExoMars sites was clay: fine - grained sediment that, on Earth, is deposited by water and is excellent at preserving the remains of ancient organisms. (science.sciencemag.org)
Palaeontologists can decipher how ancient organisms lived and interacted using taphonomy, the study of how fossils form. (newscientist.com)
This evidence is also difficult to interpret because it requires (1) sequences of sediments covering a long period of time, over a wide range of latitudes and which are easily correlated; (2) ancient organisms which survive for several million years without change and whose temperature preferences are easily diagnosed; and (3) the finding of the relevant fossils, which requires a lot of luck. (newworldencyclopedia.org)