Fossil dinoflagellates are important in "stratigraphy", the dating and correlation of strata. During their life, dinoflagellates have a mobile planktonic form and a resistant cyst phase which enables them to survive over winter in sediment. Only the cyst gets fossilised. Their first certain appearance is in the Silurian period, with a big radiation in the Mesozoic era. They are classified as "Phylum Dinomastigota" by Margulis and colleagues.