Ultrastructural and molecular phylogenetic data suggest that dinoflagellates diverged as a lineage possibly as early as the Precambrian. However, the fossil record is problematic before the Mesozoic. From the mid Triassic, though, the fossil record of dinoflagellates is a rich source of information on Mesozoic-Cenozoic dinoflagellates, especially the gonyaulacoids and peridinioids. From the sequence of appearance of species and tabulation types and the impression of early morphological experimentation and later stabilization, the early Mesozoic radiation of dinoflagellates appears to be a real evolutionary event: indeed, dinoflagellate morphology as we know it today may originate in that event. This would explain why it is so difficult to interpret earlier fossils as dinoflagellates. However, that the dinoflagellate lineage existed in some form in the pre-Mesozoic is supported by biogeochemical data, early results of which indicate that certain early Paleozoic acanthomorph acritarchs may belong to the lineage.