Lower Jurassic (Hettangian–Toarcian)
Earliest Jurassic assemblages are generally of low diversity, often monospecific, but throughout the Lower Jurassic we see diversity increasing steadily with the establishment of what would become typical Mesozoic, marine ostracod assemblages with radiations in all cytheroidean families, particularly the Cytheruridae. The most notable evolutionary event within this period is the global extinction of the Metacopina in the earliest Toarcian that appears to be broadly coincident with a major perturbation in the global carbon cycle at this time.
Although this period is characterized by fluctuating sea levels most Lower Jurassic ostracod assemblages occur in fully marine facies. Shallow-water, condensed sequences are not uncommon in some areas during the Toarcian, as are occasional periods of bottom-water dysaerobia. In general, the Lower Jurassic ostracod record marks the first real recovery and diversification in post-Palaeozoic assemblages following the end-Permian extinction.