Like the Palaeozoic Era, the Mesozoic Era has been divided into smaller units of geological time called period. Triassic is the oldest period of Mesozoic followed successively by Jurassic and Cretaceous Periods.

• 如同古生代一样，中生代也可划分成一些叫纪的较小的时间单位。三叠纪是中生代中最老的一一个纪，然后依次为侏罗纪和白垩纪。