The Calymmian Period (from, meaning "cover") is the first geologic period in the Mesoproterozoic Era and lasted from Mya to Mya (million years ago). Instead of being based on stratigraphy, these dates are defined chronometrically. The period is characterised by expansion of existing platform covers, or by new platforms on recently cratonized basements. The supercontinent Columbia started to break up during the Calymmian some 1500 Mya. The Volyn biota have been dated to 1500 Mya.