The Calymmian (from Greek calymma, "cover") is the first geologic period in the Mesoproterozoic Era and lasted from 1600 Ma to 1400 Ma (million years ago). Instead of being based on stratigraphy, these dates are defined chronometrically.
The period is characterized by expansion of existing platform covers, or by new platforms on recently cratonized basements.
The supercontinent Columbia broke up during the Calymmian some 1500 Mya.
The breakup began about 1600 million years ago, and continued through this period ending in the Ectasian, around 1300 -1200 million years ago.
Rifts began to appear along the western edge of the north American plate (Laurentia), the east margin of India, southern margin of Baltica, southeastern margin of Siberia, northwestern margin of South Africa, and the northern margin of the South China Block. This rifting was accompanied by volcanic action along the rift sites, which led to large igneous deposits of rock in these regions.
Around 1500 Mya fossils appear of possible eukaryote. Structurally complex micro-fossils with irregularly branching and bulbous protrusions have a morphology that suggests the presence of a cytoskeleton, a character unique to eukaryotes.