The geologic time scale is the construct used to delineate the major geological events that have taken place throughout the history of Planet Earth. The largest divisions are called Eons, enormous periods of time indeed. Everyone today is living in the Phanerozoic Eon, which has lasted for some 570 million years. Phanerozoic means evident life, and so the Phanerozoic Eon is the Eon of Evident Life.
This does mean to imply that life began 570 million years ago – MYA – but only that this is the eon for which an abundant fossil record exists.
The Phanerozoic is divided into three subsections called Eras. These are the Paleozoic Era or era of ancient life, the Mesozoic Era or era of middle life, and the Cenozoic Era or era of recent life.
We are concerned with the Mesozoic Era, an interval which began about 250 MYA and ended about 65 MYA at the beginning of the Cenozoic, the current era in the planets long developmental history.
The Mesozoic Era is itself divided into three distinct periods; with the familiar sounding names Triassic, Jurassic and Cretaceous periods. The Mesozoic is also referred to as the age of dinosaurs, for it is during this interval that these fascinating creatures great and small developed, flourished, diversified and became the dominant species on land, sea and through their close relatives the reptilian pterosaurs in the air as well.
Middle life aptly describes the paleontological history of the Mesozoic. The era began with the greatest extinction event in the earth’s history, the Permian extinction or “Great Dying” which nearly ended life on the planet entirely accounting as it did for 90% of all plant and animal species with marine life being particularly hard hit. It ended with the second largest extinction event, the K-T extinction separating the Cretaceous period from The Cenozoic era which destroyed, among much else, the dinosaurs in their entirety.
Unless we count birds among the dinosaurian population as it seems we surely must, not a single representative of that great reptilian superorder saw the dawn of a Cenozoic day. This cleared the way for the opportunistic mammalian order to claim the place in the sun which it enjoys down to present times.
Great geological changes took place during the Mesozoic. Coming out of the Paleozoic all of the Earth’s significant land masses were concentrated into one great continent called Pangaea. By the end of the Cretaceous period the breakup of Pangaea into a more recognizable world map was well underway. South America was sundered from Africa; North America from Eurasia and Australia and Antarctica although still united as one land mass were heading south towards their present locations.
Plant life also underwent a great transition; the era began with conifers and non-flowering shrubs, with cycads and cycadeoids as the dominant fauna. By the late Jurassic these had declined in importance with the development and diversification of familiar flowering plant types. Some major groups like the cycadeoids becoming totally extinct.
The Mesozoic Era was a long and dynamic time range, transitioning much of the ancient world’s geography and life forms from the vastly alien into forms readily recognizable today. It remains a most fastening interval in the history of Planet earth.