Muschelkalk (Middle Triassic)
The Middle Triassic Sea was an isolated shallow marginal sea. Its German name, “Muschelkalk” (coquina, or literally “shelly limestone”), refers to the massive accumulations of abraded shells found in these rocks. The coquina composed of bivalve, gastropod and brachiopod shells that were transported and deposited by currents and storms can sometimes be traced over great distances, forming geological marker horizons.
Following the mass extinction of marine animals at the end of the Palaeozoic, 252 million years ago, the oceans were invaded by terrestrial reptiles. Within a few million years, marine reptile diversity increased extraordinarily, as did their body shapes and ways of locomotion. Ichthyosaurs started outas undulating, eel-like swimmers, but evolved an increasingly dolphin-like shape and gradually moved towards using their tail fluke for propulsion and their flipper-like limbs for steering. In contrast, sauropterygians paddled with their limbs in order to move their stiff trunks through the water. If and how the long-necked reptile Tanystropheus may have ever swum remains unknown.