Early Mesozoic Nature In and Around Tethys

At the onset of the Mesozoic Era, all of the landmasses were assembled into a single supercontinent (Pangea) that enclosed a smaller ocean, called Tethys, and was surrounded by a single vast ocean (Panthalassa). Carnivorous archosaurs and crocodile-like reptiles, together with amphibians, populated the land, but these groups witnessed a dramatic change that began during the Late Triassic. This change was the rise of the dinosaurs. During the Jurassic, dinosaurs evolved to enormous body sizes and expanded their range over all parts of a gradually more fragmented supercontinent. The break-up of Pangea resulted in extensive coastlines and embayments in which ammonites, other mollusks, and swimming reptiles thrived. The marine reptiles include groups known as ichthyosaurs, placodonts, and nothosaurs, all of which are predatory. Once the invertebrate communities recovered from the end-Permian extinction event, shallow marine settings witnessed the development of Triassic reefs. These were initially built by several different invertebrate and plant groups (calcareous algae), only to be replaced by a new group of corals, the Scleractinia, in the Late Triassic. Marine biodiversity increased when a seasonal climate prevailed along the coastal zones. These conditions affected upwelling and nutrient supplies in the oceans and altered the atmospheric circulation pattern across the land's surface. The interiors of the continents were propelled into a semi-arid or arid climate, during which time the long and warm dry period was interrupted by several humid pulses in the Late Triassic. The most significant humid pulse is called the Carnian Pluvial Episode. At about the same time, extensive basalts, known as the Wrangellia Traps, formed through effusive eruptions. It is possible that this high volcanic activity influenced climate and triggered the Carnian Pluvial Episode. During this humid interval, high extinction rates affected marine groups (ammonoids, crinoids, bryozoans, conodonts), and changes in ocean chemistry resulted in the evolution of calcareous marine nannoplankton, forming chalk. Simultaneously, on land, the number of wetland plants increased along with the expansion of conifers, among which dinosaurs and pterosaurs diversified.