The Age of Reptiles: Dinosaurs and Marine Reptiles

The Mesozoic began after the devastating end-Permian extinction. The land animals that followed included numerous reptile branches and surviving synapsids, the wider group to which mammals belong. Dinosaurs became prominent within this changing biological world.[3][18]

Some of the oldest well-established dinosaur fossils come from Brazil and Argentina, in rocks roughly 233–230 million years old. Those fossils show that dinosaurs were present by the Late Triassic. They do not identify the exact time or place where the first dinosaur evolved.[2]

Early dinosaurs lived alongside many other archosaurs, including large predators and armored herbivores on the crocodile side of the family. Dinosaur abundance differed between regions, and their species diversity, body forms, and ecological importance changed at different rates.[3]

At the beginning of the Mesozoic, most continental land belonged to Pangaea. This enormous landmass included extensive dry interiors and climates unlike today's. Connected land did not mean equally suitable habitat everywhere.[7]

Rifting began to break Pangaea apart during the Triassic, and separation continued through the Jurassic and Cretaceous. Changing coastlines and ocean barriers altered the routes by which terrestrial animals could spread. The Mesozoic landscape was continually being remade.[7]

Flowering plants changed ecological relationships

Angiosperms became increasingly diverse during the Cretaceous. Flowers, enclosed seeds, fruits, and changing leaf biology created new relationships with herbivores and pollinators. Seeds themselves were much older than flowering plants.[18]

This was a prolonged transformation. Flowering plants did not immediately replace every conifer, cycad, or fern, and their ecological influence continued to expand after the Mesozoic. More species of flowering plants in a flora did not automatically mean they already dominated its biomass.[18]

Pterosaurs were the first vertebrates known to evolve powered flight. Their wings were membranes supported largely by an extraordinarily elongated fourth finger. Birds evolved a different flight apparatus within the dinosaur branch; pterosaurs did not turn into birds.[8]

Pterosaur anatomy and fossils support varied feeding strategies, including catching insects or fish, feeding on small land animals, and filtering aquatic prey. A long-tailed Jurassic pterosaur and a giant Cretaceous form therefore represent very different lives.[12]

Were birds already replacing them?

Late Cretaceous discoveries in Morocco reveal several pterosaur groups with different sizes and proportions close to the end of the period. This challenges the simple picture of a long, inevitable decline as birds became better fliers.[12]

Such discoveries do not prove that global diversity stayed constant. They do show why claims of universal competitive replacement need more evidence than the coexistence of two flying groups.

The Chicxulub asteroid impact is strongly supported as the trigger of the end-Cretaceous mass extinction. Volcanism in India was also changing the climate around this interval, but a necessary combination of volcanic weakening and a final impact is not established.[21]

Dust and other airborne material reduced sunlight and cooled the surface. The resulting disruption of photosynthesis helps explain how a collision at one location could devastate food webs across the planet. Models continue to refine the severity and duration of that environmental crisis.[22]

Non-avian dinosaurs, pterosaurs, mosasaurs, and plesiosaurs disappeared. Ichthyosaurs had already been extinct for millions of years. Some bird lineages crossed the boundary, along with surviving mammals and other reptiles.[5][12][14][15][16][19]