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The Mezozoic Era, which began about 200 million years ago, is divided into the Triassic, Jurrasic and Cretaceous Periods. These are the dinosuar periods and they walked the earth on Delmarva as they did elsewhere. Fragments of both flying reptiles (Pterosaurus) and semi-aquatic dinosaurs (Hadrosaurus) have been found near the Chesapeake and Delaware Canals in North America.
The Mezozoic Era starts with the emergence of the first dinosaurs and mammals, and an abundance of reptiles. At the end of the first epoch, the Triassic period, about 35% of the animal families become extinct. Most of the marine reptiles vanish during this extinction. This allowed the dinosaurs to expand into many niches and become the dominant species throughout the following 140 million years. The Mezozoic waters of Europe were the probable center of dispersal of the ancestors of the present horseshoe crab species
The first birds, butterflies and bees also develop during this time period, along with the first flowering plants. Grasshoppers, ants and flies also appear and are later followed by the first small cat-like mammal. By the end of the Mezozoic era, the class of mammals known as marsupials is common. Our current-day opposum is an example of this class of mammal.
The Mezozoic Era ends with a major extinction of dinosaurs and about 50% of marine invertebrates, probably caused by an asteroid impact or by massive volcanic activity. Horseshoe crabs survived.
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