10-Introduction

The Age of Reptiles (Figure 10‑1) began with the evolution of amphibians in the Carboniferous Period and extended until the End Cretaceous Extinction. Mammal ancestors ruled the Permian. Beginning with the Triassic, archosaurs ruled the earth during the Mesozoic Era. Scientists classify archosaurs into two groups: archosaurs like crocodiles, and archosaurs like birds. Dinosaurs are in the bird group.

Figure 10‑1. The Age of Reptiles. Credit: Steven Earle. https://opentextbc.ca/geology/. Used here per CC BY-SA 4.0.

Figure 10‑2. Divergence of the mammals, archosaurs, and squamates from amphibians and amniotes. Images on right credited later in this chapter.

Amphibians moved to land in the Carboniferous (Figure 10-2) and reptiles evolved soon afterward (Section 10-2). Amphibians needed to stay near or in the water. The evolution of the hard-shelled egg and layers of dead skin in reptiles prevents desiccation of larvae and adults, respectively, and enabled reptiles to live in dry land areas and exploit food resources on land. There were two branches of early reptiles: synapsids were the ancestors of the mammals, and diapsids were the ancestors of dinosaurs, birds, crocodiles, and squamates.

After the initial evolution of reptiles in the Carboniferous, the reptilian ancestors of mammals ruled the Permian Period (Section 10-2). The end Permian extinction opened the door for the evolution of dinosaurs, birds, and crocodiles at the upper end of the food chain. The evolution of birds (Section 10-3) began in the Triassic and continued in the Jurassic and Cretaceous. Beginning with the bottom of the Grand Canyon, the Colorado Plateau (Section 10-4) contains the geologic record of almost the entire 500 million years of the Phanerozoic Eon. Petrified Forest National Monument contains the record of the early Triassic and has many crocodylomorpha fossils (Section 10-5). Dinosaur National Monument (Jurassic, Section 10-6) has, you guessed it, dinosaur fossils.

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Hesperosuchus (220 Ma) from Chinle Formation in Petrified Forest, Arizona, an early archosaur related to crocodiles. Image credits: Nobu Tomura. Used here per CC BY-SA 2.5