You might not recognize the name Mesozoic, but I’m confident you’ll have heard of the periods contained in the Mesozoic era. Today we’re talking about the Triassic, Jurassic, and Cretaceous periods, which are quite possibly the most famous periods on the Geologic time scale. The time of the “terrible lizards” spanned over 180 million years, and gave rise to some of the largest, scariest, and most fascinating creatures to ever walk the earth. It’s time to walk with the dinosaurs!

 

The Triassic (248-206 Ma) began following the Great Dying discussed in my last post, and was relatively lifeless for its first few million years. According to a UK Natural History Museum article, it may have taken up to 10 million years for the Earth to recover and repopulate after the Permian mass extinction. During the Triassic, large conifer forests covered much of the Supercontinent of Pangea, and the most common land vertebrate was a small, herbivorous, and mammal-like reptile called the Lystrosaurus.

 

Meanwhile, freshwater waterways were controlled by a group of amphibians called the Temnospondyls that resembled modern crocodiles and the first Ichthyosaurs arrived in the Earth’s oceans between 250-246 Ma. While not technically dinosaurs, ichthyosaurs were marine reptiles that ruled the sea while the dinosaurs ruled the land. Looking almost like scary, sharp-toothed dolphins, ichthyosaurs grew to enormous sizes, some even rivaling the blue whale. While I won’t have time to cover this branch of terrifying reptiles here, check out this LiveScience article to learn more about some of the most dangerous predators to ever swim in Earth’s oceans.

 

By the mid-Triassic, the mammal-like synapsid reptiles began to decline and the archosaurs, a group including the dinosaurs, birds, pterosaurs, crocodilians, and turtles, began to diversify. The first dinosaurs evolved around 240 Ma, and were small and bipedal, and pterosaurs became the first vertebrates to achieve powered flight around 228 Ma. And, luckily for us, a branch of the synapsids called the mammaliaformes evolved by 225 Ma and would later give rise to all of Earth’s mammals.

 

By the end of the Triassic, the dinosaurs had split into two groups, one of which contained the herbivorous sauropods and another which contained the carnivorous theropods and bird-like ornithischians. However, the Earth once again faced a mass extinction, likely caused by extreme volcanic activity, and all of the archosaurs except for the dinosaurs, pterosaurs, and crocodiles went extinct. Surprisingly, this mass extinction massively benefited the dinosaurs by allowing them to evolve and diversify to fill all the ecological niches created by the sudden death of much of terrestrial life.

 

As the Jurassic Period (206-144 Ma) began, the Earth’s climate transitioned from hot and dry to humid and subtropical as the supercontinent of Pangea began to break apart. According to National Geographic, Plesiosaurs took over the top of the food chain in the oceans while sauropods grew to enormous heights on land. Brachiosaurus grew to 52 feet tall and 85 feet long with a weight of over 80 tons. Other Jurassic dinosaurs included the T-rex like Allosaurus, the armored Stegosaurus, and my favorite tiny Jurassic Park character, the Compsognathus. (Check out this UK Natural History Museum site to learn about these dinosaurs and more!)

 

During this time, pterosaurs also diversified, according to the National Park Service, and a pterosaur was the largest animal to ever fly! Check out this PBS Eons video to learn about the giraffe-sized, flying, cretaceous Quetzalcoatlus!

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The Jurassic period ended with a minor extinction event, and the Cretaceous (144-65 Ma) began immediately afterwards. According to LiveScience, the Cretaceous was the time of some of the largest predators to ever walk or swim the Earth, including the 56 foot long marine Mosasaur and the Tyrannosaurus Rex, the largest ever land predator. The Titanosaurs were the most common sauropods of the time and ate plants far above the Ankylosaurus and Triceratops. The earliest fossilized bird, Archaeopteryx, flew around 150 Ma, while some small mammals hid from the much larger and more diverse dinosaurs. The Cretaceous was certainly a time of giants.

 

However, like all good things, the time of the dinosaurs had to come to an end, and it did so with a bang. The K-Pg (Cretaceous-Paleogene) extinction event occurred around 66 Ma and killed off around 75% of the organisms on Earth, including all the nonavian dinosaurs, the pterosaurs, and many marine reptiles like the plesiosaurs and mosasaurs. While some scientists argue that volcanic eruptions may have played a part, we’ve all heard the story of the asteroid that killed the dinosaurs. The asteroid hit the Yucatan Peninsula, creating a 110-mile diameter crater and many other devastating impacts. The impact may have caused a 10.1 magnitude earthquake, a 330-820-foot-tall tsunami, a hurricane-force shockwave, and may have heated the atmosphere to 2,700 degrees Fahrenheit, spontaneously igniting forest fires across the Earth. Sunlight may have been blocked for up to 16 years, drastically reducing the Earth’s temperature in the tropics from 81 F (27 C) to 41 F (5 C).

 

The large dinosaurs didn’t stand a chance as their food supplies disappeared almost instantly, but the small mammals that were able to burrow and survive as scavengers gave rise to the Earth we know today. Guess we’re lucky that asteroid swung by.

 

Thanks for exploring the Mesozoic with me, and I’ll finish my discussion of the geologic timescale next time with the Cenozoic era, also known as today!