Early Jurassic: Manifest Destiny
With the end of the Triassic Period 201 million years ago, Earth entered into the Jurassic Period, the second of three time spans that make up the Mesozoic Era. During the Early Jurassic and in the wake of the End Triassic Extinction, life on Earth shifted radically. Dinosaurs filled just about every major niche in their environments, from apex predators to megaherbivores down to smaller insectivores and tiny herbivores. At this point in history, it’s easy to assume that the environment was exclusively filled with dinosaurs from all the mass extinctions and ecological shake-ups. And while it’s true that dinosaurs were the dominant group of animals at the start of the Jurassic period, Earth was teeming with many other amazing creatures. Amphibians, lizards, and turtles still made up much of the ecosystem. And although Archosaurs had been brought to their knees by the end of the Triassic, the ancestors of crocodiles and alligators were still around. Many species had settled into the very niche they occupy today as semi-aquatic ambush predators. However, a few select groups, like Protosuchus, stayed on land to carve out a meek existence among the dinosaurs.
In the oceans, where dinosaurs were nowhere to be found, reptiles reigned supreme. Many different groups of reptiles independently returned to the ocean and, like dolphins and whales, traded their legs for flippers. The oceans were teeming with tons of unique and strange animals. Long-necked plesiosaurs ambushed fish, while the speedy and powerful pliosaurs swam after larger prey. Some even looked eerily similar to animals we see today. Ichthyosaurs adopted a body reminiscent of dolphins with long snouts and dorsal fins. Even the Thalattosuchians, the ancestors of crocodiles, returned to the seas—keeping their iconic scaly exterior and toothy snout while sporting short, stumpy flippers. The ocean became such a diverse and even deadly place that not even dinosaurs would dare venture into the waters.
In the skies, the pterosaurs had their breakout. The ecological space was theirs for the taking, and pterosaurs were found throughout the whole world. Pterosaurs like Dimorphodon were roughly the size of a Bull Terrier and featured a disproportionately large head with a set of snaggle teeth. These fuzzy fliers soared across Pangea in search of insects or small lizards.
It was during this time that the ancestors of modern mammals evolved. These mammals would have looked like shrews or chipmunks skittering around in the underbrush. And like echidnas and platypuses, they would have still laid eggs. But overall, it was now the dinosaur’s world, and it would stay that way for a long time.
At this point, the eruptions of the CAMP had ceased, and carbon dioxide was finally on the decline. Carbon dioxide was declining at a very slow rate, suggesting that carbon sinks were doing their job; plants were absorbing carbon dioxide and releasing oxygen in return. This period saw a new abundance of oxygen that hadn’t been seen for millions of years. It’s estimated that oxygen levels were much higher than they were in the Triassic and they were similar to if not slightly higher than what we experience today. In fact, the long stretch of cooler temperatures during this time has led us to dub it the Early Jurassic Cool Interval. But while oxygen levels are correlated with a cooler temperature, carbon dioxide is the main driving force when it comes to our climate. Carbon dioxide is considered a greenhouse gas, but oxygen is not necessarily an “anti-greenhouse gas.” It’s simply a byproduct of plants’ absorption of carbon dioxide. As plants absorb carbon dioxide to produce oxygen, there’s less greenhouse gas in the atmosphere to keep the planet warm. Thus, high oxygen levels are often related to a decrease in temperature.
Even though this period was identified as a Cool Interval, it’s a relative term compared to the whole Mesozoic. This Cool Interval only reached about 19 degrees Celsius at its coldest point, which is hotter than today. To quote the American geologist and paleogeographer Christopher Scotese: “Not all climate modes can be neatly pigeon-holed as either a steaming hothouse world or a frigid icehouse world.” Yes, the dinosaurs lived in a warmer world than our current one during the Mesozoic. The entire Triassic Period was unique in that it was extremely hot and dry as a result of the immense carbon emissions from the Siberian Traps at the beginning of the Triassic. In fact, the poles had no permanent ice caps during the Early Jurassic like we do today. But it was significantly cooler than the Triassic Period: cool enough that it could snow in the winter (even if it would never stick around for more than a month or two). And the temperatures were moderate enough to support life throughout the globe.
Earth’s climate states can be broken up into five major groups: Hot Ages, Warm Ages, Transitional Ages, Cool Ages, and Ice Ages. Paleoclimatologists consider the Early Jurassic to be one of these transitional states in Earth’s climatic history. The Earth has been in transitional climates like this for roughly 27% of the time that complex life has been around. That’s more than Hot Ages and Ice Ages, which only make up 18% and 13% respectively. This leaves Warm Ages at around 23% and Cool Ages (like our current climate) at 18%. Thus, Earth is usually warmer than what it currently is.
So, if Earth has a track record of usually being warmer than it is today, what does this say about our modern world? Is global warming inevitable? Are we set on a course that can’t be stopped? Global warming as a whole is probably inevitable. Just from simple probability based on those percentages, it’s very likely our planet will get warmer in the future. But that’s okay. Life has survived the harrowing Hot Ages of the past for millions of years and we are still here. Not only are we still here, but life is still extremely diverse, showing us that many animals, plants, and microorganisms are capable of surviving the worst this planet can throw at them. Humans even lived through the Ice Age, another relatively extreme climate. If life has had to spend millions of years in extreme climates, it’s doubtful that any climate state could make this planet unlivable. And, more importantly, our current climate crisis is not inevitable.
