gastropods, brachiopods, and echinoderms were still common.
All was hunky-dory till about 252 million years ago, when the Permian period ended with “the Great Dying” — Earth’s most extreme extinction event ever. Ninety-six percent of all marine species and 70 percent of all terrestrial species were wiped off the planet. Talk about a sad ending! This mass extinction event was most likely caused by one or more of the following:
Climate change due to a mass release of methane into the atmosphere from the oceans
Volcanic activity/eruptions in Siberia
A really big rock slamming into the Earth (a large crater matching the age of this event has been found near the Falkland Islands off the coast of Argentina)
And on that happy note, we come to the end of the Paleozoic era … but think about all the good times we had: the diversity of life-forms exploded, plants and animals spread from sea to land, and everyone’s favorite super-continent, Pangea, was formed. It also set the stage for … wait for it …
Gaining momentum in the Mesozoic era
Sorry, are you still waiting? The Mesozoic era (from 252 to 66 million years ago) is known as the Age of Reptiles or, for you terrestrial loving tree-huggers, the Age of the Conifers, not to mention the freaking DINOSAURS!!!!! This era is divided into only three periods (phew).
Spoiler Alert: If you want to skip the next few sections, here’s the plot summary: Early mammals, birds, amphibians, bees, and flowering plants evolved. Dinosaurs took over the planet (woohoo!), then all non-avian (not birdlike) dinosaurs went extinct (booo!). The Mesozoic marine revolution occurred, during which shell-crushing marine reptiles evolved, causing organisms with shells to grow stronger, spinier shells in self-defense, which sparked an evolutionary arms race that continues to this day.
The Triassic period
The Triassic period (from 251.9 to 201.3 million years ago) was a time of rebuilding. After the Great Dying, few creatures remained. (Talk about a shrinking dating pool!) So, who was left? Temnospondyls (early amphibians) and therapsids (early relatives of mammals) made the cut, as did some fish, including some small sharks, and some mainly amphibious marine reptiles. Most animals living in shells fared poorly due to ocean acidification, which hinders the ability to form shells (more on that later). Hinged brachiopods, crinoids (sea lilies), and some ammonites (early mollusks) did okay.
During this period of rebuilding:
The first ichthyosaurs (fishlike reptiles) evolve, probably from terrestrial ancestors that returned to the ocean; for example, the cartorhynchus (say that three times fast) has flippers like a seal and a tail like a lizard. Ichthyosaurs were predators, feeding on fish, shellfish, reptiles, and other ichthyosaurs. Some were ram feeders, like today’s whale shark, swimming toward their prey with mouth open to engulf the prey and the water around it. Others could crush shells or bore into them.
Archosaurs (which means ruling reptiles) appeared on the scene. Broadly classified as reptiles, this group includes all extinct dinosaurs along with birds and crocodiles.
The archosaurs split into two groups: Saurischia (lizard-hipped) and Ornithischia (bird-hipped).
The plesiosaurs arrive — large marine reptiles with four flippers, a long neck, and a long tail (see Figure 3-5).
Source: National Park Service, Public Domain
FIGURE 3-5: An average, everyday plesiosaur.
This period abruptly ceased with the End-Triassic Mass Extinction, also known as the Triassic-Jurassic (T-J) Extinction event. Around the same time, Pangea started to break apart, with North America drifting off first. These were triggered by massive shifts in the tectonic plates which caused an enormous spike in volcanic activity and a resulting increase in greenhouse gasses, which really cranked up the heat. This particular mass extinction caused the loss of the temnospondyls (sort of like salamanders), therapsids (mammal-like reptiles), and most ichthyosaurs (though some larger species survived).
The Jurassic period
The Jurassic period (from 201.3 to 145 million years ago) is everyone’s favorite (thank you Michael Crichton and Jeff Goldblum!). Because you’ve probably seen the movie, we’ll keep it simple … ish. During this period:
Ornithischians (vegetarian dinosaurs with bird hips) begin to spread and diversify.
Adaptations to leg and pelvic bones allow for a larger gut and, hence, larger teeth to help fill that gut, ultimately resulting in larger dinosaurs.
Megazostrodon, the first true mammal appears, but unfortunately it’s as ugly as today’s possums … even though they are kinda cute.
Some mammals become aquatic (such as the castorocauda, a beaverlike critter) and some take to the air (such as the volaticotherium, similar to today’s flying squirrels, definitely cute).
Paravian dinosaurs appear, sporting feathers in place of scales (très chic), becoming, unbeknownst to them, ancestors of modern birds.
Large marine reptiles rule the seas as main apex predators (top of the food web with nothing big enough or mean enough to eat them). These marine reptiles include plesiosaurs and ichthyosaurs (which look like plesiosaurs without necks) and, later, pliosaurs (close cousins of ichthyosaurs) and marine crocodiles.
Diverse ecosystems develop complete with ammonites, gastropods, and fish.
Freshwater turtles put in an appearance.
The Jurassic period ended with a relatively minor extinction event (what a disappointment). Pangea continued to go to pieces, and sea levels began to rise, creating shallow seas in North America and Europe. These rising seas coupled with a change in climate caused by an increase in volcanic activity in the Pacific may have been the cause.
The Cretaceous period
Life literally started to bloom during the Cretaceous period (145 to 66 million years ago), during which time flowers first appeared in the form of a genus of plants known as Archaefructus. Of course, plenty of other activity was going on as well:
Tyrannosaurids (early tyrannosaurus relatives) evolved.
Ceratopsians (those dinosaurs that look like rhinos) grew their head gear.
Several awesome dinosaurs stepped on stage, including: Ankylosaurs (sort of a cross between a horned toad and an armadillo), titanosaurs (the reptile version of a giraffe), hadrosaurs (hard to describe), and azhdarchids (giant flying pterosaurs).
Mosasaurs appeared, looking like a cross between a whale and an alligator.
Rays and modern sharks became common.
