Mesozoic

Since we define it according to the K-Pg boundary, I'd say the Mesozoic ended when the iridium started landing on the earth to create that layer. More on reddit.com

Historically, many geologic time designations were distinguished based on significant and obvious features preserved in the geologic record. The Cretaceous is named in reference to large chalk deposits observed in Europe in the 19th century. The Jurassic itself is named for the Jura Mountains, which Humboldt observed to preserve a unique limestone formation. The precise boundaries of many geologic time periods came to be defined by significant extinction events that had occurred at the time, but as only five major mass extinctions have occurred through the current eon (and which we are doing our best to make six), not every era, period, or epoch can be defined by a mass extinction. That being said, there was an extinction event at the end of the Jurassic, just not a major one. Renne et al. (1992) mentions a "faunal mass extinction associated with the Jurassic-Cretaceous boundary" in the abstract. Tremolada et al. (2006) points to global climate warming at the start of the Cretaceous leading to diversification of calcareous phytoplanton, hence all the chalk that gave the period its name. Vršanský (2005) is a rather intriguing study that identifies deformities in wing structure across a broad population of Mongolian insects at the J/K boundary, which he concludes is evidence of wide-scale environmental stresses lasting on the order of hundreds of thousands of years. While I have no reason to be skeptical of this study, I've also never read anything like it so I can't really speak to its merit. All in all, these studies do identify some level of biological change occurring on global or regional scales at the time of the J/K boundary. Remember, the periods were initially identified and named by 19th century geologists with very limited understanding of the processes and timescales they were observing. The Cretaceous was not named with the extinction of the dinosaurs in mind (in fact it was named before the word dinosaur was ever dreamed), but to designate chalk-rich French rock formations. The Jurassic was originally just used to refer to layers of limestone in the Jura Mountains, also in France. In the Paleozoic, the Cambrian, Ordovician, Silurian, and Devonian are all named after locations in the UK. Seeing a pattern here? Early geologists were nearly all European and were just categorizing the parts of the world they knew. That doesn't make geologic time designations irrelevant or useless elsewhere, but it does mean that clearly defining boundaries on global scales can be difficult and imprecise. Today, we take into account paleoecological and paleoclimatic data to help us understand when precisely these boundaries can be placed, but at the heart of it we're still looking at changes in rock formations and fossil assemblages that are preserved in the geologic record, just with more nuanced methods and understandings. Now all that being said, something very important happened in the early Cretaceous which has come to be a hallmark of the period, though it can't quite be used to define its beginning: flowers! Angiosperms, or plants that reproduce by means of flowering buds, fruits, and pollinators, first appear in the early Cretaceous and absolutely radically alter every single terrestrial ecosystem on the planet. Next time you eat...anything...thank angiosperms and their happy little bee friends. More on reddit.com

Triassic is good enough. "Our journey continues in the period denominated the Jurassic. The supercontinent of Pangaea has split into two landmasses - Laurasia and Gondwana. While not as arid as the Triassic, the climate is still hot and humid, with forests extending into the poles. Following a brief extinction event, out of literal ash, the dinosaurs rose like a phoenix, quickly dominating their environment in all shapes and sizes. Long-necked behemoths known as sauropods browsed above clashes between formidable carnivorous theropods and spiked stegosaurian prey. Flying reptiles known as pterosaurs patrolled the skies, and giant marine reptiles had their own dynasty. Their success will grant these species to rule some more million years, in which they will diversify even more." "Our journey ends in the Cretaceous period. Continental drift continues, as our world begins to take on a more familiar shape. The climate is still considerably warm. Dinosaurs remain in power, and have hit their evolutionary peak. Theropods continue their legacy with the largest carnivore to ever walk the Earth - Tyrannosaurus rex. Mighty herbivores known as ceratopsians fight for their survival with all numbers of horns. Some tiny dinosaurs have taken to the air, but live under the shadows of the biggest flying animals to yet exist. However, it was not to last forever. An asteroid the size of Mount Everest strikes near the Gulf of Mexico with the energy of 10 billion Hiroshima bombs. The impact unleashes global wildfires, in turn enveloping the planet in a firestorm. The ensuing 15-year period of freezing darkness would've finished off any survivors above 25 kilograms. Small, unassuming critters known as mammals, which previously lived in fear of the dinosaurs, were given the opportunity of taking their place. While this was the end of the dinosaurs' rule, it wasn't the end of their kind. One lineage of flying dinosaurs survived, rapidly diversifing in spite of the mammals' rise to power. They still exist with us today, where we now call them birds." More on reddit.com

It was probably even sadder than that. The rising sea levels drowned millions of creatures, and the smog from the explosion killed pretty much all plant life, meaning the rest essentially starved to death

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