A catastrophic mass extinction more severe than the Permian hits Earth, effectively wiping out more than 99.5% percent of all living species. In other words, hand sanitizer for the planet. All extant tetrapods, along with many fish, plant, and invertebrate clades don't stand a chance. Any animal longer than a centimeter or two was toast. Underwater did better at 30 cm. The surviving groups struggle to live in a wasteland, until the plants return and multiply, and everything returns back to normal. Everything except the fauna, that is.
The Holocene Mass ExtinctionEdit
For starters, a 22-km asteroid is revealed to be on a collision course with Earth. It's still a ways off though, so a prototype rocket is launched, designed to grab the asteroid and nudge it away from Earth. But the outcome is not certain, so several bombs and missiles are prepared to blow it up if necessary. It's far off enough that the most severe damage expected from it exploding is some totaled cars and vandalized building. Then as government funds are being built for people to claim if they have meteorite problems, a GRB hit Earth for 5 minutes and 26 seconds, centered around eastern china.
This pretty much killed half the ozone layer as well as nearly the entire continent of Asia. Society almost collapses completely as panic wraps the globe. Scientists leave their posts, not knowing that the prototype failed. By the time they figure it out and launch the backups, it's too late. They hit the asteroid with enough force to split it in three major pieces and many smaller ones. The first one, 8 km in diameter, slams into Cape Verde. A few hours later as a firestorm spreads and Hypercane Tobias develops and prepares to swallow Africa and Europe, a 4-km asteroid devastates Yellowstone, effectively triggering the overdue supervolcano eruption. And, as the ash cloud spreads over North America and nearby volcanoes go off, a 6-km asteroid breaks up over the Western Antarctic Ice Sheet. Half of it cracks off over the next few days. The remaining asteroids rain down in various locations, including the Pacific, South and East Africa, and the Mediterranean. An eruption of the Vesuvius supervolcano combined with the tremors released by Yellowstone cause the remaining volcanoes to follow suite. Earth's tectonics plates had a temper tantrum and all of them began to move at a pace similar to the crawling of echinoderms. The firestorm that followed killed the majority of things that lived above water.
If that wasn't enough, a solar flare hit the weakened atmosphere and made it through to the surface. After the onslaught, the land was mostly a wasteland of dead plants with a few lucky survivors. The extinction was so perfect, it didn't seem real. Not like that mattered to people anymore, all of whom were dead, dying, or doomed.
In many cases, it is mostly pointless to list the species that survived a mass extinction. You just list off the groups that went under and the major that survived, but the amount of dying at the end of the Holocene means that only about 45,000 species of eukaryotes survived.
As the smartest invertebrates, (and certainly smarter than all remaining vertebrates) cephalopods seem likely to inherit the earth. But with their overall lack of skeletal structure, moving onto land gave cephalopods the developmental delay lungfish and arthropods needed for their moment to shine.
The great subclass of Nautiloidea has been reduced to 6 species at present day. Having survived all five mass extinctions prior to present day and surviving better than it's sibling cephalopods, it wasn't going to give up the ghost quite yet. Only one species survived though, the bellybutton nautilus. It's only hope was to radiate in as many different directions as fast as possible. Lucky for them, it was the largest predator on New Caledonia.
Several noteworthy adaptations set it on the path for success. From Nautulocetaceae and it's filter-feeding abilities to Ammonitomimus and it's strong resemblance to ammonites, the Nautiloids are numerous again.
Ram's Horn Squid Edit
Spirula is often thought to be the closest living relative of the extinct belemnites. What does evolution have in store for the 4 cm cephalopod?
A few species of small cuttlefish survived in the North Sea and Mediterranean. With their main competitors, the malacostracans, reduced to shrimp and amphipods, these 10 cm cephalopods have an interesting future.
Almost no incirrate octopuses survive the mass extinction and all remaining species live deep down.
With an endoskeleton and reputation for colonizing the land, the coelocanths seem to have the biggest advantage. However, the last time they made the transition, the only cephalopods around were little more than specialized nautiluses. The presence of squid and octopi surely put a wrench in their evolution.
The most in store for insects and others is a 16-million year period resembling the Carboniferous, called the Oxygeniferous due to it's high oxygen level of 30.2 % and relatively low CO2 amount. Land arthropods will reach sizes of around 100 cm at most before the even briefer invasion of the Sarcopterygii cause them to return to previous sizes. But relics remain.
For some examples, some horseshoe crabs have become amphibious and are a common sight in Indonesia, while the 1-meter Bornean land shrimp can be found almost anywhere on the island. Descended from mantis shrimps, it attacks any creature that stumbles by it's burrow with a crack that can be heard for miles.
During the Oxygeniferous, many obscure groups of animals managed to diversify and grow in size. Some quickly lost that diversity, but there are a few that have managed to keep their relative dominance into the Age of Cephalopods.
North America Edit
The Giant Tardigrade and mega-rotifers have evolved side by side, developing many traits to keep up with each other. Two of the most notable changes are continued cellular growth throughout their lifespan and respiratory systems that allow them to reach unheard-of sizes. By this, I mean 20 cm and 3-8 cm respectively, which is still massive, considering you need to have a microscope to see their present-day counterparts.
The West African Kinorhynch has taken up the niche that earthworms would usually fill, consuming detritus and aerating the soil.
MORE COMING SOON!