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EDIT: As many have pointed out, the whole Lunar Collision aspect I originally envisioned is probably more extreme than I predicted. I don't know where to start as far as the math goes, although my gut instinct says that a series of smaller collisions could make it work, but I could take or leave the Moon getting enough mass to be habitable. That's just a thought I had mostly for spectacle. The important part is that in modern times, a K-Pg level impact or series of impacts occurs, so what organisms are likely to survive that?

Original Main Body:

An origin I'm considering for the Science Fantasy world I'm building is that in the not too distant future, next Sunday A.D., a foreign celestial body or cluster of asteroids/comets joins Earth's orbit, then collides and merges into the Moon, adding enough mass to it that it eventually gets oceans and an atmosphere, and spreading debris around that peppers the Earth with meteorites for the eons to follow. The first wave of meteor crashes causes a mass extinction event comparable to the K-Pg impact.

So here's my question: What notable modern organisms will have died out or diversified after a new K-Pg type event, which also includes the moon's mass drastically increasing?

I of course searched for more generally related threads before asking. I'm not talking about:

Would animals really mutate in the post-apocalypse? This thread is just talking about radiation-induced mutation.

What is the next dominant species? This is asking what non-primates might become like humans, not quite what I'm looking for.

What aquatic creatures would survive a large-end mass extinction? This is too specific to the ocean, and also assumes that no life larger than an insect survives the impact. My apocalypse isn't quite that extreme.

In my scenario, humans just barely survive, evolving into at least 8 species and reverting from modern/futuristic to stone-age tech until about 10,000 years before the actual story begins, and they make some pretty drastic adaptations to make it through, even with their intelligence. So with that as a benchmark, what other life forms will make it, and which ones won't?

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    $\begingroup$ an impactor large enough to add that much mass to the moon will blow a huge amount of material out, the moon will start over as a molten ball or magma. The amount of material raining down on earth will make the dinosaur killer look like a spit ball. the question is not what life survives on earth the question is IF life survives on earth. $\endgroup$ – John Sep 6 '19 at 18:32
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    $\begingroup$ I mean, some bacteria will almost definitely survive somewhere, but I'd be very surprised if anything multi-cellular did. $\endgroup$ – Gryphon Sep 6 '19 at 18:36
  • $\begingroup$ @John I think its not so much an asteroid colliding with the moon, but more of another moon-sized planetoid being gravitationally captured by the earth, and then the moon colliding with that. What's left won't really be the moon anymore. $\endgroup$ – Starfish Prime Sep 6 '19 at 18:57
  • $\begingroup$ You’re pretty much rewriting Seveneves by Neal Stephenson, and should certainly read it before going any further with your own story. $\endgroup$ – Mike Scott Sep 7 '19 at 9:20
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    $\begingroup$ @Gryphon What makes you so sure bacteria would survive? The amount of energy we're talking about here is enough that I would be surprised if less than half of the Earth's crust was turned molten. $\endgroup$ – forest Sep 7 '19 at 11:25
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Since you mentioned K-Pg: All animals above a certain (small) weight went extinct. They could not hide fast enough nor could meet their food demands after the event.

But your event will be much worse then K-Pg was, I don't think anything above bacteria, some tough bugs like the tardigrades or fungi will survive. It won't be a single rock, but thousands of huge, Sudbury Basin level impactors.

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Anything on land larger than a cat will die

No ifs, ands, or buts. In the actual K-T, large animals across the board were wiped out with little fanfare. These include things like terrestrial pseudo-tortoises, cat-sized multituberculates and marsupials, and other members of groups that otherwise had low losses.

So your small generalists like raccoons, rabbits, and the Virginia opossum? They will die. They're just too large to survive in a post-apocalyptic neo-K-T hellscape.

This includes humans. Humans will not survive a K-T scale extinction, whatever you throw at them. It's thought that after the meteor impact you didn't have any new plant growth for as little as a few years to as much as a century after the impact. Most plants likely regrew from dormant seeds in the Earth, and even then it took a long time on a human timescale for "normal" vegetation to reassert itself (the immediate aftermath of the K-T is characterized by a "fern spike" where weedy plants like ferns were the dominant plants until trees and shrubs got established). Imagine years with no new plants. Humans barely survived one year without a decent growing season. Our species isn't going to survive off of seeds in the earth, hibernating squirrels, and whatever we can dredge out of rivers. Our megafaunal butts put too much demand on a post-crisis ecosystem for us to survive long-term.

The largest proportion of survivors will be from freshwater aquatic ecosystems

Freshwater ecosystem are what are known as "brown" food webs. They are food webs that are less dependent on the sun than either marine or terrestrial food webs, because if the sun is cut off they can survive off of dead or decaying organic matter and aren't intrinsically tied to photosynthesizing land plants or phytoplankton. Semi-aquatic land animals that are otherwise tied to freshwater food webs are more likely to survive. For example, in the K-T most crocodilians actually went extinct, the ones that survived were either those that lived in freshwater all their life (eusuchians) or returned to fresh water to breed (dyrosaurs). Even sebecids, which were mostly terrestrial in the Cenozoic, are suspected to have survived in the form of semi-aquatic freshwater representatives and re-evolved terrestriality, some of the earliest Cenozoic sebecids are semi-aquatic. Modern birds are suspected to largely be descended from waterbirds that could feed off of brown foodwebs until plants re-established themselves. Notably, modern birds (Neornithes), were a minority in Cretaceous ecosystems, whereas the dominant bird groups (Enantiornithes, Hesperornithes, Omnivoropterygidae, etc.) all died with no survivors.

This is why the fact that freshwater ecosystems are disproportionately affected by human industrialization and pollution is such a big deal, because we are basically killing our back-up drive for the biosphere. Expect higher than usual casualties compared to the K-T because rivers, lakes, and wetlands are some of the most degraded ecosystems in the present day, and therefore will be less robust to mass extinction events. I'd still rather be a shorebird than a land bird in this scenario, though.

Mass casualties even among the groups that survive

One thing that is often underappreciated in the K-T is it is not as if the dinosaurs and other megafauna were selectively removed from the environment and the mammals all got through with little casualties. Many of the groups we see as "victors" of the IRL K-T barely squeaked through the extinction. For example:

  • Crocodilians suffered mass casualties in the K-T. Only generalized freshwater taxa survived. Casualties included the herbivorous Simosuchus, the mahajangachampsids, the terrestrial baurusuchid sebecosuchians, the small, generalized allodaposuchids, etc. Specialized freshwater taxa like the turtle-eating Brachychampsa also went extinct.
  • Only one clade of birds (Neornithes) and possibly as few as five lineages (ancestor of modern palaeognaths, ancestors of modern ducks and geese, and Qinornis)
  • Upwards of 75% of all mammal species died in the K-T, including many groups of multituberculates, placentals, and marsupials
  • Insects won't care. Insects barely even blinked in the IRL K-T, the only mass extinction to ever significantly affect them was the P-T. You might get a big die off among insects that specialize in parasitizing or living symbiotically with particular plants and animals, like ichneumon wasps, bees, etc.

Generalized mammals will be more likely to survive than specialists

This is one that's kind of a no-brainer, but it bear repetition. Before the K-T extinction in North America marsupials and multituberculates were actually both more diverse in terms of species and ecological niches than placentals, which were mostly omnivorous generalists. Marsupials were the ones that included specialized fruit-eaters, most of the specialized carnivores, and even semi-aquatic and burrowing colony-dwelling forms. However, marsupials and multis were disproportionately affected by the extinction because of this (As well as the few specialized placental lineages like the zalambdalestids), and as a result it was the few generalized placental lineages that took over the place after the meteor hit.

Disproportionate extinction of arboreal animals.

Arboreal animals did really, really bad across the K-T boundary. Mostly because the forests they lived in all burned down.

Most survivors among mammals will be hibernators or can go into torpor

This is something that has actually been studied in some detail. One factor that has been suggested to have aided in mammal survival across the K-T, especially since so few of them are associated with brown food webs, is the fact that they could just hibernate through the extinction. So mammals that can hibernate or go into torpor to conserve energy will do better than those that cannot.

Similarly, mammals that burrow or dig their own burrows will have a disproportionate survival rate, since they have ready-made shelter for when the impact hits.

Complete collapse of ocean food webs

The real K-T showed a complete collapse of ocean food webs, with animals with a planktonic lifestyle or dependent on plankton as the base of their food web disproportionately affected. For example ammonites, which were the dominant free-swimming animals and had a planktonic larval stage, were wiped out with no exceptions. The dominant reef-builders (rudists and inoceramid clams) also all went extinct. With the exception of sharks and some marine crocodilians, all of the marine megafauna present in the late Cretaceous (ichthyodectids, carnivorous pachycormids like Protosphyraena, protostegid turtles, mosasaurs, plesiosaurs) was wiped out. Large filter-feeders (pachycormids) were completely wiped out, and notably none of the living large filter feeders (whales, basking sharks, megamouths, manta rays, etc.) seem to have origins that predate the K-T.

Wherever the meteor actually hits will be disproportionately impacted

You can see this in North America. Prior to the K-Pg North America had a diverse fauna and flora which included a large number of non-teleost actinopterygians, marsupials, multituberculates, gingko trees, and dawn redwoods. Then the meteor hit and just about sterilized the North American continent, made worse by the fact that the angle of impact would have showered North America with the worst of the debris. Most of the groups that had North America as their stronghold of diversity pre K-T are reduced to relict groups today largely because of that impact, whereas North American ended up being mostly recolonized by groups that had their centers of diversity in Asia (like placental mammals).

Winners

  • Shorebirds (esp. seagulls
  • Crocodilians (it's time to reconquer Earth...again)
  • Rodents
  • Small ground squirrels like chipmunks are of particular note here, as they are 1) hibernators, 2) burrowers, and 3) omnivorous
  • Shrews
  • Smaller opossums like Marmosa
  • Native marsupials and rodents reclaim Australia from the foxes, cats, rabbits, and other Holarctic invaders
  • Sharks (they won't win, but they won't suffer huge losses either. Just keep trucking on.)
  • Grass (will do better than most plants because they are highly resilient and wind-pollinated)
  • River turtles
  • Amphibians (surprisingly enough despite predictions of acid rain there are little to no extinctions among amphibians across the K-T)
  • Freshwater fish (no more humans to pollute their water, plus an entire ocean that's just begging to be re-invaded)

Losers

  • Ungulates
  • Xenarthrans (armadillos have a better chance than most, but they're still too large)
  • Primates (no forests, mostly frugivorous, large)
  • Carnivorans (the smallest weasels or civets might survive, but it's unlikely)
  • Marine mammals
  • Tortoises
  • Coral reefs (will probably go completely extinct and be refounded by some other reef-building organism in the aftermath)

EDIT: With regards to your moon question, it just won't work. The moon right now isn't habitable, it has no atmosphere, isn't large enough to maintain it's own magnetic field, and unlike Titan or Europa isn't in orbit around a gas giant where tidal flexing can provide those things for it. Adding enough mass to make the moon habitable will make the moon stop being the moon, it's gravity would be so great it would either end up impacting the Earth (which will probably end all life on it by turning the entire surface into molten slag, akin to the Thea impact), or it will break free of Earth's gravity well and establish its own orbit (this seems less likely based on my understanding of physics, but I'm not a physicist or astronomer). Either way losing the moon means Earth loses its habitability, as it no longer has a large orbital body to stabilize its orbit.

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  • $\begingroup$ Molluscs such as oysters can build reefs, so I'd consider them the most likely candidates for our recovering planet's reef-formers. As a bonus, some of them actively purify the water they live in. $\endgroup$ – Karst Jan 16 at 21:22
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With the amount of energy in a collision required to drastically increase the mass of the moon, the amount of matter raining down on Earth would be enormous. A significant portion of the Earth's crust would become molten (bye bye Oceans). We say life always finds a way, but there are no reservoirs of life in the mantle. The chances are that no life would survive, unicellular or otherwise.

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Assuming the impacting body has about the same mass as the Chixulub asteroid, the effects are going to include massive global cooling (one or more "years without a summer", followed by a long-term trend of significantly colder weather), reduced sunlight levels from atmospheric ash, and widespread forest fires in the area of the new impact basin. In the K-Pg event, the ecological niches that were hit the hardest were large land-dwelling animals and shallow-seafloor biota. Animals with a smaller overall body size (fewer calories needed to survive) and that were adapted to colder climates tended to survive and diversify. For instance, birds (being dinosaurs, taxonomically) have diversified so much over the Cenozoic that there are as many extant species of Dinosauria now as there were in the late Cretaceous. So, the fauna that would be most likely to survive this hypothetical mass extinction would be deep-ocean and arctic species, as well as generalists- anything that wouldn't be wiped out by the loss of one particular food source.

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