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Before the Permian extinction synapsids occupied large land animal niches. After the permian extinction some synapsids occupied large land herbivore niches, but most large land animal niches were filled by archosaurs and their relatives, and after the Triassic no synapsids would occupy large land animal niches until after the KPG extinction. Before the KPG extinction dinosaurs occupied large land animal niches, and after the extinction some of the surviving avian dinosaurs continued to occupy large land animal niches but most of the large land animal niches were filled by mammals.

In the case of the Permian extinction synapsids didn't need to go completely extinct to get replaced in terms of what was filling the large land animal niches, and similarly dinosaurs didn't need to go completely extinct for mammals to largely replace them in terms of filling large land animal niches.

After a mass extinction caused by a nuclear war, in which enough nukes are detonated to wipe out 99% of plant and animal life including humans, would mammals likely still be the animals occupying large land animal niches or would they get replaced by other animals? If mammals got replaced as the animals filling large land animal niches what would most likely replace them?

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  • $\begingroup$ Speculative evolution is an active genre of scientifically informed art and speculative fiction. The Wikipedia article has a list of recommended books. $\endgroup$
    – AlexP
    Jun 16 at 12:00
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pick what you want.

A mass extinction of that level might well leave no vertebrate species behind, or it might leave thousands. Its impossible to say with any certainty what would survive other than to say they will mostly be small generalists that breed quickly. Within that constraint say whatever you want survived and no one can argue.

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-Quadrupedal animals can grow larger before their legs and spines collapse, the tallest humans ever were all short lived, the largest humans ever live below average life expectancy but they still reproduce, Bipedal animals can grow larger if their spine is horizontal and not vertical, like Terror birds.

-Skinny animals or animals living in cold climates can grow larger. Elephants are skin and bone, so are giraffes, you can feel the muscles and tendons of a horse through their skin, they have no fat.

-Animals with an efficient digestion grow larger, before an animal even dreams about evolving into something giant, they first must either find an infinite amount of high calories food or develop the ability to eat rotting food, dirt and mud without getting diseased. T-Rex could eat decaying carcasses filled with maggots, smaller dinosaurs had a harder time doing it.

as far as I know, most mammals in this planet had a giant counter part, even moose and sloths ...hell even pigs and primates.

There isn't really any hard requirment to develop gigantism successfully, even people could grow giant we started walking on all fours like a baby.

oh and upping the oxygen levels worldwide helps, more oxygen means less required food.

Giant herbivorous dinosaurs didn't eat as much as grazing mammals, in proportion, but they have incredibly complex lung systems to make good use for all the extra oxygen.

Think of bird lungs and such.

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After a mass extinction (any cause), what we tend to see, and probably expect to see is:

  1. The survivor species will be random to an extent, but generally, most species fail because they are too dependent on a temporarily broken food chain or other inimicable transient effects. Those that survive tend to be the ones that can endure the few years until food and light (or whatever is missing to re-stabilise the food chain/biosphere) are available. So they are likely to be smaller, more adaptable/less specialist, maybe small burrowers or living in sea areas that provide a "buffer" against the physical catastrophe, and so on. Look for smaller generalists, such as rodents,insects, small seabed scavengers, and so on.
  2. Within a few million years, we see adaptive radiation /evolutionary radiation. These overlapping terms refer to the way that, if there is a sudden "opening" in the ecospheredue to a mass extinction or other huge change, existing life forms will explosively adapt and diversify to fill it.

Both of these can be seen for example, when the non-avian dinosaurs died. At that time, as Wikipedia describes it, "the placental mammals were mostly small, insect-eating animals similar in size and shape to modern shrews." They were exactly the kind of life one might expect to survive - small burrowing opportunist generalists, able to "make do" with whatever was available (item #1). At that time, mammals had already been around for about 160 million years (from about 225 mya). But after the extinction event, within only about 10-30 million years after the mass extinction (the Eocene), "they had evolved into such diverse forms as bats, whales, and horses." (Item #2).

We can't say what survived your event, and what didn't. There are tendencies for small opportunistic generalists, maybe in sheltered environments, to have better odds, as they are less sensitive to ecosphere disruption, but beyond that there's no way to know which species make it, which don't, and which ones inherit the earth.

Fast forward a few tens of millions of years, and the inheritors could be anything from more mammals, to squid and cuttlefish, to .... well, pretty much anything.

On a side, I'd note that your scenario is a bit different from historical mass extinctions, as its a nuclear conflict. That suggests two changes - radiation which can linger, and lack of "deep earth" effects.

  • Radiation decays with a half life, and over geological time its very temporary - high levels of radiation will be over within centuries, to 100k of at most millions of years, and most likely most places are not nuked, so most places (90% of earth) recovers sooner. Some individuals will die, some won't. Live evolves, and radiation won't really make much difference.
  • For all its ferocity, nukes are fundamentally surface or air weapons. They don't give rise to volcanism, tectonic movement, deep oceans, or any other huge scale changes such as a meteor impact or basalt flow event might do. That means there is a lot more scope for sheltered niches to exist and persist.
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