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Inspired by this comment:

What if every animal as large as, or larger than, a ice-age vintage horse, except for those that are acclimated to cold climates, went extinct during the last Ice Age?

Ice Age I'm talking about, which killed off the American horses

Size is quantified in terms of body mass.

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    $\begingroup$ Few additions - the extinction event you are referring to is known as the Quaternary extinction event or more specifically The Pleistocene extinction event, which is a time frame extending about 2.5 million years back to current, though the extinction event is around 60'000 years ago, pending continent. It was a period defined by glacial events (11 major) and saw a good number of extinctions...australia lost 12 of 13 large mammals, and South America lost some pretty unique species from the smilodon (sabertooth cat) to the Stegomastadon (a 13'000 lbs elephant) to the Hippidion (horse). $\endgroup$ – Twelfth Apr 21 '15 at 0:34
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    $\begingroup$ Interesting line - this could effect climate. From the wiki on mammoth extinction: A 2010 study suggests that the decline of the woolly mammoth could have increased temperatures by up to 0.2 °C (0.36 °F) at high latitudes in the northern hemisphere. Mammoths frequently ate birch trees, creating a grassland habitat. With the disappearance of mammoths, birch forests, which absorb more sunlight than grasslands, expanded, leading to regional warming $\endgroup$ – Twelfth Apr 21 '15 at 0:39
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    $\begingroup$ Size of a modern horse, or size of an ice-age-vintage horse? $\endgroup$ – Mark Apr 21 '15 at 1:11
  • $\begingroup$ Need to throw in a correction....acclimated to cold climate or not, you are still at risk in these extinction events. The cold isn't what kills and causes these mass extinctions $\endgroup$ – Twelfth Apr 21 '15 at 18:26
  • $\begingroup$ @Twelfth - ... Oh... they might still go extinct due to food depletion as a side effect of other extinctions? $\endgroup$ – Malady Apr 21 '15 at 18:33
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That is really an "it depend's" sort of question. Early horses were rather small, so if we are talking about creatures of that size in OTL, then the planet is now overrun with creatures mostly smaller than modern day ponies.

Since you stipulated that the Ice Age was the proximate cause of the extinction, we have another issue; many creatures capable of surviving and thriving in cold environments are actually rather large (mammoths, walrus', polar bears), so vast niches in the ecosystem will be left empty. I am also presuming that Homo Sapiens and our cousins like the Neanderthals and Denisovans are still around.

The first thing that will happen is that creatures will begin pushing into empty niches in the ecosystem. Although true speciation will take perhaps hundreds of thousands to millions of years, we will start seeing various sub species growing (or shrinking for that matter) to take advantage of the new conditions. Some of the megafauna of the last ice age illustrate this perfectly; giant ground sloths, for example. We will see species of deer rapidly expanding in size to fill the niches of elk and moose, small predatory animals growing in size to take over the niches of apex predators like the short face bear or sabre tooth tiger (even if apex predators were under the size limit, the sudden extinction of their primary food source would drive them to extinction or major modification as well), and most likely a flowering of the rodent family since they are extremely adaptable already (sabre tooth rats, anyone?)

Humans and their cousins will make things more difficult, since humans are generalists, so the sudden loss of large game animals won't be quite as devastating for Homo Sapiens. (There is some question about the lifestyle of the Neanderthals, who do seem to have subsisted on large game animals, but that might have been a cultural preference.) Larger animals will have a harder time evolving to fill the niches, since humans will find them easier to hunt. Evolving into megafauna may not be possible (and indeed there is a very limited selection of megafauna on Earth today). Animals in general will evolve to be much quicker and stealthier because humans are capable of tracking and hunting virtually all animals (and it may be true that today's animals are much better at hiding and running away then their ice age ancestors).

So in a world without humans, small creatures will eventually begin to specialize in the niches that the larger creatures once inhabited. Given enough time, they will eventually evolve into new species and cover the same range of sizes and behaviours that the extinct animals once had. In a world with humans, specialization to fill the niches of the former large animals will be suppressed by the ability of humans to preferentially hunt larger animals, and most creatures will have selective pressure to evade detection and predation by humans.

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  • $\begingroup$ Thank you, I have now added an extinction exemption for creatures that have adapted to the cold, as they would most likely not be rendered extinct due to an Ice Age. $\endgroup$ – Malady Apr 21 '15 at 0:18
  • $\begingroup$ Adapted to cold doesn't help much, they'll still get caught up in the extinction event. It's the melt that kills, not the cold. $\endgroup$ – Twelfth Apr 21 '15 at 18:26
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Extinction cause:

First note...large creatures actually fare decently in ice age environments and it's not the ice age that claims them. It's coming out of the ice age that gets them. This isn't hard fact, but I like the theory on it. Lake Agassiz is a Pleistocene lake that formed on top of the glaciers in Northern Canada...this lake grew out of the melting glaciers to several times the size of the great lakes (we can still somewhat make out what was once the extent of its beaches on today's globe, we're talking a fresh water lake that's larger than all of today's fresh water lakes combined). In an event known as a http://en.wikipedia.org/wiki/Glacial_lake_outburst_flood Glacial Lake outburst Flood, the ice walls that contain this giant lake fail and the majority of the lakes contents (fish and all) drain out across the continent. This flood washes over nearly all land leaving many of the land features we know today (http://en.wikipedia.org/wiki/Missoula_Floods Missoula floods is a good example read on it). It's this giant flood event that wipes out the large mammals, not the ice age itself. A little more of a violent end. Incidentally, in a similar event, one of these lakes drained into the hudson bay and the resulting mass of water being dumped into the ocean initiated the flood of what we now call the Black Sea (we inhabited the land prior). Remember that the Quaternary period (includes the Pleistocene era mentioned above) had a minimum of 11 major glacial cycles and this massive flood could have occurred multiple times during the extinction event time frame.

Small creatures can ride out these floods...hidden away underground, in trees above, living in and around the lake that floods the continent, or even riding out the wave. Large creatures don't have the options and we're left with a single event that takes out all large mammals, but only (only?) devastates the smaller mammal population but leaving behind enough to repopulate...it also notably leaves behind mountain creatures and other mammals capable of living in arctic conditions simply because they aren't caught up in the flood.

Our effects:

Hunting would not / could not sustain us to the amount it has and we would have been put into more of a foraging role early on prior to agriculture. When agriculture comes around, we'd be looking to alternative beasts of burden to tend to plowing and other heavy strength tasks that early agriculture demanded.

There would have been many more forests and less plains...the Woolly mammoth is thought to heavily feed on birch trees and their presence added to the deforestation of area's in favor of great plains. Lacking the large mammals feeding on them, forests would have survived significantly better and there would be far less grassy plains.

The horse was heavily responsible for our early transportation and communication. I suspect 'running networks' would have been formed similar to that of the Incan trails (Incan people had an amazing communication network through mountains and entirely on foot). It's possible that as a people we would have maintained our running physique significantly longer as we would have remained much more dependent on our running as a mode of transportation.

It is notable that a few area's are pretty immune to the flooding (Africa for example) and it's possible that Africa would have supplied our stock of beasts of burden (wildebeest? elephant?). These area's would be able to export their beasts of burden to the rest of the world. After trade between larger distances starts occurring, these African large mammals would start to take on the roles our now extinct horse and other beasts would have for us.

And from there, I'm not too sure how big of a deviation from current history we'd experience. Assuming thousands of years, I believe large mammals from Africa could eventually obtain the status horses have in todays world.

As a funny end note: A popular notion out of European scientists in early colonization days derided America in quite a few ways and used the lack of large mammals found in America to judge it as weak (small penis's fit in here too...European science at this time made some petty claims). If European and Asian large mammals had also died off, the European scientists that be would have had to find another reason to come to the conclusion that all American creatures had small penis's.

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