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Because nature is never straightforward, there are different levels of carnivores. On the lowest rung of the ladder are the hypocarnivores, in which meat can't take any more than 30% of their caloric intake. Ursidae (bears) and maybe Amphicyonidae (bear-dogs) stand on that rung. On the opposite side are hypercarnivores. Here, 70% is the minimum requirement. And that is where we'd find Felidae, the cat family.

In an alternate Earth, Ursidae never existed, so more amphicyonids filled in that niche instead. Fast-forward to five million years ago, and a sudden, dramatic drop in temperature wiped out half of all plant and animal species, including all of the bear-dogs and any cat species bigger than a puma. Using our knowledge of latest Miocene felids, if this push came to this particular shove, would it be possible for any latest-Miocene felid species as big as or smaller than a puma to transition from hypercarnivory to hypocarnivory?

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    $\begingroup$ Sure, this is a "yes" or "no" question and "no" would require knowing the dietary requirements of ALL Miocene felids when we do not even know all of the species, so the answer MUST be "yes, it is possible"? $\endgroup$ – Ville Niemi Jan 15 at 1:55
  • $\begingroup$ Keep in mind felids had diverged quite a bit by this point so you are only going to change one or two lineages. Felids are also isolated on different continents too so if you want it in the Americas and Asia you need it to evolve twice. $\endgroup$ – John Jan 15 at 4:04
  • $\begingroup$ @John Could you clarify or expand on that? $\endgroup$ – JohnWDailey Jan 15 at 4:18
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    $\begingroup$ a picture is worth a thousand words wildcatfamily.com/felidae-evolution the line in the middle of the chart is the end of the Miocene $\endgroup$ – John Jan 15 at 4:50
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    $\begingroup$ Cats can have little a plant. $\endgroup$ – Muzer Jan 15 at 14:14
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Meet the red panda, Ailurus fulgens.

Red panda

A red panda at the Cincinnati Zoo. Photograph by Greg Hume, available on Wikimedia under the Creative Commons Attribution-Share Alike 3.0 Unported license.

The red panda lives in the eastern Himalayas and eats mostly bamboo. Red pandas belong to the family Ailuridae, in the superfamily Musteloidea, making them close relatives of the hypercarnivorous weasels. Wikipedia gives the temporal range of the Ailuridae as Oligocene to present, so the evolution from carnivory to herbivory must have happened somewhere along this time.

So, yes, it is possible.

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    $\begingroup$ This answer would be improved by mentioning the red panda is believed to have diverged around 3-4Mya, which puts it right at the end of the Miocene, so your time frame is workable. $\endgroup$ – John Jan 15 at 4:04
  • $\begingroup$ Showing that it is possible for one particular species to transition doesn't conclusively answer whether another particular species can. $\endgroup$ – Acccumulation Jan 15 at 19:10
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    $\begingroup$ @Acccumulation: It absolutely does show that it is possible, as in, not impossible. The question does not actually designate a species, it only asks if, in general, it is possible for a carnivore species to evolve into a herbivore species. $\endgroup$ – AlexP Jan 15 at 19:25
  • $\begingroup$ It asks for a felid species. I'm pretty sure the red panda isn't felid. $\endgroup$ – Acccumulation Jan 15 at 21:44
  • $\begingroup$ It's not, it's a musteloid. As I have written. $\endgroup$ – AlexP Jan 15 at 21:45
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Sure

Say that there is an illness, that only infects the prey animals. It doesn't kill them, but it does make them skinnier, and slower to reproduce. They don't die off immediately. Instead, their numbers lessen. And they provide less meat to the big cats that hunt them.

Over time, less meat will drive the hypercarnivores to eat plants to make up for the reduced prey. The ones how can handle the improved diet will live longer and reproduce more, so over the course of generations, the offspring that can thrive on the new split diet will become dominant.

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If you're looking for some subspecies of Felidae that might "make it", Viderridae are your best bet, as they are omnivorous to begin with. They're the least specialized family of Felidae, and the most primitive, which might not be what you're looking for in your worldbuilding.

As for "the big cats", I'd say possible, but unlikely.

While not exactly everything, but many things are possible in nature, so I won't say "impossible".

However (emphasis mine):

...a sudden, dramatic drop in temperature wiped out half of all plant and animal species... ...would it be possible for any latest-Miocene felid species as big as or smaller than a puma to transition from hypercarnivory to hypocarnivory?

The bigger, more advanced families of Felidae are very much specialized as (ambush) hunters, much more so than Caniformia (dogs, bears, seals, racoons, weasels, and that rather special red panda). From ambush hunter to plant scrounger with the occassional prey of opportunity is a long way to go. It's not just the digestive tract and the teeth, it's being good enough at it to find a niche among the much more effective plant eathers.

Evolution works rather slowly. Which is fine if adapting to slowly changing climate / environment like at the end of the Miocene... But you explicitly mentioned a sudden and dramatic change, within one generation or two or even due to a singular catastrophic event (?).

A transition would take many generations of evolutionary pressure, but those hypercarnivores are starving today. It would be much more likely that big hypercarnivores would get displaced by species more fit to cope with the sudden change.

You could come up with mitigating factors, which would allow your hypercarnivore Felidae to survive, likely in much reduced population numbers and / or species variety, and indeed "make it" into this new age of your world.

If however your ultimate idea is having those felidae be / become once again a "ruling" family of any description, that would take quite some time, and lots of mitigating factors.

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    $\begingroup$ "Evolution works rather slowly." Tell that to the mammals who ballooned from two pounds to a hundred just one million short years after the fall of the dinosaur empire. $\endgroup$ – JohnWDailey Jan 15 at 13:12
  • $\begingroup$ "within one generation or two or even due to a singular catastrophic event (?)." More like 40,000 years. Ice ages, after all, don't really happen overnight. $\endgroup$ – JohnWDailey Jan 15 at 13:13
  • $\begingroup$ "It would be much more likely that big hypercarnivores would get displaced by species more fit to cope with the sudden change." Hence the puma size maximum. The puma is not terribly big, and thus a likelier candidate for surviving big change than the pantherines. $\endgroup$ – JohnWDailey Jan 15 at 13:18
  • $\begingroup$ @JohnWDailey: 1) Mammals did get a kickstart from the dinosaurs no longer occupying all those "get big" niches. Because, you know, the dinosaurs couldn't really cope with the sudden changes brought by the Yucatan impact... 2) An ice age occurring for natural reasons (orbital / axial shift for example) doesn't really qualify as "sudden" and "dramatic" in my book. 3) The Puma is still "a big cat", an ambush hunter that would simply die if prey would no longer suffice to support him. Which gets us back to 2), what kind of "sudden" do / did we talk of. I'll leave this A for completeness...? $\endgroup$ – DevSolar Jan 15 at 13:32
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    $\begingroup$ To summarize, a lot of what makes Felidae the most effective hunting mammals of Earth (!) would make them very poor omnivores, and it would take a long time for evolution to change that to a sustainable degree. They would have to survive for that long. Which makes the suddenness of the change the crucial variable for "possible" / "highly unlikely". I'd guess they'd fare much better by "simply" getting even better at hunting in the new environment... hard to say for certain without hearing much more background on your world's past, that change's present, and your story's desired future. $\endgroup$ – DevSolar Jan 15 at 13:37

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