Some 55.8 million years ago, Earth underwent a really dramatic heat wave known as the Paleocene-Eocene Thermal Maximum, or PETM. What happened, exactly? We don't know how it happened, but we do know that within 20,000 years, the temperature climbed by five to eight degrees Celsius (or nine to 14 degrees Fahrenheit) and persisted for 200,000 years afterwards. Many animals responded to this change by downsizing, not only to better maneuver around the spreading jungles, but also to lose more heat more quickly.

You might think that this would be good news for the carnivores. While small miacids and viverravids were around long enough to witness the PETM, they did not become the modern order Carnivora until after the fact, 42 million years ago. Until then, they'd have to stay in the shadows of two other groups of carnivores--the creodonts, a group popularized by such megapredators as Hyaenodon and Simbakubwa; and the meat-eating ungulates known as mesonychians. The former died out 11 million years ago due to overspecialization, limited mobility and smaller and simpler brains, which made them unable to deal with the changing climate from one side and the onslaught of smarter, more flexible carnivorans on the other. The latter died out early in the Oligocene probably due to competition with the creodonts.

In this alternate Earth, the PETM lasted longer than in our timeline--three to four times longer. Many lifeforms couldn't take the extra heat. Coral reefs died out, taking another 25% of all marine species with them. Ginkgophyta, Cycadophyta and Gnetophyta became extinct with no living descendants, whereas only 38% of the conifer species and one-third of all angiosperm orders were gone. And the extra heat forced many animals--like the creodonts and mesonychians--to get smaller and smaller. Would this sort of downsizing make the creodonts and mesonychians smaller and more flexible, perhaps evolve into carnivores analogous to cats, dogs, bears, pandas, skunks, weasels, raccoons, linsangs, civets, hyenas and mongeese, thus surviving well into the present day?

  • $\begingroup$ Just curious why cycads go. I thought they like it hot. $\endgroup$
    – Willk
    May 5 '20 at 20:08
  • $\begingroup$ There's another issue you may want to look at it: higher temperatures make larger ectothermic predators more viable. The crocodillians may make a comeback. $\endgroup$ May 6 '20 at 4:28

If it gets hot, the protocarnivorans win sooner.

The ancestors of the carnivores probably already had their better brains. They likely dominated their niches soon after the Cretaceous if not before - catching and eating small things.

In our world, to outcompete the larger orders and eat their lunch (so to speak) the protocarnivorans need to get big enough to eat it. A genet-like viverravid cannot take down a big grazer that is prey for a hyaenadon. It has to evolve larger forms until it can take on the specialized predators on their own turf to cause them to go extinct.

But if big bodies suddenly become metabolically expensive, there goes the big grazers that big carnivores eat. Those big carnivores cannot live on bugs and lizards. But grazers will be ok - smaller grazers will thrive in a grassy hot world. Smaller, quicker creodonts that can subsist on smaller prey might evolve but now must take on the carnivore predecessors on their turf. They will lose that contest, sooner than their descendants lose it in our timeline.


... died out 11 million years ago due to overspecialization, limited mobility and smaller and simpler brains, which made them unable to deal with the changing climate from one side and the onslaught of smarter, more flexible carnivorans on the other.

This is the root cause of the decline of the Creodonts, and I don't see where the length of the PETM has any impact on that. Those are genetic limitations of the Credontic line and they were going to get wiped out by something more adaptable and intelligent sooner or later no matter what.

So... probably not.



So first off, creodonts didn't have brains that were that much smaller than carnivorans. They had smaller brains than modern carnivorans, but then contemporary Paleogene carnivorans had brains about the same size as creodonts as well.

What did the creodonts in was their dental morphology. Carnivores have cheek teeth with one pair of carnassials and then one to two pairs of grinding teeth (except things like cats and hyenas). Creodonts have three pairs of predominantly shearing carnassials and no grinding teeth. The creodont arrangement is good for being a hypercarnivore. However, because there is no division of labor in the tooth row, they cannot have both grinding and shearing teeth in the jaw. They have to modify their molars all at once, which creates a trade-off between omnivorous and carnivorous features.

I.e., the shearing trigonid fights for space with the grinding talonid. An increase in one results in the reduction of the other, and in creodonts you usually can't change the proportions between teeth in the tooth row. Carnivores have one of their teeth (the m1) be dominated by a huge shearing trigonid, and then the rest of the molars have no shearing blade at all.

This causes creodonts to run into a phenomenon called the macroevolutionary ratchet, where animals can become more specialized for carnivory but have a hard time going the opposite direction and becoming more omnivorous, mostly because being more carnivorous involves loss of complex structures that cannot easily be regained. This leads to the specialized carnivore lineages, which are often less dietarily flexible, going extinct, and is the general reason why the history of carnivorous mammals consists of groups repeatedly becoming hypercarnivores and then becoming extinct shortly thereafter.

Carnivorans aren't better meat-eaters (indeed creodonts usually dominated the hypercarnivore niches when they coexisted), they're just better at occupying the small, generalized, civet or raccoon-like niches that tend to survive extinction events and climate change better. After the extinction, a branch of the small carnivorans then takes over the niches of the now-extinct hypercarnivores (whether creodont or carnivoran) and the cycle repeats itself. Small creodonts tended to be biased towards hypercarnivory and struggled with the trade off between shearing (meat-eating) and grinding (omnivory) features of their dentition.

The only way to save the creodonts is to make them have some major evolutionary breakthrough that lets them able to balance carnivorous and omnivorous adaptations like carnivorans do. This would probably have to be in Africa during the Paleogene because everywhere else your next-gen creodont will compete with incumbent carnivorans and will not get off the ground. You now have a small, weedy creodont that competes for niches with civets, raccoons, mongooses, and skunks, and can survive extinctions easier to repopulate the carnivore guilds with creodonts.

Mesonychians have it even worse. Their teeth are barely built for shearing at all. They crush bone but are almost no good for anything else. It's not even clear how effective of predators they were, though they do seem to be predators.


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