Would the Muroid Niche Be Occupied by Just One Clade?

Question

The muroids are a superfamily of rodents consisting today of mice, rats, voles, hamsters, lemmings and gerbils. There are at least 1750 different species of them, proof apparent of their environmental versatility. That, for me, is the problem. Rodents like the muroids are so successful that in a speculative evolution scenario, they've become one of the subgenre's most tiring cliches. In my observation, no one else has explored the possibility on how life might look if rodents never existed in the first place.

Today, we're exploring one possibility, one that was around long enough to witness the collapse of the dinosaur empire 66 million years ago--Eulipotyphla, the order consisting nowadays of hedgehogs, shrews, moles and solenodons. They are similarly small, and though primarily insectivorous, they are known to tackle other food items.

Considering that we are talking a minimum of 1750 species here, the question isn't whether or not it's possible for eulipotyphlans to occupy the muroid niche, but would they be the only clade to fill it?

Answer 1

It depends

During the Paleocene-Eocene there were a lot of different groups filling the "rodent" niche that muroids mostly do today. Multituberculates, plesiadapiform primates, other Glires like Gomphos, apatemyids, and more. The first two were the main competitors for a "rodent-like" niche. On southern continents you have the niches of muroids filled by argyrolagoids, paucituberculatans, ektopodontids, polydolopimorphians, small possums, and a bunch of other groups.

The fact that muroids today represent the dominant small mammal clade on every continent (including Australia) says a single group definitely can occupy all those niches. As does the fact that multituberculates were the dominant group of rodent-like mammals for most of the Mesozoic. But the presence of many rodent-like groups in the Cenozoic says it's not guaranteed. And to be honest nobody knows why muroids are so diverse compared to other small mammals (even other rodents). The eulipotyphlans from the currently known fossil record don't have any key adaptation that would make them better at a rodent-like niche than anything else.

To be honest, I would look at plesiadapiform primates and multituberculates filling the niche of rodents before eulipotyphlans would.

Not to mention you have a bunch of other rodent clades, scuiromorphs (squirrels) and hystricomorphs, that might just evolve to fill a muroid niche if muroids never evolved.

Answer 2

Let us consider aspects of the rodents that led them to be so successful:

1: Placental mammals

2: Flexible diet, able to adapt to herbivory, carnivory and omnivory.

3: Flexible body plan as regards size

Now let us consider: after rodents (which have the most species), which mammal lineage has the greatest number of extant species, and so the greatest capacity for evolutionary radiation to fill available niches.

Bats.

https://en.wikipedia.org/wiki/New_Zealand_lesser_short-tailed_bat

Depicted: The New Zealand short tailed bat, a burrow dwelling omnivorous bat which occupies a niche filled by rodents in most parts of the world. There are extinct terrestrial bats which presumably outcompeted non placental mammal lineages but were supplanted in ancient times by the rodents.

There are more existing species of bats than any other mammal group except rodents. Bats have a flexible diet. They have a body size with a range comparable to rodents - and could probably achieve a capybara-like size if a species lost flight. Importantly, the ability to fly means that after the Cretaceous collapse, bats could quickly spread over the earth and establish themselves with their and ancestors evolving to fill niches that in our world are destined for the rodents. Birds are the masters of the daytime air, but bats have given the birds serious competition. In a world absent the masters of the nighttime earth, I assert the bats would do very well against the hedgehogs and other placental lineages.

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This proposal was sparked by Dailey's comment on "tiring cliches". I take from this that what is desired here is something possible (clearly it is possible) but also novel and awesome. Bats!