"Serina" is a popular speculative evolution project in which, apart from a long list of fish, invertebrates and plants, the only terrestrial chordate to colonize this terraformed moon is the canary. The project's creator, "Sheatherius", then explored how this one species could branch out into multiple species occupying various niches.
There are fans of Serina, if not the very idea of it, but the one who worshiped it like a god is an Australian scientist who was vehemently fed up with the feral and invasive species plaguing his home soil. In the foreseeable future, transdimensional historians have found an alternate universe that has not one alternate Earth, but whole systems of Earthlike planets, all of which in the Archean stage of evolutionary development. One of these systems include nine habitable worlds, most of which are the size of Venus, orbiting a binary system of middle-aged red giants (each one having been red giants for 542 million years--time travel is a requirement for the Aussie's plans to succeed.) Unfortunately, the scientific community has banned all travel and study to the alternate universe--categorized as "AU #9"--because all of the alternate Earths have been terraformed illegally, thus ruining the scientific community's Prime Directive beyond hope of repair.
The ecoterrorist in question, being a fanatic of Serina, had decided that each of the terraformed alternate Earths should be colonized by plants, fungi, fish, invertebrates, and only one terrestrial vertebrate. This particular person had a real beef with the feral and invasive species of his native Australia, but the one serious beef he had were the house cats. There are 600 million of them worldwide, and they are responsible for the deaths of trillions of small animals every year. So in revenge, the ecoterrorist transported all of them--he never cared where he got each particular cat from--and dumped them into only one of the terraformed alternate Earths.
Like the canaries of the original Serina, the cats are the only terrestrial chordates in this terraformed world. So in the first of what I'd hoped would be a series akin to "Anatomically Correct", my first question would be this--Can the house cats, if they have ended up being the sole land vertebrates of this new world, occupy the niches of at least all of the other placental mammals?