One of the most recurring elements of paleofantasy is anachronism.
For instance, this still is from the 1966 film One Million Years BC, and yes, that is a Triceratops fighting off a Ceratosaurus, a theropod that had been dead for over 80 million years longer.
An even more extreme example of my point is this still from the 2019 Adult Swim program Primal, showing a tyrannosaur fighting a mammoth. Due to niche constraints, these sorts of fights couldn't possibly happen in real life.
In an alternate Earth, there is only one landmass surrounded by one ocean. The only inhabitants on this planet are microbes, including photosynthetic cyanobacteria. Here, a terraformer has seeded this world with modern species of plants, fungi, algae, birds, rodents, insects, land snails, rabbits, hedgehogs, worms, amphibians, squamates, marsupials, turtles, spiders and the following extinct genera:
- Microraptor, the smallest dromaeosaur and lauded as "The Four-Winged Dinosaur"
- Dilong, the smallest tyrannosaur
- Europasaurus, the smallest sauropod
- Aquilops, the smallest ceratopsian
- Minmi, the smallest of the armored dinosaurs
- Tethyshadros, the smallest hadrosaur
- Gasparinisaura, the smallest of the ornithopods
- Magyarosaurus, the smallest of the titanosaurs
(Marine species are still under thought construction and therefore not a factor in this question.)
But for this to work, one question could either make or break--would the prehistoric species feast on the modern species?