There exists a planet with a similar biological history of Earth that experienced a sudden inundation of the undead during it's equivalent of the middle Pleistocene. These undead are capable arising from the death of any animal, with the relative violence of their demise determining the speed at which they return. Violent death results in a resurrection event occurring within minutes, whereas death by age or starvation would take hours, up to a day at most.
Once the fresh undead is reanimated, it may be assumed to be capable of moving without energy input, but is still reliant on the musculature of the original animal, restricting its power output and mobility. Undead are also capable of thermoregulation, running at temperatures similar to most endotherms, but are incapable of healing themselves and providing an immune response to infection. Thus, the undead population is controlled by the rate at which musculature rots and bones are worn and broken with use.
Unfortunately for the traditionally alive species of this planet, becoming undead also imbues animals with a predatory instinct, causing them to act similar to other predatory species in their respective environment might. Thus, a deer dying would later act like a wolf, and hunt prey as a wolf might. The exception to this is when an undead is freshly risen, or in the presence of a heavily wounded or freshly dead animal, at which point the undead goes into a "bloodfury", where the animal becomes highly aggressive and demonstrates exaggerated control over its form. This includes the ability to move and relocate bones and muscle tissue within its body, perform limited repair to any existing wounds, and merge bodies with any other animals nearby, living or undead.
It may be assumed that all undead possess an animal-like intelligence, with problem-solving skills being limited to that of predators in their respective environments. Undead in a bloodfury may experience greater problem-solving skills more similar to those of a chimpanzee or an octopus.
Lastly, the undead possess a supernatural ability to sense the direction of nearby animals, living and undead. This ability degrades with distance and relative health of the target, meaning that the undead most capable of sensing a wounded, nearby animal.
So, my question is what would the resultant biosphere look like given almost a million years? Things I assume would be true:
- Predatory species and most obligate carnivores would be out-competed by the undead
- Scavengers would feed exclusively on animals dying of causes other than predation
- Breaking bones would become an even more favorable strategy for fighting
- Animals would be hesitant to kill rivals for mating or territory privileges