I'm thinking about the evolutionary history of life on a world very similar to earth in many ways, carbon based life, life began in the seas, creatures like cephalopods, and arthropods, evolve.
But, there is something different about the world, or something that happens that prevents anything like vertebrates from ever taking up much of the animal biomass on the planet. There may be some small creatures with notochord-like structures, but they never get very large, or perhaps they flourished for a time ... only to go almost totally extinct with only a few species of more obscure animals with internal skeletons extant at the time of the story.
This planet is, of course, dominated by giant arthropods, insect-like creatures, gastropods, cephalopods etc.
But what are some environmental conditions, natural disasters, or features of an ecosystem that creatures with internal skeletons are more vulnerable to than those with exoskeletons? (It might not be directly related to skeletal structure, but instead some of the knock-on effects of how a creature has evolved to support its mass.)