So I'm working on an inhabited planet with around half the gravity of Earth (assume everything else is the same, just the gravity differs). While thinking about what kinds of terrestrial creatures this world could have, hydrostatic skeletons came to my mind and now I must ask: with a lower gravity, could vertebrate-sized creatures evolve with a hydrostatic skeleton instead of an internalized solid skeleton like ours?
yes, but not that much bigger - weight increases cubic, while strength of materials increases squared
So if you decrease gravity by 50%, then you still can't double the size of organisms. Also currently breathing seems to be the worst problem for that kind of "low" organisms on earth - so probably they'd need more oxygen in the atmosphere to survive?
Meanwhile planets with half gravity have other problems - like trouble keeping water (due to the hydrogen escaping to space)
Also our planet has that gravity due to it's metallic core - that's needed for different things, like building a EM-Shield for repelling radiation from space.
Earth has tons of variables set exactly correct to support our kind of life - adjust some of them and life would have to look totally different or not work at all.