They’re arthropods, so in terms of the appearance in the fossil record, it’s pretty much what they are completely. Idk how far you wanna go in terms of evolutionary ancestry, but they’ve conserved most of their featured for a quite a long time:
https://www.frontiersin.org/articles/10.3389/feart.2020.00098/full
You’re looking at a photo of a horseshoe crab. Still very much alive and complete.
Yeah but like, what if their ancestors were different and they just evolved it all away?
They’re arthropods, so in terms of the appearance in the fossil record, it’s pretty much what they are completely. Idk how far you wanna go in terms of evolutionary ancestry, but they’ve conserved most of their featured for a quite a long time: https://www.frontiersin.org/articles/10.3389/feart.2020.00098/full