Is there any consensus as to the internal organs/stuff which maybe doesn't fossilize well? Like, did they just evolve a bitchin' chassis but they're constantly tinkering with the internal bits?
Fossilisation has a bit more to do with deposition conditions than the creature itself. It's why we know next to nothing about jungle dinosaurs and ones around mountains. Most of our knowledge comes from marine and wetland dinos.
They would most likely look the same but there are still evolutionary changes deeper in the DNA that aren't visible like metabolic and digestive functions. A modern horseshoe crab would not be able to reproduce with one from a 100 million years ago despite looking virtually identical.
You may not like it, but this is what peak evolution looks like.
