Ah, so the key here is that the fossil record is probably more complete than I'd expect.

A few years I went down the rabbit hole of geology and palaeontology, and the thing I had trouble wrapping my head around was the incomprehensibly long time scales involved, and how there's a massive amount of stuff from different eras, but timescales that long have a tendancy of homogenizing the evidence out of existence because there's just so much stuff smushed together.