It makes sense. I just wasn't sure how likely it would be for species to evolve in significant ways over a long time without obvious changes to the shape of their fossils. Difficult to spot evolution happens a lot, apparently:

Cryptic, or sibling, species are discrete species that are difficult, or sometimes impossible, to distinguish morphologically and thus have been incorrectly classified as a single taxon. Cryptic species are found from the poles to the Equator and in all major terrestrial and aquatic taxonomic groups [2, 3]. For example, a recent meta-analysis yielded 2,207 articles reporting cryptic species in all metazoan phyla and classes, including 996 new species in insects, 267 in mammals, 151 in fishes and 94 in birds [2].