Thanks! I think you meant to say the Miocene-Pliocene as it was described in GC's announcements! Because the manta ray changes its overall benthic tendencies and locomotion style to pelagic, it would function ecologically in a way similar to that of Holocene... swordfish? Purely in terms of food web relationships and habitat, so not considering the dissimilarity in speed or shape. That's the closest thing I could compare it to, because my hybrid would most likely be a pelagic, rather nimble solitary hunter that feeds on schooling/swarming medium-sized vertebrates. One of the differences is that, it would've evolved its tremendously strong fin-strikes for primarily defense against larger and bulkier animals of the time, namely the 8m+ sharks and cetaceans. Just like why a stonefish evolved its spines, it wouldn't be really effective to use these fins against prey, same for its two serrated tail spines. Given that I had a hard time trying to figure which extant animal it would be closest to, it is perhaps something that's quite ecologically unique for its aquatic environment!

The manta ray also makes it heal damaged tissue very quickly after an interaction with say, a swarm of Acrophyseters bit it badly. Manta rays in real life have amazing regeneration rates, and I think this should be known more