I'm pretty sure the tirtouga line is inspired by
Archelon, a very famous genus of ancient sea turtles:
https://en.wikipedia.org/wiki/Archelon . For what it's worth, the leatherback is its closest living descendant though...relicanth's rock typing is already a tongue and cheek allusion to how it's a "living fossil", but it's not extinct so it shouldn't qualify as a fossil. Tirtouga and carracosta are extinct so they qualify as fossils. It's simple as that. Sure you can revive them, which makes them technically non-extinct, but they don't naturally exist in the current time period. You get the idea lol.
What I would like to see though is maybe a revamp to the fossil system. Change it up a bit. The first sets of fossils have been ancient invertebrates and reptiles, but the trend is that they have been modernizing. Trilobites and ammonites (along with the ancient crinoid genuses and
Anomalocaris) came before dinosaurs (cranidos and shieldon), which came before
Archaeopteryx (which technically was a dinosaur, but a fairly late genus and is actually more there to represent the dawn of the modern birds) and
Archelon (more there to represent the dawn of modern reptiles since
Archelon differed very little form modern sea turtles).
T. rex and
Amargasaurus were dinosaurs that came around the later end of the Cretaceous as well. Aerodactyl is the odd one out here.
As you can see, there is an "age of invertebrates" chunk (1-3), and an "age of reptiles" chunk (4-6), so we should have an "age of mammals" chunk next if moving down the timeline sequentially...
Therefore, it would be cool for the next "fossils" to be ice age mammals, the ones that come next naturally. Furthermore, this will allow us to do away with the obligatory rock typing because instead of fossils, why not revive them from frozen remains? Replace rock with ice instead...we can have a "frozen fang" that gets revived into an ice/ fire type
Smilodon and a "frozen tusk/ pelt" that gets revived into an ice/grass mammoth or something...