So I have some theories but I want to hear what you have to say on this.
So the latest “fossil” Pokémon clearly never actually existed (nor should they now, either), but the most interesting thing about them is none of them are rock type. In your standard reputation of reading heavily into this as world building and not Game Freak not giving a $#!+ about maintaining any sense of consistency, what would be your reasoning for this and why? If you have multiple theories, feel free to share more than one!
I’m already on the record as thinking that we shouldn’t read too much into the fact that all the previous fossil Pokémon have been Rock-types: I don’t think it means that all prehistoric Pokémon were Rock-types, and I don’t think it means that the process of fossilisation and reanimation turns them into Rock-types. I actually favour thinking that Rock Pokémon are more likely to become fossilised, because they have tougher bodies and stronger bones (or other highly resilient bodily structures – several of them probably don’t have “bones,” since they’re based on animals that don’t). This is a real thing in both archaeology and palaeontology – species with large bones or thick calcareous shells are overrepresented, and it can be very difficult to learn anything at all about soft-bodied invertebrates except when you get highly favourable preservation conditions. If I were to work along those lines, I would say that what we see in Sword and Shield is the result of unusual geology that’s specific to Galar, allowing the survival of fossils that are not preserved in other parts of the world (and in fact Britain does, in reality, have some extremely rich fossil deposits – that’s one of the reasons British researchers were so influential in the beginnings of modern palaeontology). Maybe these species existed in other regions too, but no fossils have survived there!
An alternative reading, though, is that these new fossil Pokémon are Rock-types.
Think about it. There are four fossils, two “upper body” ones and two “lower body” ones, that can combine into four different Pokémon, each with two types: Electric/Ice, Electric/Dragon, Water/Ice, Water/Dragon. There’s clearly one type for each fossil: the Fossilised Fish (-vish) is Water-type, the Fossilised Dino (Arcto-) is Ice-type, the Fossilised Bird (-zolt) is Electric-type and the Fossilised Drake (Draco-) is Dragon-type. But we never see what any of these Pokémon would have looked like “whole” – we don’t see, for instance, the drake’s head or the bird’s tail. Maybe the original prehistoric Pokémon were, respectively, Water/Rock, Ice/Rock, Electric/Rock and Dragon/Rock, and the types we see instead are only the result of unnatural combinations? It’s a possibility I wouldn’t rule out.