Upon
a discussion about the complete form of Galar Fossil Pokemon, and how it's likely not ever happening, that got me thinking why the Galar Fossil Pokemon differ so much from previous Fossil Pokemon even though their fossils seemingly look more complete. I came up with this possible theory:
We're likely never going to see the "originals" (of the Galar Fossil Pokemon). Like, the strange thing about Galar fossils is that not only do they need to be combined, they are ABLE to be combined. Yes, they do make abominations, but it still works which shouldn't be possible if there wasn't something odd going on with them that makes them different from previous gen fossils.
Consider this, previous gen fossils weren't complete either:
- Omanyte just has its helix shell.
- Big Aerodactyl comes from a small Old Amber, a reference to Jurassic Park where they got dino DNA from bloodsucking insects which got stuck in tree resin. So either they cut the middlemon and just had a batch of Aerodactyl have parts of themselves get caught in tree resin OR Old Amber has a extinct species of bloodsucking insect Pokemon which primarily prayed upon Aerodactyl; which means we get Aerodactyl from just a few droplets of its blood!
- Anorith just has its claw.
- Cranidos & Shieldon just has have their heads.
- Tirtouga is also just its shell.
- Archen is just from a feather.
- Tyrunt only has its lower jaw.
- Amaura only has the fin from the top of its eye.
YET, despite that, we still get a complete Pokemon when we revive them. Whatever is missing the rejuvenation machine is able to have what DNA strands it gets to regrow it (with a possible side effect of them becoming part Rock-type as the Pokemon's organic matter is mixed in with inorganic matter of the fossil).
So, how come the Galar fossils then need to be combined, especially when the Galar Fossils are more complete than some of the fossils I mentioned above? Heck, if the Old Amber does contain an ancient bloodsucking insect Pokemon, why doesn't IT get revived instead of or along with Aerodactyl (or we get some sort of freak Bug-type insectoid Aerodactyl)? Also think of this: in the real world we have fossils of thousands of prehistoric animals; surely this must be also true for Pokemon yet we only have these handful of Fossil Pokemon we can bring back?
It has to be more than just having the fossil. It could very well be the Fossil Pokemon we're able to bring back is because scientists got lucky and were able to completely map out the DNA of those Pokemon. Maybe they found a batch of complete or nearly complete fossils with most of their organic matter preserved in a recoverable state (via the Pokemon world's super science). They map out the DNA and body structure, program the schematics into the rejuvenation machine, and now any significant fossil piece of that Pokemon can bring it back to life (or at least create an expy of it); it's just for player convenience that all Fossils for one Pokemon are the same. And this is probably a RARE thing, like they may have complete fossils of other ancient Pokemon but unfortunately there's something preventing the Pokemon's DNA from being sequenced. Like if the Old Amber does contain a bloodsucking insect Pokemon with Aerodactyl blood (BTW we know there's normal Aerodactyl fossils so they were able to get a complete DNA sequence of Aerodactyl from that and not the Old Amber, though the blood probably did help), it could be the tree resin damages the insect so much during the preservation process it loses vital organic information thus why it can't be revived along with Aerodactyl.
So, what about the Galar fossils? Not sure where I mentioned it, but I have a theory that the reason we only have half the Galar Fossil Pokemon is maybe because their other half is made from a non-preservable matter. Maybe their other half is entirely made from a soft tissue, or maybe it was entirely made from a solid form of energy. Whatever the case, this missing half cannot be simply recreated from sequencing the DNA of the other half thus the original Galar Fossil Pokemon can't be revived. HOWEVER, Cara discovered that this subset of Fossil Pokemon can have their halves combined with another as lone as one was the head/front and the other was the hind. Of course Cara Liss is careless so, instead of realizing she's combining two different Pokemon together, she thinks she actually figured out what these Pokemon looked like and it just so happens they evolved to have similar parts.

Thus, we get the Galar Fossil Pokemon we have now, Fossil Pokemon that on their own are incompletable but have a genetic quirk that lets you "combine" parts of them "together".