Omastar does two things:
1. Setup lead (spikes, stealth rock, etc.)
2. Shell smash sweeper (shell smash, surf, white herb, etc.)
Basically identical to Cloyster except typing is rock / water. The only difference is that the typing really sucks and there's no rock special attack STAB move for it to actually use unlike Cloyster which can fight anything once it gets shell smash off.
The reason I saw typing sucks is that it has a 4x weakness to grass and is weak to earthquake and fighting. Also lightning. Even if you manage to shell smash, unless the opponent is weak to water, you're gonna die in the next turn most likely. Setup lead is okay but you will never get off spikes + stealth rock before exploding.
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Kabutops is a swift swim sweeper.
You use it when there's rain, so its speed is doubled and water STAB moves are doubled. Then you use STAB stone edge and waterfall / priority aqua jet.
It also has rapid spin, which can be pretty useful.
Same shitty typing as Omastar. Even more fragile. STAB stone edge is pretty scary though.
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If I were to rank the fossil pokemon, it'd be like this in descending order:
1. Aerodactyl - Has mega evolution and is scary as shit with that. Pretty meh otherwise.
2. Aurorus - It's abilities are really scary, both of them. Other than the 4x weakness to fighting, it can do some major damage.
3. Carracosta - It's like a much better version of Omastar
4. Archeops - It's super good if it's over 50% hp. Once it goes below that, it's more useless than garbage.
5. Tyrantum - It's okay as a lead. Dies too easily before it can do anything though. You'd think it'd be scary, but it relies on bite moves to do stuff, so no stone edge / etc.
6. Rampardos - One hit wonder. It basically exists to die. But before it dies it does SOOOOOOOOOOOOOOOO MUCH DAMAGE.
7. The rest - They're all just different versions of crappy