This card has an identity problem - and I think part of it is being an instant rather than a sorcery. At least if you feel the casting restriction should be part of the card.

It kinda reads like a combat trick that usually isn't active during combat (where the most dying of creatures is going to happen). The flavor implies more that this is meant as a post-combat play that gets you the counter plus something afterwards - which would fit with a sorcery.

Comparing the card to Fungal Infection also doesn't really favor it. This is already a card that is in its interaction entirely restricted to fossil creatures. That's like a card "Cast this spell only if a creature with shadow died this turn. Target creature with shadow you control gets a +1/+1 counter. Target creature with shadow an opponent controls gets -1/-1 until end of turn."

Wouldn't you consider such a spell incredibly niche? If your opponent doesn't use the block mechanic of fossil/shadow, the card is dead. If your opponent uses removal to control your fossil creatures and keeps them to zero, this card is dead unless you are about to lose your second creature already. If there aren't enough fossil creatures that each player controls one after one has already died, this card is dead.

Honestly this was a top-down design based on the concept of a large dinosaur like a T-Rex scavenging another dinosaur's kill- the one who stole the kill eats well and gets better (the counter), while another creature goes hungry. Instant just seemed like the natural card type, so that the player had the option to use it during an opponent's turn if the player wished.

I wasn't aware of Fungal Infection, but based on that, this could probably be reduced to just .

The idea with Gaddgar and the fossil supertype is that constructed decks would have to figure out how to balance two decks within itself, since focusing entirely on either fossil or non-fossil would leave the player unable to answer to whichever they didn't focus on. So that being said, it'd be a dead card against a deck that wasn't using any fossil creatures, but unless the other deck was fossil removal, the opposing deck should be vulnerable to your fossil battlefield. Also, something so basic as