From Bulbapedia, the community-driven Pokémon encyclopedia.
The Dome Fossil (Japanese: こうらのカセキ Carapace Fossil) and Helix Fossil (かいのカセキ Shellfish Fossil) are fossils which appear in the Generation I games, their Generation III remakes, and in the Generation IV games.
These key items are found in Mt. Moon after defeating a Super Nerd. While both are present, players are allowed to choose only one, as the other one will be taken by the Super Nerd. If brought to the laboratory on Cinnabar Island, the Dome Fossil can be regenerated into a Kabuto, and the Helix Fossil can be regenerated into an Omanyte. The Pokémon extracted from the Super Nerd's fossil is seen in the zoo just outside the Safari Zone.
In the anime, Dome Fossils are seen in Shell Shock. They make up the structure of an island, and are revived into Kabuto in red moonlight, causing the whole island to collapse and sink.
Both fossils appear on the artwork for the Mysterious Fossil card. However, the artwork depicts the Dome Fossil as the fossil of a Kabutops rather than a Kabuto.
The fossils are also available, like the Claw and Root Fossils, in Sinnoh's Underground after the National Pokédex is acquired. Helix Fossils are more common in Diamond while Dome Fossils are more common in Pearl.
In HeartGold and SoulSilver, the Dome and Helix fossils can be found randomly by smashing rocks at the Ruins of Alph, and can be revived at the Pewter Museum of Science. The Dome Fossil is exclusive to the SoulSilver version while the Helix Fossil is exclusive to the HeartGold version.
Dome Fossils, as seen in the anime