Trilobite

The Trilobite is a standalone fossil in the introduced in  that appears in all subsequent games to date.

In
After donating the fossil in, its plaque in the museum will read:

"Similar in appearance to a wood louse, the trilobite was an ancient sea-dwelling arthropod. It could range between 4 mm and 70 cm in length, depending on various factors. The trilobite was one of the first creatures to have eyes and the ability to detect enemies and prey."

Real-world information
Trilobites were one of the earliest groups of arthropods, and one of the most successful groups of early animals. They first appeared 521 million years ago in the Cambrian period, already highly diverse. Their biodiversity, combined with wide geographic ranges and easily fossilized exoskeletons, lend to an extensive fossil record, with over 20,000 species in ten orders described over their almost 300 million years in existence. They suffered a slow decline through the upper Devonian; only four families from the order Proetida survived the Devonian period, and by the end of the Permian, only four genera from two families (Brachymetopidae and Phillipsiidae) remained. These last two families went extinct during the Permian-Triassic extinction 252 million years ago, which was the largest mass extinction in the history of Earth, killing 96% of marine species and 70% of terrestrial vertebrate species.