noun
index fossil; guide fossil
Technical term in geology and paleontology for a fossil used to define and identify geological periods.
示準化石は地層の年代を決めるのに役立つ。
Index fossils are useful for determining the age of strata.
Compound of 示準 (shijun, 'indicating standard') and 化石 (kaseki, 'fossil'). The exact derivation is uncertain, but the term is a modern technical coinage.