living fossil
Refers to a species that has remained unchanged for millions of years and closely resembles fossils of its ancestors. Used both in biology and figuratively for something that seems ancient or unchanged.
シーラカンスは生きた化石と呼ばれている。
The coelacanth is called a living fossil.
この古い風習は、まさに生きた化石だ。
This old custom is truly a living fossil.
Compound of 生きた (ikita, 'living') and 化石 (kaseki, 'fossil'). The phrase is a direct translation of the English term 'living fossil', which was coined by Charles Darwin.