This is the 2645th most frequent Japanese word.
最古
"Oldest" or "ancient."
In this sentence, 最古 is used as a descriptor to denote being the oldest among a group, here referring to the shrine.
この神社はこの地域で最古のものです。
This shrine is the oldest in this region.
Here, 最古 identifies the fossil as being the oldest historically or temporally among similar items.
この博物館に展示されている化石が最古であるとされています。
The fossil displayed in this museum is considered to be the oldest.
In this context, 最古 qualifies the records as the earliest or first known in a chronological order.