from or referring to the period of time between about 56.5 and 35.4 million years ago

Fossils show that until the end of the Eocene period a land bridge connected Eurasia with North America. 化石显示，直到始新世末期，欧亚大陆和北美洲之间仍有一座陆桥相连。

rocks of Eocene age 始新世时期的岩石

a period of time between about 56.5 and 35.4 million years ago. It is between the Palaeocene and the Oligocene epochs (= periods of time)

During the age known as the Eocene, the earth experienced a great expansion of mammalian species. 在被称为始新世的时代，地球经历了哺乳动物物种的大规模扩张。

Fossils of sub-tropical species such as crocodiles and turtles have been found in the Canadian high Arctic dating from the early Eocene, 55 million years ago. 在加拿大北极高纬度地区发现了亚热带物种的化石，如鳄鱼和海龟，可追溯到5500万年前的始新世早期。