[5][6][7] The Ypresian is additionally marked by another warming event called the Early Eocene Climatic Optimum (EECO).

The EECO is the longest sustained warming event in the Cenozoic record, lasting about 2–3 million years between 53 and 50 Ma.

[14][15] The Ypresian Stage was introduced in scientific literature by Belgian geologist André Hubert Dumont in 1850.

The base of the Ypresian Stage is defined at a strong negative anomaly in δ13C values at the PETM.

The top of the Ypresian (the base of the Lutetian) is identified by the first appearance of the foraminifera genus Hantkenina in the fossil record.