RhymeZone: hettangian definitions

Definitions of Hettangian:

noun: (geology, paleontology) The earliest age of the Jurassic period, from 199.3–201.3Ma
noun: The corresponding lowest stage of the Jurassic system, marked by the lowest strata in which the neoammonite genus Psiloceras appears.

(Definitions from Wiktionary)