Noun
Tremadocian (uncountable)
(geology, paleontology) A subdivision of the Ordovician period.
The upper boundary is defined as the appearance of Iapetognathus fluctivagus which marks the beginning of the Tremadocian and is radiometrically dated as 485.4 ± 1.9 million years ago. Source: Internet
The boundary chosen for the beginning of both the Ordovician Period and the Tremadocian stage is highly significant. Source: Internet