Ordovician period vs Ordovician

Ordovician period and Ordovician both are nouns.

Word	Noun	Adjective	Verb	Adverb
Ordovician period	Yes	No	No	No
Ordovician	Yes	No	No	No

As nouns, Ordovician period and Ordovician are synonyms defined as:

Ordovician period and Ordovician: the period from 500 million to 425 million years ago; characterized by conodonts and ostracods and algae and seaweeds

Ordovician period (noun)	Ordovician (noun)
the period from 500 million to 425 million years ago; characterized by conodonts and ostracods and algae and seaweeds	the period from 500 million to 425 million years ago; characterized by conodonts and ostracods and algae and seaweeds