Answer:

triassic

Answer	Crossword Clue
TRIASSIC	Period in which dinosaurs appeared
TRIASSIC	Dinosaur time period
TRIASSIC	Early dinosaur period

TRIASSIC	Period when dinosaurs appeared
TRIASSIC	Very old
TRIASSIC	Mesozoic Era period
TRIASSIC	Period of the first dinosaurs
TRIASSIC	Arizona's Petrified Forest dates from this period
TRIASSIC	Period bookended by extinction events
TRIASSIC	Period when mammals first appeared