Mesozoic Era grades
A: Jurassic. 58 million years, neatly divided into three roughly equal ages.
C: Triassic. A nice clean 50 million years, but who portioned this era out? Why is 70% of the Triassic "Late Triassic"
F: Cretaceous 77 million years??? And what do you mean there is no MIDDLE?
about 7 hours ago