Originally published 3/9/11, 10:45 am.
It seems apparent that in chapter 1 the Mishnah was unaware of any nighttime obligation to read the M'gillah because the villagers gathered to read only on Monday or Thursday during the day time - Yom hakknisah, see 1:1. 1:2
Kehatti emphasizes this in 2:10 when pointing out the absence of M'gillah as a nighttime mitzvah. Here. he quotes Rabbenu Tam and the Ra"n that the iqqar reading is in the day.
This pretty much proves the argument that it was R Y'hoshua ben Levi's Meimra that is the earliest source to require reading the M'gillah at night.