According to Sanhedrin 1:6, the Sanhedrin in Jerusalem had either seventy or seventy-one sages. But according to Zevachim 1:3, Yadayim 3:5 and 4:2, there were seventy-two sages in Yavneh on the day they replaced Rabban Gamliel with Rabbi Elazar ben Azariah. Why was there an extra sage?
Note: I am open to the possibility that the extra sage was R' Elazar ben Azariah, who based on the Talmud's drosh in Berakhot was actually too young to have been a sage prior to that point, but I would like to see a source that says so. Also, note that Rabbi Eliezer, who had been on the council previously, was in Lod that day, so can't be included in the total (Yadayim 4:3).