sam2- we already answered your kasha ????
patur. rambam 2:1 (?”? ?????)
??? ??? ???? ????? ??????, ???? ??????: ?? ?? ?? ???? ???? ?????, ???? ????? ??????? ?? ???? ???????? ??? ?? ???? ??????
Isatis means indigo kayidua.
than in halacha 8 rambam says
???? ???? ??’ ???? ???? ??? ??? ???? ???? ??’. ???? ???? ???? ???? ??? ??? ???? ??????? ??? ?? ????? ???? ???? ???? ?????.
so all rambam (and k”m) means if something else besides isatis is domeh to techeiles it has the din of kala ilan (i.e. it isn’t techeiles, can’t be used for lavan on tallis shekula techeiles) But it doesn’t mean that in the times of the gemara they had something else. Just lidoros if they find something else it has this din.
And of course the rayos from the gemaros are very muchach that there was nothing else known to be like indigo. The Murex was around Bizmanum memeila Ba harug braglov