Why is it Endodontic (-ic suffix) but Dental (-al suffix)?

Solution 1:

The adjective-forming suffix -al is originally from Latin (in the form -alis). The adjective-forming suffix -ic existed originally in both Latin (in the form -icus) and in Greek (in the form -ικός), but Greek -ικός was used more freely to form new adjectives than Latin -icus. So a large proportion of -ic words are from Greek, while a smaller proportion of -al words are from Greek. (The combined suffix -ical developed later as an extension of -ic, so like -ic, -ical also tends to occur in words with a Greek origin.)

Dental is built on the Latin word for tooth, dens (with the stem dent-), while endodontic is built on the Greek word for tooth, ὀδούς (with the stem odont-).