What is the origin of prothalamion?

Prothalamion, “a song or poem written to celebrate a marriage,” is modeled on epithalamion “a song or poem in honor of a bride and bridegroom.” Epithalamion is the neuter singular of the Greek adjective epithalámios “bridal, nuptial,” literally “at the thalamus,” i.e., the inner chamber at the rear of a house, woman’s room, bedroom, storeroom. Epithalamia (plural of epithalamion) were traditional features in Greek weddings and were therefore a very ancient custom. The epithalamia of the Lesbian lyric poet Sappho, the Athenian comic playwright Aristophanes, and the tragedian Euripides were famous. Edmund Spenser coined prothalamion in 1597, apparently intending his coinage to mean “a song or poem celebrating an upcoming wedding,” the Greek prefix pro– here meaning “before in time,” not “before in space.”