Among the earliest literature, we have in Rig-Veda 9 a reference to the marriage of the Sage Syavasva to the daughter of King Rathviti Darbhya. The Satapatha Brahmana10 describes the story of the marriage of the old sage Chyavana, a Bhargava or Angirasa and Sukanya daughter of King Saryata. 

Gautama declared 11: " children born in the regular order of wives of the next, second or third lower castes become Savarnas, Ambhashthas, Ugras, Nishadas, Daushyantas or Parasavas." Hence the Parasava is the offspring or a Brahmana from a Sudra wife.12 Among those unfit to dine at a Sraddha, Gautama13 mentions a Brahmana who has married a Sudra wife. Narada places a Parasava son higher than Nishadas (son of a Kshatriya husband from a Sudra wife.) 14

Pratiloma marriage:

The Talagunda Pillar Inscription of Kakusthavarman15 can be taken as an example of Pratiloma marriage.

Kakusthavarman belonged to Kadamba families who were Brahmins. Mayurasarma, the founder of the Kadamba family, went with his preceptor to Kanchi Pura, where he joined the Pal lavas and exchanged the sword for learning. Kakusthavarman, the fourth from the founder gave his daughters in marriage to the Gupta and other kings who were evidently Kshatriya. Here is an example where women descending from Brahmin Ancestors married Kshatriya without the censure or ban of society.