While baptisms for Jerez, Zacatecas have been indexed/extracted, marriages have not been. Marriage records exist from 1712, and are online at Family Search. Since Tomas’s baptismal record says he was a legitimate son, a church marriage should exist. In the Jerez marriage records there is a gap in the register from 1745 to 1761, according to the catalog at Family Search. This is an unfortunate gap as the marriage you need may be in this period. A search strategy would be to start at the end of the volume that contains 1745 and work backwards (film 440059); also scan the whole volume as pages are sometimes out of order and this may not be reflected in the catalog. Scanning through the next film, 440060, may be worthwhile for the same reason. Sometimes you can find missing marriages in the matrimonial investigations, unfortunately, the catalog says they begin in 1785, which is too late to be useful, but sometimes these are out of chronological order and other years mays be included. It is worth a look.
Also, the baptismal records for Tomas states he was a mulatto, that is a person of mixed race. This then may be the end of the line for this family and earlier records may not exist. Mixed race people where near the bottom of social strata in Colonial Mexico, so the paper trail often disappears.
As far as ancestors from Spain, sometimes the early baptisms will note if a parent was from Spain and name the town. The same with the marriages records. But, in this latter case if the groom is from Spain a dispensation would be required since he was an “ultramarino.” The records of these are in the diocesan archives in Guadalajara. These are online at Family Search. privately made indexes do exist both in print and in online resources (but not at Family Search).
Sometimes you can find the very early immigrants to Mexico in the Pasajeros a Indias. These volumes exist in print, and are online at the PARES website (Spanish Archives).
