Complex sentence: The series was formed at the dawn of the'pony car' era, and was derived from the SCCA's A & B Sedan amateur Club Racing classes, Early Trans-Am Series racecars were just modified production cars.
Split into simpler sentences: The series was formed at the dawn of the'pony car' era and was derived from the SCCA's A & B Sedan amateur Club Racing classes, based upon commercially produced cars which had been modified for racing competition. At first, Trans-Am Series racecars were just modified production cars.