Complex sentence: Upon his return from Northern Europe in 1981 Callahan founded Piltdown Productions as a means to sell his many publications, supplies, and instructional materials, as well as the stone tools and knives he produced.
Split into simpler sentences: Upon his return from Northern Europe in 1981 Callahan founded Piltdown Productions. The company was founded as a means to sell his many publications, supplies, and instructional materials, as well as the stone tools and knives he produced.