Thin monodromy in Sp(4)

We show that some hypergeometric monodromy groups in Sp(4, Z) split as free or amalgamated products and hence by cohomological considerations give examples of Zariski dense, non-arithmetic monodromy groups of real rank 2. In particular, we show that the monodromy group of the natural quotient of the Dwork family of quintic threefolds in P-4 splits as Z * Z/5Z. As a consequence, for a smooth quintic threefold X we show that the group of autoequivalences D-b(X) generated by the spherical twist along Ox and by tensoring with O-X(1) is an Artin group of dihedral type.