On preperiodic points of rational functions defined over Fp(t)

Let P is an element of P_1(Q) be a periodic point for a monic polynomial with coefficients in Z. With elementary techniques one sees that the minimal periodicity of P is at most 2. Recently we proved a generalization of this fact to the set of all rational functions defined over Q. with good reduction everywhere (i.e. at any finite place of Q.). The set of monic polynomials with coefficients in Z can be characterized, up to conjugation by elements in PGL(2)(Z), as the set of all rational functions defined over Q. with a totally ramified fixed point in Q. and with good reduction everywhere. Letp be a prime number and let F-p be the field with p elements. In the present paper we consider rational functions defined over the rational global function field F-p(t) with good reduction at every finite place. We prove some bounds for the cardinality of orbits in F-p(t) for periodic and preperiodic points.