If Prthu M is an avatara why did he take birth from a material body?

In the fourth canto, in the chapter on King Vena, it is described how King Prthu is born from the dead body of King Vena. King Prthu is a direct incarnation of Lord Visnu (4.16.2). Then should he not have a spiritual body? Why does he take a material body which has birth, disease, old age and death. Does that not put him in under the control of maha-maya?