Why did Jenne architecture change by the ninth century?

Roderick and Susan McIntosh excavated at Jenne-jeno and neighboring sites in 1977 and 1981 and returned in 1994 for coring and more survey, with funding from the National Science Foundation of the United States, the American Association of University Women, and the National Geographic Society (1994). This research formed the basis of their Ph. D. dissertations at Cambridge University and the University of California at Santa Barbara, respectively. The McIntoshes have published two monographs and numerous articles on their archaeological research in the Middle Niger. They are professors of anthropology at Rice University in Houston, Texas, and they continue to collaborate with Malian colleagues from the Institut des Sciences Humaines on research along the Middle Niger.