de Villiers, Mark; Hirtle, Sheila (July–August 2007). "Space, Time, and Timbuktu". Natural History. Archived from the original on 27 September 2007. Retrieved 17 December 2014.
de Villiers, Mark; Hirtle, Sheila (July–August 2007). "Space, Time, and Timbuktu". Natural History. Archived from the original on 27 September 2007. Retrieved 17 December 2014.