A sudden increase in the production of elephant ivory artifacts on the Mediterranean coast of North Africa occurred in the tenth century. Historians explain this increase as the result of an area opening up as a new source of ivory and argue on this basis that the important medieval trade between North Africa and East Africa began at this period. Each of the following, if true, provides some support for the historians' account described above EXCEPT: ['In East Africa gold coins from Mediterranean North Africa have been found at a tenth-century site but at no earlier sites.', 'Documents from Mediterranean Europe and North Africa that date back earlier than the tenth century show knowledge of East African animals.', 'East African carvings in a style characteristic of the tenth century depict seagoing vessels very different from those used by local sailors but of a type common in the Mediterranean.', 'The many surviving letters of pre-tenth-century North African merchants include no mention of business transactions involving East Africa.']