Excavations in the ancient city of Gao on the River Niger near the southern fringes of the Sahara, have unearthed evidence which suggests that at least some of the medieval world's ivory trade was based not on elephant tusks, but on hippopotamus teeth (also known as tusks).
Ivory was the basis of a massive industry in medieval Islamic Spain and north Africa. Vast quantities of ivory-inlaid furniture and caskets, as well as items of scientific and medical equipment were made - and most experts in Islamic art and technology have always assumed that the ivory came from elephants.
And from the late 13th and 14th century, craftsmen in Paris, London and Cologne started working in what academics have always believed was also elephant ivory. But the Gao discovery may cast a totally new light on much of the medieval ivory trade.
Excavating in what was the mercantile half of Gao, Cambridge archaeologist Timothy Insoll discovered a hoard of 50 hippopotamus teeth weighing in total at least 100lb. Each tooth is up to 2ft long and is made of very white ivory which, to the naked eye, looks almost identical to elephant ivory.
Until now, virtually no Islamic or European ivory has been identified as coming from hippopotamuses - but then very few pieces have ever been scientifically examined.
Gao was a major trade centre in medieval times. In the very late medieval period it became the capital of a vast imperial system - the Songhai Empire (1450-1580) - covering some 800,000 square miles - more than eight times the size of modern Britain. Prior to that, the city formed part of the Mali empire (1290-1400).
But at the time that the hippopotamus teeth were buried - probably by merchants hiding their products, Gao was almost certainly the capital of a small independent Songhai kingdom, possibly functioning under the influence of the west African Almoravid empire, which conquered Spain in the mid-11th century. The twin metropolis of Gao consisted of two cities some four miles apart. The oldest, perhaps established as early as the 5th century AD, was a royal city covering some 85 acres by the 10th century, and was deserted by around 1300.
The second city - where the hippopotamus teeth were found - was the mercantile complex and covered around 500 acres, some 25 per cent of which is now occupied by buildings of the modern town of Gao, which has grown up and developed since the beginning of this century.
Mr Insoll's excavations also unearthed extensive evidence of medieval trade with Egypt, Spain and Tunisia. Huge quantities of smashed high- quality pottery and glass, and bronze jewellery manufactured in those areas were found in a dense layer almost immediately above the teeth.
At the time the teeth were deposited - presumably for safekeeping - Gao was officially Muslim, and the whole of the mercantile city would probably have been almost entirely Muslim.
The great medieval kingdoms and empires of west Africa are in the main a poorly researched area. In the Songhai capital, very little archaelogical work has been carried out, and in the royal part of the twin city, the archaeological resource is disappearing at an alarming rate.
Tragically, 80 per cent of the 26ft-high occupation mound of the royal city has been pitted with shafts dug by looters in search of beads to sell to tourists in Mali and neighbouring Mauritania.
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