The earliest known sculpture of large size in the Sudan is that produced in pottery by the Nok culture, which flourished extensively in northern Nigeria from the 5th century BC into the early centuries AD. These people were the first known manufacturers of iron in western Africa, furnaces at Taruga having been dated between the 5th and early 3rd centuries BC; they continued, however, to use stone tools. Of well-fired clay, their sculptures represent animals naturalistically; human figures, however, are depicted with heads that are usually tubular, but sometimes conical or spherical, and with simple tubular trunks and limbs. The art of Nok indicates the antiquity of many basic canons of West African sculpture, but the precise relationship between ancient and modern forms is obscure.