Texier PJ, Porraz G, Parkington J, Rigaud JP, Poggenpoel C, Miller C, Tribolo C, Cartwright C, Coudenneau A, Klein R, Steele T, Verna C. (2010). "A Howiesons Poort tradition of engraving ostrich eggshell containers dated to 60,000 years ago at Diepkloof Rock Shelter, South Africa". Proceedings of the National Acadademy of Science, Stati Uniti d'America, 107: 6180–6185, doi 10.1073/pnas.0913047107, PMID 20194764