Esper was the first to record the finding, in Gailenreuth Cave, of human bones alongside the remains of unknown and probably extinct animals. The implications of this dramatic observation published in a color plate book about unusual fossil animal bones seem to have been unnoticed by the scientific establishment. Also French translation, Nuremberg, Knorrs, 1774. 

Esper's book also included the first published description of disease in ancient bones, a possible bone tumor affecting a fossil cave bear.