Inscriptions on objects often provides clues. The burial of Apis bulls began in the reign of Amenhotep III. The bulls were killed in a sacrifice and placed in tombs. The details of each bull were included, as well as details of the rulers. It is possible to estimate dates of regnal years using this information. The bulls were still being buried in Ptolemaic times. There is a significant gap in the record between Ramesses XI and the 23rd year of Osorkon II. Poor documentation of these finds in the Serapeum also makes it difficult to use these records.