The petrous bone contains high concentrations of osteocytes: one possible reason why ancient DNA is better preserved in this bone.

The characterization of ancient DNA in fossil bones is providing invaluable information on the genetics of past human and other animal populations. These studies have been aided enormously by the discovery that ancient DNA is relatively well preserved in the petrous bone compared to most other bones. The reasons for this better preservation are however not well understood. Here we examine the hypothesis that one reason for better DNA preservation in the petrous bone is that fresh petrous bone contains more DNA than other bones. We therefore determined the concentrations of osteocyte cells occluded inside lacunae within the petrous bone and compared these concentrations to other bones from the domestic pig using high resolution microCT. We show that the concentrations of osteocyte lacunae in the inner layer of the pig petrous bone adjacent to the otic chamber are about three times higher than in the temporal bone, as well as the cortical bone of the femur. The sizes and shapes of the lacuna in the inner layer of the petrous bone are similar to those in the femur. We also confirm that the petrous bone lacunae do contain osteocytes using a histological stain for DNA. We therefore conclude that one possible reason for better preservation of ancient DNA in the petrous bone is that this bone initially contains up to three times more DNA than other bones, and hence during diagenesis more DNA is likely to be preserved. We also note that the osteocytes in the inner layer of the petrous bone may have a function in hearing.