Developmental distribution of osteocyte lacunae in the limb bone cortex of Musivavis amabilis with a review of bone microstructure adaptations in Enantiornithes

The growth strategy of enantiornithine birds differs from other stem-group birds due to the slow and protracted growth phase that is usually recognized by the deposition of parallel-fibered bone, mostly avascular, and the appearance of resting line/s. In this study, we aimed to investigate the osteohistology of the miniature bohaiornithid-like enantiornithine bird Musivavis amabilis. The midshaft cortex of the left tibiotarsus of Musivavis consists of almost entirely avascular parallel-fibered bone; neither indicators of a faster growth period nor cessation growth marks are microscopically visible. To decipher the growth dynamics of this seemingly monotonous histostructure, made of relatively abundant osteocyte lacunae, we developed a new methodological algorithm in our laboratory and tested it. In principle, 2D-quantitative lacunar parameters were processed using statistical clustering methods to detect any transition between two different local densities of osteocyte lacunae. We found that the avascular cortical profiles of Musivavis exhibit a hidden pattern recording a short dynamic regime through a restricted transitional stage to gradually lowered dynamics in the distribution of the osteocyte lacunae. We conclude that novel mathematical tools are to be invented in studies of bone samples that look histologically simple as the substantial biological information can be overlooked by classic study approaches.