Bone microstructure records complete growth history of captive springbok (Antidorcas marsupialis)

Bone microstructure contains records of life history events, growth patterns, and biomechanical function. Physiologically stressful events such as birth, weaning, and the onset of unfavorable seasons can result in the formation of cortical growth lines. Osteohistological examination of modern taxa with known life histories aids in interpreting bone microstructure of wild and extinct animals. Despite this utility, in-depth evaluations of mammalian bone histology remain relatively scarce. Here, we describe forelimb and hindlimb long bone microstructure of a limited ontogenetic series of captive-raised springbok (Antidorcas marsupialis). Our sample consists of animals of known age for directly comparing cortical growth mark counts to age at death. The bone cortices of springbok long bones generally consist of an inner cortex of endosteal lamellar tissue with regions of coarse compacted cancellous bone. The mid-cortex is fibrolamellar bone with laminar to plexiform vascularity interrupted by annual growth marks, though coarse compacted cancellous bone extends into the mid-cortex regionally. The outer cortex is an external fundamental system composed of avascular parallel-fiber tissue. This outer lamellar tissue marks the attainment of skeletal maturity. Birth lines, weaning lines, and lines of arrested growth are present throughout most of the sample, though medullary cavity expansion and remodeling processes have obliterated growth marks in some areas. Growth marks are found within the mid-cortical fibrolamellar tissue and the external fundamental system. The onset of the EFS aligns with the attainment of skeletal maturity based on mid-cortical growth mark counts. Mid-cortical growth marks in springbok appear to be a reliable indicator of age prior to skeletal maturity.