Insights into dwarf stegodon (Stegodon florensis florensis) palaeobiology based on rib histology

Abstract

Stegodon florensis florensis is an example of a diminutive insular stegodon from the early Middle Pleistocene bonebeds of Mata Menge, So’a Basin (Flores, Indonesia). We report the first insights into this Stegodon's bone histology, which furthers our understanding of this species’ palaeobiology, and evaluate the micro-anatomy and metabolic microstructural products retained in well-preserved rib bone tissue. A proximal rib fragment selected from a larger assemblage was sampled at three anatomical regions from which histological sections were produced. The sampling regions differed in cortical width size ranging from relatively thick to thin (17.94 mm to 6.31 mm). Bone tissue arrangement and histomorphometric variables measuring histological products of adult bone remodelling processes (secondary osteon population density and area, bone area, and Haversian canal percentage) were examined. The preservation of bone histology was very good, showing widespread Haversian bone with indication of several generations of remodelling processes, multiple resorption cavities, and the presence of ‘super’ (unusually large) osteons. Secondary osteon density and geometry varied with cortical width through the serial sections whereby the section with the narrowest cortex produced the highest secondary osteon density but also the largest osteon area. The effects of medullary cavity infilling on the endocortical region in the Stegodon rib may also relate to the adjacent periocortical region. The rib histology of this intermediate-sized dwarfed species shows, for the first time, a degree of secondary bone active remodelling in a Stegodon that indicates this was possibly a long-lived individual. Microscopic bone characteristics shared with other extinct and extant proboscideans may be useful in identifying Stegodon from fragmented mixed fossil assemblages.