title: Rib remodelling in Pleistocene insular elephants and stegodons
Creators: name: Basilia, Pauline G
Publisher: name: Griffith University
Year: 2023
Subjects: subject: fossil proboscidean
subject: insular dwarfing
subject: palaeohistology

description: Pleistocene insular dwarf proboscideans are among the most notable examples of the ‘Island Rule’, an ecological trend of body size reduction (dwarfing) and increase (gigantism) under insular conditions. While population-oriented studies, such as in Island Biogeography Theory, have focused on demonstrating the generality of the Island Rule, there is less attention on describing the biological process of dwarfing. Macro-anatomical analyses of fossils have provided data on morphological modifications that may have accompanied insular body size reduction. In insular dwarf proboscideans, morphological modifications, such as low-gear locomotion for navigating rough terrains, reveals adaptations to insular conditions beyond body mass. However, macro-analysis is limited by methodological concerns, such as preference for complete bones and rejecting fossil fragments that comprise the majority of fossil assemblages. Fossil fragments can be examined using micro-anatomical methods to generate more data on the effects of insular body size adaptation. Palaeohistology, the study of bone tissue microstructure, can provide important information on insular adaptation at a microstructural level. In this thesis, I provided the first analysis of rib histology in fossil Elephas maximus from Sumatra, dwarf Stegodon florensis florensis from Flores, dwarf Palaeloxodon creutzburgi from Crete, and large Palaeoloxodon antiquus from mainland Greece. My project explores remodelling in the cortical bone of size-reduced Pleistocene insular proboscidean ribs. Bone remodelling is executed by basic multi-cellular units (BMUs) through bone resorption (osteoclasts) and deposition (osteoblasts). BMU activities result in the formation of secondary osteons (hereafter ‘osteons’) which consist of Haversian bone tissues. From the thin-sections I prepared, I examined remodelling in periocortical Haversian bone tissues by measuring osteon population density (OPD), osteon (On.Ar) and Haversian canal area (HCa.Ar), bone volume (B.Ar), and Haversian canal percentage (%HCa). To simulate rib cortical size variation, I corrected all values by the cortical width (Ct.Wi). I overcame the lack of data on proboscidean bone microstructure by designing a multi-staged methodology. First, I determined the best skeletal element that would effectively show non-biomechanical influences (Chapter 3). Comparing histology of fossil against extant Asian elephants in the same size-range, I found that limb bones had statistically similar histomorphometry, in contrast to the rib bones. Since the elephants were of comparable size, I suggested that the weight-bearing function of limb bones heavily influenced bone remodelling. Differences in the histomorphometry in the ribs of same-sized conspecifics could be indicative of influences unrelated to weight-bearing as experienced by limb bones. Next, in the fossil Asian elephant, I described unusually large osteons, super osteons (&gt; 100,000 μm2), previously recorded in Asian elephants, and discovered extreme osteons (&gt; 500,000 μm2) in the elephant humerus (Chapter 4). Large cortices in the rib, vertebrae, and humerus could have promoted formation of unusually large osteons. To test the effects of cortical area on remodelling, I examined remodelling in varying cortical widths from the same rib of an insular dwarf stegodon (Chapter 5). I found that remodelling differed between cortical widths, with the thinner cortex displaying the largest osteons. My results suggested that cortical remodelling may be also be affected by other factors, including non-BMU-related activities, such as trabecular bone tissue formation. Finally, I compared remodelling between insular dwarf elephants from Crete and their closest phylogenetic ancestral species from the Peloponnese Peninsula, Greece (Chapter 6). My key finding is that rib remodelling between the Cretan dwarf and the large elephants significantly differed, with the dwarf elephant exhibiting larger osteons. The formation of larger osteons in the periocortical region of the insular P. creutzburgi ribs might be linked with ancestral bone traits. All proboscideans in this thesis exhibit trabecular infilling of the medullary cavity, which adds tensile and compressive strength to bones. Insular dwarf proboscideans could have inherited trabecular infilling from their ancestral species and, consequently, exhibit remodelling affected by trabecular formation. By examining rib bone microstructures of insular elephants and stegodon, my thesis provides new insights into insular dwarfism in proboscideans at a microstructural level. The outcome of my project may be indicative of insular adaptations resulting from retention of ancestral traits. My thesis adds to a better understanding of the insular dwarfing process experienced by fossil proboscideans. I suggest further histological analysis of fossil proboscidean rib bones comparing insular dwarf species with their ancestral species.