Histological description and bone compactness analysis of fossil ( Aldabrachelys sp.) and extant Aldabra giant tortoise ( Aldabrachelys gigantea ) shell bone fragments

Aldabra Atoll, north of Madagascar and part of the Seychelles Outer Islands, is home to the Aldabra giant tortoise, Aldabrachelys gigantea, the only survivor of several species of giant tortoises that were once widespread on islands across the Western Indian Ocean. Aldabra Atoll preserves an extensive fossil record of giant tortoises, providing a unique opportunity to compare the shell bone microanatomy of extinct and extant forms. This study presents the first detailed histological investigation of fossil Aldabrachelys sp. shell fragments and a preliminary comparison with extant A. gigantea. Additionally, bone compactness is determined. Most fossil specimens exhibit the typical diploe structure of tortoise shell bones, comprising external and internal cortices surrounding cancellous bone, although considerable variation in cortical thickness, vascularization, remodeling, the presence and abundance of Sharpey's fibers, and preservation was observed. In contrast, extant specimens generally display a more homogeneous microanatomy with less distinct regions. This variation is also reflected in the bone compactness. Fossil Aldabrachelys sp. fragments range between 45.1% and 84.9%, and those of extant A. gigantea fragments between 45.8% and 86.1%. The substantial intra- and interfragment variability observed in both histology and compactness complicates the identification of general microanatomical patterns and highlights the importance of considering regional shell variation. Our study establishes a baseline for future investigations of giant tortoise evolution, ecology, and insular adaptation via histological characterization of shell bones.