A Previously Unreported Macroscopic Structure in Palaeoloxodon Tusk Fossils: A Right-Handed Double-Ridge Spiral Band

This paper describes three tusk fossil fragments collected from construction spoil in the Lunan region, Shandong Province, China. Based on morphological and microscopic observations, the specimens are tentatively assigned to the tusk of Palaeoloxodon sp. The specimen surfaces bear a macroscopic structure not previously documented in the published literature: two raised ridges extend spirally along the long axis of the specimen, running nearly parallel with relatively uniform spacing. The ridges are separated by a narrow groove on their inner sides and bordered by wider, deeper grooves on their outer sides; the two ridges together with the narrow inter-ridge groove form a band-like structure, here provisionally named the "right-handed double-ridge spiral band." The handedness is determined with the root-to-tip direction as the reference, and both ridges show right-handed winding. One specimen is used for macroscopic observation, and the other two are sectioned separately for transverse and longitudinal sections to verify the internal dentinal structure. Morphologically, this structure differs markedly from vertebrae, Daimonelix trace fossils, and narwhal tusks. This paper describes the macroscopic morphology and preliminary microscopic features of the structure, discusses the evidence for identifying the specimens as tusk and for the tentative assignment to Palaeoloxodon, and proposes a preliminary hypothesis on its origin; detailed argumentation will be presented elsewhere.