Medusa’s gaze: Cell traces and fibrils but no collagen in permineralized Jurassic ichthyosaur bone

Bone is formed by specialized cells whose activity allows bone to grow, change shape, and repair itself. Its composite structure of collagen fibrils and bioapatite nanocrystals gives bone exceptional mechanical strength. Using scanning electron microscopy, we show in fossil ichthyosaurs, 150 to 200 million years old, from the Jurassic of France and the UK, abundant and direct evidence of cellular activity on the fossilized forming, resting, and resorbing surfaces of bone trabeculae, as well as bone fibrils, Sharpey fibers, and cartilage fibers. These features are identical to those observed in fresh deproteinized mammalian bone, including human bone. Despite the striking similarity of the fibrils to those in modern bone, we found no evidence of collagen preservation. Fossilization removed non-mineralized components and exposed trabecular surfaces at the mineralization front. Cellular activity in skeletal tissue, familiar to any medical student, is preserved for >200 million years, and probably longer in vertebrate fossils.