Fibrous tissues and microstructures recovered from IRSNB 1624.

• (A) Secondary electron micrograph of acid etched cortical bone showing fibrous tissues and what appears to be part of the osteocyte-lacunocanalicular system (osteocyte-like entities at arrows).
• (B) Osteocyte-like structure in lacuna within fibrous tissues.
• (C) Isolated osteocyte-like form visualized with fluorescent dye.
• (D) Topographic image of the same specimen as in C to illustrate the three-dimensional arrangement of the presumed cytoplasmic protrusions.
• (E, F) Light micrographs of fossil microstructures that are consistent in size and morphology to osteocytes or pericytes enfolding the outer surface of two vessel fragments.
• (G) Light micrograph of demineralized mosasaur bone tissues showing possible remains of the osteoid associated with vessel-like structures (C = cortex, M = medulla).
• (H) Light micrograph of an isolated fiber bundle.
• (I) TEM-image of demineralized mosasaur bone showing parallel-oriented fibrils. The spacing of the arrows indicates a 67 nm axial repeat D-banding pattern, which in modern bone is characteristic of collagen.
• (J) Transverse section (TEM-image) of a blood vessel from cortical bone of an extant monitor lizard humerus (LO 10298). Note the hair-like bone matrix fibers that are coiled around the canal wall.
• (K) Corresponding structures in the mosasaur humerus.
• (L) Histochemical staining of demineralized mosasaur bone suggesting the presence of connective tissue (blue) in the hair-like fibers that line a partially ruptured canal wall (the fracturing presumably occurred during the preparation of the sample).
• (M) Light micrograph (thin section) of untreated mosasaur bone showing fibers embedded in hydroxyapatite.