Fig. 1

Histology of vertebra (A–G), rib (H, J) and gastralium (I, K, L) of Askeptosaurus italicus. A, Transverse section of laterally compressed tail vertebra (PIMUZ T 4839_vert_a) in polarized light. B, Detail of neural spine displaying a medullary region filled by endosteal lamellar bone and surrounded by well and poorly organized coarse parallel-fibered bone of the periosteal region in polarized light. C, Collapsed trabecular structure of the inner centrum (PIMUZ T 4839_vert_a) consisting of endosteal lamellar bone (white/bright areas) and patches of calcified cartilage (some marked by arrows) in polarized light. The dark black spots are diagenetically initiated opaque mineral phases. D, Detail of medullary region and cortex of the tail vertebra (PIMUZ T 4839_vert_a) in polarized light. Dimension/extension of thin periosteal cortex indicated by double headed arrows. Single headed arrows indicate direction of Sharpey’s fibers (in bright grey/whitish and black, the latter are overgrown by opaque mineral phases). E, Detail of periosteal cortex of the tail vertebrae (PIMUZ T 4839_vert_a) in polarized light depicting coarse parallel-fibred tissue showing mainly in the upper left half a honeycomb structure of the thick and coarse scaffold of the parallel-fibred bone. F, Detail of periosteal cortex with primary osteons (arrows) of the tail vertebrae (PIMUZ T 4839_vert_a) in polarized light. G, High magnification of the periosteal cortex of the tail vertebra (PIMUZ T 4839_vert_a) with higher organized and less coarse parallel-fibred tissue in polarized light. The arrow marks Sharpey’s fibers. H, Cross section of the rib of PIMUZ T 4839 in polarized light. Note the dense central medullary region filled by endosteal bone and the nearly avascular, compact periosteal cortex consisting of coarse parallel-fibred bone. I, Cross section of the gastralium of PIMUZ T 4840 in normal light. Note the compact avascular tissue and the accumulations of osteocyte lacunae. J, Detail of rib tissue (endosteal and periosteal) in polarized light as marked with the rectangle in H). Note the coarse parallel-fibred tissue and the simple vascular canals obscured by opaque mineral phases (in black). K, Detail of gastral tissue of PIMUZ T 4840 in polarized light. Note the different degree of organization of the coarse parallel-fibred tissue in polarized light. The arrow marks the direction of Sharpey’s fibers. L, Enlargement of tissue of the gastralium in polarized light. Note the coarse parallel-fibred tissue and Sharpey’s fibers and the dense accumulations of osteocytes. The arrow marks the direction of Sharpey’s fibers. Abbreviations: eb, endosteal lamellar bone; ipo, incompletely lined primary osteons, hclp honeycomb-like pattern, hpfb highly organized coarse parallel-fibred bone, ppfb poorly organized coarse parallel-fibred bone, mr medullary region, Shf Sharpey’s fibers, svc simple vascular canals