Fig 6 in The last giants: New evidence for giant Late Triassic (Rhaetian) ichthyosaurs from the UK

Fig 6. Histological overview of BRSMG Cg3178 (BAS surangular). A. Composite image of thin section under circular polarized light. B. Close-up of the external margin of the outer cortex, showing the presence of multiple growth marks (GMs), open vascular canals and cortical vascular canals with all degrees of maturity (simple canals, primary osteons and secondary osteons), supporting an ongoing active and continuous growth. Note the evident darker border of the lumen of a diagonal canal running from the top left toward the margin of the large nutrient canal (NC) showing further longitudinal vascularization. C. Concentric secondary osteon in the outer cortex under lambda filter. D. Close-up of the upper margin of the nutrient canal under crossed polarized (left) and circular polarized light (right). The growth marks appear as alternated tightly packed rows of brighter and darker periosteal intrinsic fibres (PIF). The same tight packing of the GMs occurs also deeper in the cortex. E. Lateral margin of the nutrient canal under circular polarized view. PIF are evident as bright yellow and blue coiled structures. The presence of simple canals alongside osteons, indicates primary deposition of bone along the margin of the large nutrient canal. F. Concentric secondary osteon in the trabecular bone under transmitted light. It is evident the high amount of osteocyte lacunae and the presence of plump irregular shaped ones in the lamellar bone. G. Trabecular bone under circular polarized view. The presence of primary matrix and concentric secondary osteons indicate that the trabeculae are secondary, produced from compact bone made cancellous. White arrows (D, E, G) point at PIF; white arrowheads point at resorption lines in concentric osteons (C, F, G); white dotted lines indicate borders of primary osteons (C, F, G); yellow arrow heads (B, D) point at rows of GMs. Abbreviations. LB, Lamellar bone; NC, Nutrient canal; OC, Open periosteal canal; PO, Primary osteon; RC, Resorption cavity; SC, Simple canal; SO, Secondary osteon. https://doi.org/10.1371/journal.pone.0300289.g006