Fig. 2

The microstructure of the long bones of Ceresiosaurus. Transverse mid-shaft section of a the pachyosteosclerotic femur of C. calcagnii (T5622) and b the osteosclerotic trunk rib of C. lanzi (T5454). c Higher magnification of the femur of C. calcagnii. The inner periosteal cortical region comprises a layer of embryonic bone that is composed of woven-fibred bone (wb). The thick cortex consists of lamellar-zonal bone tissue type (lzb) with undulating growth zones, annuli and lags. d–f Higher magnifications of the rib sample of C. lanzi. d The medullary cavity is filled with a calcified cartilaginous core (cc) that is remodelled by resorption and deposition of endosteal bone (ed). e, f The crystallites around the vascular canals are aligned in funnel-shaped arrangements (e in normal light and f in polarised light with lambda compensator). Further abbreviations: po primary osteon, svc simple vascular canal