Fig. 2

Histology of femora of Askeptosaurus italicus. A, Midshaft cross section of the left femur of PIMUZ T 4839 in normal light. Note the overall radial organization of vascular canals and different growth phases as indicated by distinct lines of arrested growth and/or changes in color. Note the shifts of the growth layers indicating compaction (white arrows). Black arrows indicate LAGs. B, Midshaft cross section of the right femur of PIMUZ T 4839 in normal light. Vascularity is less radial and more longitudinally organized. Note the shifts of the growth layers indicating compaction (white arrows). Black arrows indicate LAGs. C, Patches of calcified cartilage embedded in endosteal lamellar bone in the medullary region of the right femur. D, Strong radial organization of simple vascular canals and incompletely lined primary osteons in the outer cortex of the left femur. Note the coarse and loosely organized parallel-fibred tissue alternating with more with more higher organized and more finely organized parallel-fibred tissue. E, Detail of cortex of the right femur depicting alternating coarse and loosely organized parallel-fibred tissue and with more higher organized and more finely organized parallel-fibred tissue. F, Enlargement of outer cortex of the left femur. Note the simple vascular canal (white arrow) that opens into the surface and the coarse nature of the parallel-fibred tissue. G, Region with primary osteons that are completely and incompletely lined by parallel-fibred bone in the left femur. H, Enlargement of coarse parallel-fibred tissue with simple vascular canals. I, Midshaft cross section of the left femur of PIMUZ T 4840 in normal and J, in polarized light. Note the principally radial orientation of the mainly short radial and longitudinal vascular canals. Vascular density is lower when compared to the femora of PIMUZ T 4839. K, Detail of the cortex from the postaxial bone side in polarized light. Note that the outer cortex shows more vascular canals than the inner cortex. The entire cortex is stratified by growth marks. L, Detail of well vascularized (by simple vascular canals) and poorly organized inner tissue visible in I and J dorsally to the medullary region in polarized light. Note the coarse nature of the loosely organized parallel-fibred tissue. M, Radially organized vascular canal system at the dorsal bone side. Note the funnel-shaped arrangement of tissue along the simple radial vascular canals in polarized light. N, Detail of cortex depicted in K in normal light. Note the accumulation of osteocyte lacunae in the left part of the picture. Abbreviations: cc, calcified cartilage; ipo, incompletely lined primary osteons; mr, medullary region; svc, simple vascular canals