Rhachiocephalus, Seeley, 1898

Botha-Brink, Jennifer & Angielczyk, Kenneth D., 2010, Do extraordinarily high growth rates in Permo-Triassic dicynodonts (Therapsida, Anomodontia) explain their success before and after the end-Permian extinction?, Zoological Journal of the Linnean Society 160 (2), pp. 341-365 : 351

RHACHIOCEPHALUS

Portions of an ulna, radius, femur, and tibia from one individual (SAM-PK-3714) and a rib fragment from another specimen (SAM-PK-10099) were available for thin sectioning. As none of the midshaft regions was preserved, the cortical thickness could not be calculated for any of the limb bones. The bone tissue of the ulna and radius consists of highly vascularized fibrolamellar bone interrupted by annuli of lamellar bone and/or lines of arrested growth (LAGs; Fig. 3C ). Growth rings were not observed in the femur and tibia. The vascular canals are arranged in an orderly network of circumferential rows in the ulna and longitudinal primary osteons with short radial anastomoses in the radius, femur, and tibia. The average channel density (based on the femur and ulna) is 14% and there is no decrease in vascularization in any of the limb bones. Enlarged channels are observed in the ulna. Sharpey’s fibres are prominent in the radius, femur, and tibia. Secondary remodelling is also prominent and numerous secondary osteons are present in the perimedullary regions of all elements. Although the bone tissue of the rib is similar to that of the limb bones, parallel-fibred bone was observed at the periphery.

Figures

Figure 3. Permo-Triassic dicynodont bone histology. A, subadult Cistecephalus ulna NMQR1465b; B, early subadult Dicynodontoides tibia NMQR479a; C, adult Rhachiocephalus ulna SAM-PK-3714a; D, subadult Tropidostoma tibia SAM-PK-K9960c; E, adult Oudenodon femur SAM-PK-K4807a; F, adult Aulacephalodon femur NMQR3016. Arrows indicate enlarged midcortical channels in Rhachiocephalus and Aulacephalodon. Scale bars: A, D, E = 500 mm; B, C, F = 413 mm.