Dicynodon

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 : 352

DICYNODON

Thin sections of Dicynodon were acquired from specimens SAM-PK-5576, SAM-PK-5146, NMQR3665, and NMQR3633. SAM-PK-5576 includes a femur, tibia, radius, humerus, and rib. All the elements exhibit highly vascularized, uninterrupted fibrolamellar bone. The vascular canals, which exhibit an average channel density of 30% ( Table 2), are arranged in longitudinal primary osteons in circumferential rows. The medullary cavity is generally infilled with trabecular bone and surrounded by prominent enlarged channels ( Fig. 4A ), which extend into the midcortical region. Only the tibia has a sufficiently distinct medullary cavity to calculate the cortical thickness, which is 33%. The vascular canals decrease slightly in size towards the periphery in some of the elements. The rib is slightly less vascularized than the limb bones and some small secondary osteons are observed near the medullary cavity. Growth rings are not observed in any of the elements.

The limb bones of SAM-PK-5146 (femur, tibiae, fibula) also exhibit fibrolamellar bone, but the tissue is interrupted by annuli and/or LAGs. Although several sections were taken in the midshaft regions, the medullary cavity is not free and is instead completely filled with trabecular bone. All the elements of SAM-PK-5146 are less vascularized (12.2% average; Table 2) compared to those of SAM-PK-5576. The vascular canals are haphazardly arranged in primary osteons with short anastomoses or sometimes in circumferential rows. Enlarged channels are observed in the mid and outer cortex of the tibia.

The ulna of NMQR3665 is similar to SAM-PK-5146, but is better preserved. Enlarged channels are present in two circumferential rows in the midcortical region and several appear randomly distributed in the perimedullary region. NMQR3633 includes a humerus and a rib. The humerus also contains enlarged channels in the perimedullary regions, which become narrower towards the periphery ( Fig. 4B ). A single annulus was observed near the subperiosteal surface. The rib of NMQR3633 is similar to rib SAM-PK-5576e. A thick cortex surrounds a medullary cavity that is completely infilled by bony trabeculae. The cortical bone tissue consists of moderately vascularized fibrolamellar bone, interrupted by several prominent LAGs. The vascular canals are arranged as longitudinal primary osteons. The channels in the perimedullary region are fairly large, but become progressively smaller and less abundant towards the periphery of the bone.

Figures

Figure 4. Permo-Triassic dicynodont bone histology. A, juvenile Dicynodon humerus SAM-PK-K5576d; B, subadult Dicynodon humerus NMQR3633a; C, subadult Lystrosaurus maccaigi ulna NMQR3663b; D, juvenile Lystrosaurus declivis tibia NMQR735b; D, E, adult Lystrosaurus humerus (possibly Lystrosaurus declivis) NMQR3678; F, late subadult Kannemeyeria femur NMQR2674b. Arrows indicate enlarged channels in all genera. Scale bars: D, E = 500 mm; A, B, C, F = 413 mm.