Barunlestes butleri

Chinsamy, Anusuya & Hurum, Jørn H., 2006, Bone microstructure and growth patterns of early mammals, Acta Palaeontologica Polonica 51 (2), pp. 325-338 : 331-332

Barunlestes butleri ( Fig. 9 )

Rib.—A complete transverse section of a rib was available. The bone wall is fairly thick, with a RBT of about 31%. It largely comprises of parallel−fibred bone tissue, in which several rest lines are recognizable. The bone is generally poorly vascularized, except for a localized area in the region of drift, the osteocytes tend to be haphazardly oriented, and a few channels for blood vessels are visible in the compacta ( Fig. 9A ).

Femur.—The Barunlestes femoral sample examined comprised only a small fragment. The section shows an outer parallel−fibred bone tissue with several LAGs ( Fig. 9B 1 , B 2 ). In a localized part of the section, there appears to be a bone tissue with more globular haphazardly oriented osteocyte lacunae

( Fig. 9B 2 ), which may be correlated with a faster rate of bone deposition.

Extant eutherian

Heterocephalus glaber ( Fig. 10 ).—Cross sections of the femur display a rather thick bone wall, with an average thickness of about 0.8 mm and a RBT of about 30%. Two distinct regions are clearly visible in the bone wall: a poorly vascularized outer circumferential layer (OCL) that consists of a parallel−fibred bone tissue, and an inner, richly vascularized region consisting of a more woven type of bone tissue. The perimedullary region of the bone is uneven and highly resorptive. Several radially oriented vascular channels extend from the medullary cavity to the mid cortical part of the bone wall. The vascular channels lack osteonal development around them. Although some enlarged erosion cavities are visible in the compacta, no secondary osteons were observed ( Fig. 10 ).

Figures

Fig. 9. Barunlestes butleri Kielan−Jaworowska, 1975,?late Campanian, Baruungoyot Formation, Khulsan, Nemegt Basin, Gobi Desert, Mongolia. ZPAL MgM−I/77, section catalogue number, ZOO/UCT/189. A. Transverse section of a rib, showing the poorly vascularized compacta and the parallel fibred nature of the compacta, as well as several lines of arrested growth (arrows). B. The femur from the same ZPAL specimen, section catalogue number ZOO/UCT/189 (on same slide as rib). B1. Partial transverse section of the femur on the same slide showing the similarly poorly vascularized compacta of parallel fibred bone in the bone wall. B2. Higher magnification of the femur in B1. Small arrows in B2 indicate lines of arrested growth; large arrow indicates the more globular haphazardly oriented osteocyte lacunae.

Fig. 1. Cladogram showing the phylogenetic relationship of the taxa studied (simplified from Kielan−Jaworowska et al. 2004).

Fig. 2. Tritylodon sp. Early Jurassic, Massospondylus Range Zone, Elliot Formation, South Africa. A. Humerus, BPI/5671 section Ty4B2, showing the fibro−lamellar nature of the compacta. B. Radius, BPI/5167, section Ty1BII3, showing the peripheral development of parallel fibred bone with rest lines (arrows).

Fig. 10. Heterocephalus glaber Rüppell, 1842. Recent, ZOO/UCT no number; femur, section catalogue number, ZOO/UCT/192, showing the wellvascularized compacta and the narrow outer region of parallel fibred bone (OCL).