Microscopic analysis of bone microstructure and mineral infilling geometry
The microscopic analysis of the bone samples show fibrolamellar bone and Haversian bone as the main histological bone types (Fig. 3A). Osteocyte lacunae are usually well preserved and commonly display more or less pronounced remains of the attached canaliculi (Fig. 3B). Lacunae and canaliculi as well as the Haversian canals usually are infilled with pyrolusite; quartz fills only a few Haversian canals. The Haversian canals often do not have sharp margins indicating a slight etching and replacement of the bone apatite by pyrolusite during the process of mineralization infilling (Fig. 3B).
Some Haversian canals are not completely filled by pyrolusite, forming miniature spirit levels with a plane perpendicular to the vector of gravity (Fig. 3C). The remaining volume above the pyrolusite usually is filled with quartz. Surprisingly in a few bones two generations of the pyrolusite filling and geologic spirit levels are visible, intersecting in an acute angle (Fig. 3D). Measurements of the orientation of these levels in 237 Haversian canals in two bones yield one predominant plane in each specimen and one or multiple additional orientations for the associated level planes (Fig. 4A, B). This clearly indicates that the bones were rotated one or several times after the first generation of pyrolusite levels had formed. Succeeding generations of bubble levels formed in the reoriented bones.
All bone specimens have two systems of microcracks, both mostly filled with pyrolusite. Firstly, short central radial cracks radiate outward from the walls of the Haversian canals (Fig. 3E). Secondly, several short peripheral radial cracks can be found at the outer margin of the secondary osteons (Fig. 3F). The latter do not bridge the wall of the secondary osteons and unlike the radial microcracks do not spread into the regions beyond. The latter are usually found in Haversian bone fossilized under aquatic conditions (Pfretzschner, 2000, Pfretzschner, 2004 and Pfretzschner, 2006). The peripheral radial cracks start at the circumference of the secondary osteons and spread a few micrometers centripetally into the osteon. Additionally, most secondary osteons are isolated from the surrounding bone matrix by circumferential cracks that follow the mineralized cement line along the outer surface of the seconday osteones.