Fossil dinosaur bone microstructure

This specimen is from the Judith River Formation along the Milk River in southern Alberta. It is Campanian (Late Cretaceous) in age, which corresponds to about 80 million years old. It is an example of ideal preservation, and shows all the microstructural features typical of most fossil vertebrate bone. Little or no alteration of the hydroxyapatite appears to have occurred, and a significant amount of the pore space is not infilled with minerals and retains its "spongey" texture.

In hand specimen, the polished cross section is a dark brown colour, while the outer surface has weathered to a light tan colour. Even by visual inspection without the use of microscope, the pores of the haversian canal system are clearly visible in the inner "marrow", with smaller examples in the surrounding, denser bone.

hand specimen, polished cross section surface

Some abrasion appears to have removed the outermost layers of the bone, exposing some of the haversian canals on the outer surface, and revealing the complex pattern of anastamosing tube-shaped haversian canals.

outside surface with haversian canals Scale bar in millimetres.

In thin section with transmitted light, the structure of the bone is more easily seen, and the bone material aquires a deep yellow-brown colour. Blue colour is from an epoxy resin used to strengthen the specimen and make pore space more visible.

thin section of bone, showing haversian canals

Note the occurrence of distinct zones to the density of bone and the size of the haversian canals.

At higher magnification, the laminar structure of the bone surrounding the haversian canals becomes visible. Some of the bone has also been reabsorbed in the process of growth, as evidenced by cross-cutting relationships between some of the circular laminated bone around the canals.

dinosaur bone thin section, from "marrow" (left) to outer surface (right).

At still higher magnification, the cavities left by individual bone cells are visible, forming dark spots that are elongated parallel to the laminations around each canal (filled with blue epoxy). Also present are very fine (several microns) anastamosing channels extending from each cell cavity, known as caniculi. These are just barely visible as light-coloured filaments radiating from each dark cell cavity in the highest-magnification image. Most are oriented in a radial direction, perpendicular to the laminations of the bone around each haversian canal.

close up of cell-related structure around individual haversian canals

In summary, fossil bone has a very distinctive microstructure, consisting of tubular, branching haversian canals, each of which has a laminated, concentric structure, and in well-preserved material, cavities and tiny caniculi left by individual bone cells are visible at high magnification. Some of this distinctive structure is visible by examination with the naked eye or a hand lens, but it becomes blatently obvious with modest magnifications of only 10-100x.

Perhaps the most diagnostic feature, visible even at low magnification and with poor preservation, is the clear tubular geometry of the haversian canals. They are also clearly segregated with respect to size, with the largest, millimetre-sized tubes in the centre of the bone, and diminishing in size towards the margins. Even without the other features, this should be easily recognizable, although there would be potential for confusion with some types of vascular plant structure if the preservation were too poor. The laminated structure and cavities left from the bone cells would be one distinguishing feature, and most vascular plants also have comparatively thin cell walls with distinctive pores (tracheids). In addition, many plant remains will have some carbonized plant material representing the cell walls, while bone is completely mineralized, whether it is unaltered or not.

Back to the evaluation of Carboniferous bone
Andrew MacRae