Sample TH96-001 -- interpretation

NOTE: Some of these images are rather large. Check the sizes before downloading. I could decrease the size, but only at the expense of important details. If you want smaller images anyway, let me know.

Weathered exterior view, reflected light

These images were photographed with a macro lens.

Fig. 1 -- oblique view showing flattened cylindrical shape. The two pink objects sticking out of the top are plasticine to hold the specimen -- oops :- Magnification: approximately 2.2x

Fig. 2 -- top view showing outer surface. Magnification: approximately 2.9x

Interpretation:

The exterior of the specimen is a dark reddish brown colour, plus a few areas with a thin coating of orangey material. The red-brown colour is characteristic of the weathering colour of siderite (FeCO3 -- iron carbonate). Fine, grey mineral grains (probably quartz) are the main minerals visible on the surface, but there are also some small, flat grains with metallic lustre, probably a mica mineral. Weathering appears to have penetrated more deeply on the flattened cylindrical surface, and less so on the relatively flat surfaces on either end, suggesting they were recently broken. The end surfaces have some indication of concoidal fracturing, which is consistent with the relatively well-indurated (hard) condition of the sample. It is obvious the sample is well cemented by some mineral, likely the siderite, but perhaps other minerals as well. The orange coating appears to be limonite or some other iron oxide or hydroxide (i.e. basically "rust"). NOTE: when the specimen first arrived, it was coated with an adhering black mud, which has been washed off with soap and water.

Polished cross section, reflected light

These images were photographed using a reflected light microscope.

Fig. 3 -- cross section (186Kbytes), polished, sawed surface. Opposite end from that in Fig.1. Top of Fig.2. Digitally reconstructed from two images. Magnification: approximately 6.6x.

Interpretation:

The cross section has concentric structure, roughly parallel to the outer surface. A reddish-brown layer occurs within about 3-5 millimetres of the surface. Inside this, the specimen is grey, but in two zones -- an outer, dark grey zone, and an inner, lighter grey zone. The central zone is about 5-6 millimetres high, and about 1.6 centimetres long, again corresponding roughly to the shape of the outer surface. Contacts between these three zones are diffuse. At the junction between the outer dark grey and inner light grey zone are small (less than 0.5 mm) yellowish-green grains with metallic lustre. These are likely pyrite (FeS2, iron sulphide).

At higher magnifications, details of the structure in the grey zones are visible, particularly if the surface of the specimen is wetted. There is no apparent change in grainsize between the zones. All variation in colour appears to be due to changes in the matrix (material too fine-grained to resolve). The grains in the zones seem to be predominantly grey-coloured quartz, averaging about 100 microns (0.1 mm) in size. Also present are dark-grey, angular lithic (rock) fragments and opaque, black, grains with vitreous lustre which appear to be small coal fragments. Some of these approach 200-400 microns in size, but they are relatively uncommon in comparison to the smaller quartz grains, at least at low magnification and in reflected light.

The coloured zones appear to be due to alteration of the finer-grained matrix between the quartz and other visible grains. The outer zone is an alteration rind probably produced by the weathering of siderite (FeCO3) to iron oxides and hydroxides. The inner zone represents another alteration front, probably related to hydration of micas or other mineralogical changes.

That these zones represent alteration rinds, and not primary structure, is consistent with the observation of indistinct, parallel, laminar concentrations of quartz grains less than a millimetre thick, oriented parallel to the long axis of the elliptical cross section, and crossing the colouration zones with no change in orientation. These appear to be sedimentary bedding laminations.

Thin sections, transmitted light

The first image (Fig.4) was photographed with a macro lens attached to a camera and a light box to provide illumination. The remainder were photographed with a Leitz polarizing light microscope (i.e. a petrologic microscope).

Fig. 4 -- thin section (125Kbytes) approximately corresponding to the sawed, reflected-light surface in Fig.3 (within a few millimetres). The image is mirrored horizontally because the thin section is being viewed from the opposite side to that in Fig.3. Magnification: approximately 5.4x.

Interpretation:

In thin section, the laminations observed in reflected light become much more obvious, and are visible as horizontal zones of lighter colour, representing concentrations of quartz grains (see below). The thin section also confirms that these laminations are not affected by the alteration of the non-quartz mineralogy of the specimen. The dark brown "blob" to the upper left of centre is some iron oxide weathering that has penetrated from the broken surface at the end of the specimen. This is clear from the expansion of the "rust blob" on the slab remaining from the preparation of the thin section.

Fig. 5 -- thin section (322Kbytes) at low magnification (photographed at 20x), showing finer structure located to the lower left of centre in Fig. 4. Plane polarized light. In plane polarized light, the specimen looks much like it would in normal light, with the exception that pleiochroism (change in colour with orientation) may be observed in some minerals as the specimen is rotated.

Interpretation:

At this scale, the horizontal laminations are resolvable as concentrations of quartz grains. Quartz grains make up a significant proportion of the mineralogy, but other mineral grains and rock fragments are also present.

Fig. 6 -- thin section (183Kbytes) at moderate magnification (photographed at 50.4x), showing finer structure. Plane polarized light.

Interpretation:

The completely transparent (i.e. white) grains are quartz. Note their angularity. The pale green, relatively transparent grains appear to be the mica chlorite (e.g., the large, elongate elliptical grain to the lower right of centre). Also present are small, brownish, high relief grains of siderite (difficult to see at this scale), and a substantial amount of what is likely finer grained quartz, micas, and/or clays making up a dark grey-brown "matrix" between the larger mineral grains.

Fig. 7 -- thin section (334Kbytes) at moderate magnification (photographed at 50.4x). Plane polarized light (PPL).

Fig. 8 -- thin section (334Kbytes) at moderate magnification (photographed at 50.4x). Same location as Fig.7. Crossed nichols (XN).

Interpretation:

Clear (in PPL) and grey (in XN), angular to subangular quartz grains are clearly visible. Also visible are chlorite micas (e.g., the foliated grain to the upper right), which have a slightly greenish colour in PPL, higher order colours (e.g., a full spectrum) in XN, and low relief. Yellowish-brown coloured grains infilling space between other grains and with high order colours are some type of carbonate, probably siderite (but maybe also some calcite or dolomite in small amounts). Also present are opaque (black) minerals, probably pyrite. Between the grains is a fine-grained matrix that can not be resolved at this magnification. Examination of polished thin sections under the SEM could resolve exactly what this material is as well as give elemental composition information.

Fig. 9 -- thin section (124Kbytes) at high magnification. Similar location to Fig.6, but higher magnification (photographed at 128x). Plane polarized light.

Comparison

Comparison of specimen TH96-001 with dinosaur bone microstructure. All illustrations are thin sections in plane polarized light. Each pair of images is at the same magnification.

TH96-001 dinosaur bone low magnification (photographed at 20x).

TH96-001 dinosaur bone moderate magnification (photographed at 50.4x).

Interpretation:

Not much to say here. The difference is stark. There is no indication of bone microstructure in specimen TH96-001. It is a well-cemented (probably by siderite), fine-grained, lithic-fragment-rich sandstone with traces of coal and pyrite. At most, the shape of this specimen could represent some sort of fossil structure, but the structure is completely inconsistent with fossil bone.

For example, the shape could represent:

a passively sediment-infilled burrow (e.g., of Thalasinoides sp. type)
the sandstone-infilling (cast) of a Carboniferous plant root or trunk; or
some sort of rhizolith (cementation of sandstone around a root)

In the absence of sedimentary context and branching at larger scale, it is impossible to definitively determine whether this structure is a burrow (1) or rhizolith (3). There is certainly no specific evidence to support those interpretations other than gross shape, and most rhizoliths are vertical (versus the laminations in this specimen, parallel to the axis of the cylinder, indicating it was horizontal), and they usually have less cemented cores (i.e. tubular). Most plant root and trunk casts from the Carboniferous (2) have some sort of rootlet or leaf scarring on their outer surface in a characteristic spiral pattern (See Stewart and Rothwell, 1993). These are absent.

The most likely explanation (4) is that this specimen does not represent a fossil, and is only a sedimentary structure -- a siderite-cemented concretion. The shape is equally consistent with this explanation as with any fossil possibility.

Although SEM examination could resolve the petrology of this specimen further, it seems pointless given the obvious inconsistency between its structure and bone at even low magnifications.

Back to the evaluation of Carboniferous bone

Andrew MacRae macrae@geo.ucalgary.ca