The search for ancient life often begins with an unexpected find in the field, a piece of stone that feels strangely wrong. Paleontology relies on recognizing these fragments, which represent the preserved remains of organisms from deep time. Identifying a fossilized bone requires careful observation, as the original biological material has been entirely replaced by geological substances. This process means that a true fossil is fundamentally different from a modern bone, possessing distinct physical and visual characteristics that guide initial field identification.
The Transformation of Bone to Stone
A bone becomes a fossil through permineralization or replacement, typically beginning when remains are rapidly buried in sediment. Water saturated with dissolved minerals, such as silica, calcite, or iron compounds, flows through the microscopic pores within the bone structure. These minerals precipitate and crystallize, filling all internal voids. This process replaces the original organic components like collagen and residual fats with inorganic, lithic material. The final product retains the precise external shape and internal microscopic structure of the original bone but has been chemically transformed into stone.
Initial Visual Characteristics
The most immediate indicators of fossilization are visible in the specimen’s color and surface texture. Fossilized bones rarely retain the white or beige hue of modern bone. Instead, they display darker colors like black, gray, deep brown, or reddish-yellow, depending on the minerals present. This deep color results from mineral impregnation. The surface texture shifts dramatically from the smooth or slightly waxy feel of recent bone to a rough, cold, and distinctly stony texture.
When examining a broken end, look for preserved internal architecture, which confirms the object’s biological origin. This includes the distinct, honeycomb-like pattern of trabecular bone or the boundary of the former marrow cavity. Although the object is now stone, the original anatomical shape remains preserved, often showing recognizable features such as joint surfaces or the specific curvature of a limb element.
Testing Physical Properties
The mineral replacement that occurs during fossilization dramatically alters the physical properties of the bone, making density a reliable field test. A fully mineralized bone will feel significantly heavier than a modern bone of the same size because rock-forming minerals are denser than the original porous biological material. This increased weight is a strong indicator that the object is a fossil, as it feels like a solid chunk of rock.
Another practical test involves assessing hardness by attempting a gentle scratch with a fingernail or a copper coin. True fossils are much harder than fresh bone due to their mineral composition and will resist scratching more than recently deceased remains.
A simple test of porosity is the adhesion test, often called the “tongue test.” Modern bone remains porous and will slightly absorb moisture, causing it to adhere faintly to the tongue. A fully mineralized fossil, having had all its pores filled with dense minerals, is non-porous and will not exhibit this stickiness.
Ruling Out Modern Bone and Geological Debris
Field identification requires comparing the suspected fossil against the two most common sources of misidentification: modern bone and geological debris. Modern bone, even if old, often retains a faint organic smell and might feel slightly waxy or greasy due to residual fats and collagen. These recent remains will also fail the density test, feeling light compared to the heft of a mineralized fossil.
Geological formations, particularly concretions and nodules, frequently mimic bone fragments. Concretions are masses of mineral matter that accumulate around a central nucleus, often displaying concentric layers or a crystalline internal structure. The absence of specific biological organization, such as Haversian canals or cancellous bone texture, differentiates these geological mimics from actual fossils. Simple mineral staining can darken a recent bone without achieving full mineralization; a stained bone will still be lightweight and fail the adhesion test, distinguishing it from a true fossil.
What to Do After Identification
Once a suspected fossil has been identified using visual and physical tests, the immediate next step is careful documentation. Use a GPS device to record the precise coordinates, and take multiple photographs of the specimen in situ before removal. Ethical practice mandates respecting land ownership; unauthorized collection, particularly on federal or state lands, should be avoided. The most responsible action is to contact a local university paleontology department or a natural history museum. Providing them with the detailed location and photographic evidence allows professionals to verify the find and integrate the specimen into scientific study.