Bone and Skeletal Element Identification

Gross morphological bone identification follows the same logic as zooarchaeology: element identification first (which bone is this?), then taxon identification (which species?). Element identification is nearly always possible from a fragment that retains an articular surface, because joint shapes are strongly conserved within functional groups. A distal humerus trochlea looks like a humerus whether it is from a cat, a bear, or a human; the shape differences between them are the second question.

For tiger bone specifically, the humerus, femur, tibia, and skull are the elements most frequently encountered in trade. Tiger limb bones are distinguished from domestic cattle by the combination of felid-characteristic proportions (relatively longer, more curved shafts compared to bovid bones of similar circumference), pronounced muscle attachment ridges reflecting the locomotor demands of an ambush predator, and characteristic articular surface curvatures. The USFWS Forensics Laboratory and the Wildlife Institute of India both maintain reference skeletons and photographic atlases for these elements.

Histological examination of compact bone cross-sections has a long history in bioarchaeology and forensic anthropology for estimating age and identifying human versus non-human bone. The same methods apply in wildlife forensics. A transverse section from the mid-shaft of a long bone, ground to 80-120 micrometres and mounted, is examined under transmitted and polarised light at 40-200x. The characters of interest are osteon diameter (the outer boundary of the secondary osteon), Haversian canal diameter, osteon density (number of osteons per mm2), and the proportion of primary lamellar bone versus secondary remodelled bone.

Carnivores (Felidae, Canidae) generally show larger, less dense osteons than ungulates (Bovidae, Suidae) at comparable body sizes. Tiger cortical bone shows osteon diameters averaging 180-220 micrometres with Haversian canal diameters of 50-70 micrometres in published reference data. Domestic cattle bone shows higher osteon density and slightly smaller average osteon diameter. These ranges overlap with other large carnivores and some bovids, so histology is interpreted alongside gross morphology and, when necessary, DNA.

Osteometric discriminant analysis uses standardised measurements recorded from specific anatomical landmarks on a bone element and compares them against reference datasets built from museum skeletal collections. For a given element (say, the distal humerus), a set of five to ten measurements (breadth at epicondyles, trochlea width, mediolateral condyle depth) is fed into a linear discriminant function trained on known-species specimens. The output is a probability of membership for each candidate species group.

The Wildlife Institute of India has published discriminant functions specifically for separating tiger, leopard, snow leopard, and clouded leopard limb bones using measurements applicable to museum reference collections in South and Southeast Asia. These functions correctly classify over 90% of known specimens in published validation sets. Limits include the need for complete articular surfaces for measurement and the circularity of relying on museum collections that may themselves contain mislabelled specimens.

Avian long bones are distinguished from mammalian long bones by their thin cortex, extensive internal air spaces (pneumaticity) in the larger bones, and the overall gracile proportions consistent with weight reduction for flight. Even a fragment retaining part of a pneumatic chamber is identifiable as avian. Within birds, the size and proportions of specific elements are diagnostic to the order or family level in most cases.

The USFWS publication 'Identifying Avian Parts' (available from the Forensics Laboratory) provides photographic keys for skulls, tarsometatarsus, carpometacarpus, and sternum morphology across the major orders encountered in North American enforcement. For raptors, the tarsometatarsus and talon (ungual phalanx) morphology are the most commonly encountered trade elements in jewellery and ceremonial objects, and the guide provides element-by-element photographs at consistent scale.

Bear paws in the illegal trade are typically sold dried or frozen, with the claws attached. The five-digit plantigrade foot of bears is morphologically distinctive: the phalanges are thick and heavily built, the ungual phalanges (claw bones) are laterally compressed with a prominent flexor tendon process, and the claws are strongly curved compared to pig and domestic dog digits of comparable size.

Pig trotters (Sus scrofa) are the most frequently documented substitute for bear paw in enforcement seizures. Pig feet have two main functional digits (digits 3 and 4) and two smaller lateral dewclaws (digits 2 and 5), a different digit formula from the five-fingered bear paw. When trotters are presented with the lateral dewclaws folded or removed, the substitution requires measurement: pig ungual phalanges are narrow-based and recurved differently from bear unguals, and the phalangeal index (mediolateral width divided by length) falls in a non-overlapping range. Asiatic black bear (Ursus thibetanus) and sun bear (Helarctos malayanus) phalangeal proportions have been published for discriminant analysis.