 Solution

Skeletal anatomy can reconstruct likely resource use and test hypotheses about competition, but it cannot by itself measure the strength of an interaction between fossil populations. Resource partitioning among living sympatric primate species provides a useful source of comparative predictions. It does not establish that every difference among coexisting fossil species was caused by competition: differences may predate coexistence, reflect other selective pressures, or merely determine which species can occur together.

The first requirement is to establish the relevant community. fossils from one formation may have accumulated over many thousands of years or represent animals transported from different habitats. Stratigraphy, dating, sedimentology and taphonomy must therefore establish plausible coexistence at an ecologically relevant scale. Species boundaries, skeletal associations, age, sex and the number of individuals also need attention. A mixed collection of several time-separated populations cannot be treated as one living primate group, and an isolated limb should not be confidently assigned to a hominin species solely because its teeth occur nearby.

For diet, I would measure tooth size and proportions, crown shape, shearing edges, enamel thickness, jaw geometry and muscle attachment regions. Functional morphology connects these observations to potential food processing: slicing leaves, crushing relatively hard items and resisting repeated chewing loads impose different demands. Body size affects both mechanics and energetic requirements, so comparisons need allometry rather than assuming that a larger tooth necessarily reflects a different food category. Phylogenetic relationships also matter because inherited resemblance can produce apparent ecological overlap independently of recent competitive interactions.

These measurements estimate capability better than habitual consumption. Paranthropus boisei has a powerful chewing apparatus and very large postcanine teeth, yet a dental-microwear study found comparatively low surface complexity and anisotropy in the seven specimens preserving suitable surfaces. ^[ref] This contradicted a straightforward inference of habitual hard-object feeding from robust anatomy alone. It is compatible with the importance of mechanically demanding fallback foods, or other uses of the apparatus, without uniquely demonstrating either explanation. A morphology selected during rare food shortages need not correspond to the last meals recorded by dental microwear.

Enamel chemistry provides a partially independent test. Carbon-isotope measurements indicated substantial use of C4-based resources by Paranthropus boisei. ^[ref] This helps distinguish it from a consumer relying predominantly on C3-derived resources, but does not specify the plant part, exact food species or necessarily every intermediate step through a food web. Grasses and sedges can share the broad photosynthetic signature. Isotopic overlap therefore need not mean identical diets, just as different wear patterns need not imply complete separation. The strength of the reconstruction comes from combining evidence that integrates different biological timescales and different aspects of resource use.

For locomotion, I would examine joint shape, limb proportions, phalangeal curvature, grip-related structures, the shoulder, pelvis, vertebral column and articular surfaces. These can constrain leaping, climbing, suspension, quadrupedal walking or habitual bipedalism and hence access to canopy levels, support diameters and terrestrial resources. However, a locomotor category is not a diet. Animals with similar locomotor competence may choose different foods, while animals with different locomotion may compete for the same ground-level resource. Wear, preservation and reconstruction uncertainty need to be propagated rather than assigning every specimen to a single categorical lifestyle.

Hominins make this caution especially important. A bipedal lower limb need not imply complete abandonment of climbing. Comparative analysis of the juvenile scapula of Australopithecus afarensis supported retained climbing-related features and showed why ontogeny matters in comparison with living apes. ^[ref] This informs possible resource access, but does not quantify time spent climbing or competitive effects on another species. A fossil community containing an australopith, a species of Homo and other primates could consequently share some resources while partitioning others. The anatomical mosaic should be retained in the model rather than compressed into a single “terrestrial” score.

An explicit model clarifies what is missing. In one conventional competition model:${\alpha }_{ij}$ is an ecological competition coefficient, defined relative to the chosen density units and model assumptions. It represents the effect of another population on population $i$, not simply their morphological resemblance. Estimating it requires information about density, resource limitation and demographic responses that skeletal shape rarely provides. The same niche overlap could accompany negligible competition when food is abundant and severe competition during a bottleneck. Conversely, strong present-day partitioning could be a legacy of past competition rather than evidence that competition was always weak.

I would therefore construct quantitative, uncertainty-aware estimates of dietary and locomotor overlap, calibrated against living primates with measured behaviour. The calibration should include species occupying similar environments but differing in abundance or competitive context, and test predictions on species excluded from model fitting. A morphospace can summarize measurements, but distance in it is not automatically ecological distance. Closely related species, correlated skeletal measurements and multiple teeth from one individual cannot be counted as independent demonstrations.

To test competition more directly, compare putatively interacting species across intervals or localities with and without the competitor, while accounting for habitat, climate, ancestry and body size. Divergence specifically associated with overlap would support ecological character displacement more strongly than a difference observed at one site. Comparable shifts in abundance, resource availability or population performance would strengthen the inference, although fossil abundance itself is affected by preservation and collection. Several independent lines of evidence are needed to distinguish competition from environmental filtering or an inherited ecological difference.