The carnassial is the slicing hallmark of the order Carnivora, the modified cheek tooth pair that functions like a pair of scissors. In the upper jaw it is the last premolar, in the lower jaw the first molar, and together they shear flesh and other foods during each chewing stroke. Because these teeth meet blade against blade, their wear facets record the mechanical history of feeding with particular clarity. Dental microwear analysis samples these facets, ideally ones that occlude directly with one another, and uses the resulting texture to infer whether an animal was slicing tough meat, crunching bone, or processing a more varied diet. The method has become a standard tool for reconstructing the ecology of extinct mammals, from sabertooth cats to Pleistocene hyenas.

The practical problem that motivated the new research is one of scarcity. Fossil carnivore teeth are rare, and a usable analysis typically requires many individuals to reach statistically meaningful sample sizes. To maximize the number of usable observations, researchers routinely pool upper and lower teeth, treating the opposing, homologous wear facets as interchangeable. Previous work on fossil herbivores, however, had suggested that upper and lower teeth do not always carry identical wear signals, particularly when comparing facets that are not true functional counterparts. If the same discrepancy applies to carnivores, then pooling upper and lower carnassials could quietly contaminate dietary reconstructions with a methodological artifact.

Cecilia Loddi of the University of Florence and colleagues, including Riccardo Stefani, Lorenzo Rook, and Saverio Bartolini-Lucenti, set out to test whether the slicing portions of the upper and lower carnassials in living carnivores can genuinely be analyzed without distinction. Their question was precise: do the homologous wear facets of the upper and lower carnassials produce the same ecological evidence, or are they statistically distinct? To answer it, they turned to Dental Microwear Texture Analysis, or DMTA, a high-resolution technique that treats tooth surfaces as three-dimensional landscapes rather than flat images.

DMTA works by scanning a small patch of enamel at sub-micrometer resolution, producing a detailed topographic map of the wear surface. Software then quantifies that landscape using a set of parameters derived from scale-sensitive fractal analysis, known as SSFA attributes. These descriptors capture features such as surface complexity, roughness at different scales, heterogeneity, and the anisotropy of the texture, meaning the degree to which scratches run in a consistent direction. Complexity, for example, tends to rise when hard items like bone are processed, while anisotropy reflects the directionality of shearing movements. Together, the SSFA variables provide a numerical fingerprint of diet that can be compared across individuals, species, and, crucially for this study, across upper and lower teeth.

The team sampled four species of extant carnivores, examining the homologous wear facets of the slicing portion of the upper and lower carnassials in museum specimens. Working with modern animals whose diets are known is the essential calibration step: if upper and lower facets from animals with identical feeding habits were to show different textures, the difference could only come from the teeth themselves rather than from diet. The specimens were molded and cast following established replication techniques, and the resulting surfaces were scanned and characterized in the laboratory at the University of Florence, with the authors acknowledging curatorial and technical support from several European natural history museums in Basel, Paris, Berlin, Florence, and Munich.

The statistical approach is where the study distinguishes itself. Rather than simply running a null-hypothesis test and concluding that no difference was detected, the authors employed equivalence testing, a framework more familiar in psychology and clinical research than in paleontology. A conventional significance test can only fail to find a difference, which is not the same as demonstrating that two things are alike. Equivalence tests, such as the two one-sided tests procedure implemented in the TOSTER R package, flip the logic: they ask whether any observed difference is small enough to fall within a predefined zone of practical equivalence. Failing to reject a null hypothesis of no difference is weak evidence of sameness; formally rejecting the hypothesis of a meaningful difference is strong evidence of it.

By that stricter standard, the results were clear. Similarity tests found no statistically significant differences in the SSFA attributes between upper and lower homologous carnassial facets, and, more importantly, the parameters proved statistically equivalent. In other words, the texture signatures recorded on the upper carnassial and its lower counterpart carry the same ecological message, at least in the extant carnivores examined. The upper tooth really is the lower tooth, at least as far as microwear is concerned, and the old pooling practice survives its first formal challenge in this group.

The practical implications reach well beyond the four study species. If upper and lower carnassial facets can be combined without distortion, every fossil jaw fragment bearing either tooth becomes admissible evidence, effectively doubling the pool of available data for a given fossil assemblage. For groups where sample sizes have always been the bottleneck, this is a substantial gain. Fossil carnivores are notoriously underrepresented in microwear studies compared with the richer herbivore record, and the authors highlight this data scarcity as a central obstacle to reconstructing the dietary ecology of ancient predators. A validated doubling of usable teeth could make previously marginal assemblages analytically viable.

Some caveats remain and are worth keeping in view. The equivalence demonstrated here applies to homologous, occluding facets of the slicing carnassial in extant carnivores; earlier work on herbivores suggests that non-homologous facets and other tooth positions may behave differently, so researchers should not assume the result generalizes beyond the specific pair of surfaces tested. Extending the analysis to a broader range of carnivore species, and eventually to fossil taxa themselves, would strengthen the conclusion. Even so, the study converts a long-standing convenience into a validated protocol, giving paleontologists a firmer statistical footing the next time they combine an upper carnassial with a lower one to reconstruct what an ancient predator ate.

Subject of Research: Equivalence of dental microwear texture patterns between upper and lower carnassial teeth in extant carnivores

Article Title: Up is down: testing the equivalence in dental wear patterns of the upper and lower carnassial

Article References: Loddi, C., Stefani, R., Rook, L., & Bartolini-Lucenti, S. (2026). Up is down: testing the equivalence in dental wear patterns of the upper and lower carnassial. The Science of Nature, 113(5), Article 110. https://doi.org/10.1007/s00114-026-02158-4

Image Credits: AI Generated

DOI: 10.1007/s00114-026-02158-4

Keywords: dental microwear, Dental Microwear Texture Analysis, carnassial, Carnivora, paleoecology, tooth wear, SSFA, equivalence testing, diet reconstruction, fossil carnivores, wear facets, paleontology

The researchers built what is known as a total-evidence phylogeny of feliforms, the branch of the mammalian order Carnivora that includes cats, hyenas, civets, linsangs, mongooses and their many extinct relatives. Total-evidence approaches combine morphological data from both living and fossil species with molecular data from living ones, allowing extinct taxa to be placed on the tree with their dates of occurrence rather than being dangled loosely from unresolved branches. This tip-dated framework is crucial for studying macroevolution, because it preserves the temporal dimension of evolutionary change. Fossils are not simply decoration on a molecular tree; they are data points whose anatomy and age jointly constrain inferences about how traits evolved through deep time.

With this phylogeny in hand, Barrett and Hopkins modeled the evolution of two key traits across feliform history: body mass and what they call the dental toolkit, measured as the relative blade length of the lower carnassial, the modified shearing tooth that is the signature innovation of the carnivoran jaw. By fitting alternative evolutionary models to these traits across the tree, the authors could detect where evolution followed different rules, testing the classical idea of adaptive zones first articulated by George Gaylord Simpson in the 1940s and 1950s. Adaptive zones are, in essence, distinct regions of ecological and functional opportunity, each with its own adaptive landscape that shapes the direction and tempo of trait evolution for the lineages occupying it.

The analysis identified three adaptive zones within feliforms. The first, which the authors term the ancestral cataract of carnivory, encompasses small-bodied, ecologically flexible taxa in which dental evolution appears largely stochastic, drifting without strong directional pressure. The second, the broad ecology cursor, is associated with lineages in which forelimb dexterity is restricted and prey processing relies more heavily on the head, favoring running adaptations and cranial specializations. The third, the soft-flesh specialist, is defined by intense selection for slicing-dominated dentitions in hypercarnivores, animals whose diets consist almost entirely of meat and whose teeth have been progressively simplified into blades at the expense of crushing and grinding surfaces.

Crucially, the framework does not stop at the feliform branch of the tree. Extending their synthesis across terrestrial mammalian carnivores more broadly, the authors propose a fourth zone, the versatile omnivore, representing a distinct adaptive regime of dietary and morphological flexibility at large body size. Bears are perhaps the most familiar modern occupants of this zone, and their fossil relatives, along with giant amphicyonids and other big-bodied generalists, suggest it has been repeatedly occupied throughout carnivoran history. Large-bodied omnivores retain generalized dentition despite their size, reflecting an adaptive landscape in which dietary breadth, rather than specialization, is the winning strategy.

The most consequential finding concerns where selection on teeth actually operates. Across all of these regimes, dental morphology experiences strong, consistent selection only in hypercarnivores. In the soft-flesh specialist zone, the demands of slicing meat impose a tight functional constraint, and the length of the carnassial blade becomes a reliable predictor of diet. Everywhere else, dental evolution is closer to a random walk. In taxa with broader diets, tooth shape drifts in ways that are weakly tied to what the animal actually eats, because generalized dentitions can process many food types and individual morphological changes carry little functional cost. This single insight elegantly explains why recent attempts to infer diet from dental measurements have produced frequent misclassifications: those methods work superbly for extreme specialists and poorly, sometimes badly, for everyone else.

The finding carries immediate practical implications for paleontology. Estimates of ancient diets underpin reconstructions of past food webs, predator-prey dynamics and ecosystem structure, and they inform debates about extinction drivers, competition among sympatric predators and responses to climate change. If diet predictions from teeth are reliable only for hypercarnivores, then paleobiologists can place greater confidence in dietary reconstructions for saber-toothed nimravids, dirk-toothed barbourofelids and the most committed flesh-slicers of the fossil record, while treating generalized taxa with appropriate caution. The new framework essentially supplies a filter: it tells researchers when a morphological proxy can be trusted and when it is likely to mislead.

The study also resonates with a rich body of prior work on carnivoran ecomorphology. Decades of research by Blire Van Valkenburgh and colleagues documented the iterative evolution of hypercarnivory, particularly in canids, where repeated incursions into extreme meat eating were followed by elevated extinction risk, a pattern that underscores how specialization can be both an ecological triumph and an evolutionary dead end. Work on elbow-joint morphology by Ki Andersson and Lars Werdelin illuminated the evolution of cursorial locomotion, and analyses of skull shape and biting biomechanics by Figueirido, Tseng, Slater and others mapped the functional landscapes that different feeding strategies impose. Barrett and Hopkins knit these threads together into a single macroevolutionary model in which different trait complexes, teeth, limbs and skulls, may respond to different adaptive landscapes operating simultaneously on the same animal, a form of mosaic evolution that the authors documented previously for feliform morphological disparity.

Methodologically, the study demonstrates the power of modern Bayesian phylogenetic toolkits. The total-evidence tree was inferred using tip-dated approaches in BEAST 2, integrating fossil occurrences as sampling events along branches, and the resulting maximum clade credibility tree served as the scaffold for model comparison. Trait evolution was modeled using Ornstein-Uhlenbeck and related processes implemented in packages such as Geiger and mvMORPH, allowing the authors to compare regimes of stabilizing selection, random drift and adaptive peaks across the tree. The data, including metric measurements from museum specimens spanning living feliforms and newly examined hyaenid and nimravid fossils, along with all analysis code, have been released openly through Zenodo and figshare, making the framework transparent and extensible. Follow-up work can now ask whether the same zones can be detected in caniforms, the other great carnivoran radiation, and whether marine carnivores obey parallel rules.

Ultimately, what makes this study compelling is that it reconciles two observations that have long seemed contradictory: the remarkable convergence of carnivore form and the unreliability of form as an indicator of function. Convergence is real, but it is concentrated in the zones where selection is strongest, above all the soft-flesh specialist regime where slicing teeth are non-negotiable. Outside those peaks, morphological similarity can be coincidental, and dissimilarity can be meaningless. By mapping where in the adaptive landscape teeth are locked to diet and where they roam freely, Barrett and Hopkins have given evolutionary biologists and paleontologists alike a predictive model for one of the most iconic radiations in the history of terrestrial vertebrates, and a sharper set of tools for reading the ecological lives of predators long extinct.

The concept of adaptive zones has a long intellectual pedigree stretching back to Simpson’s foundational work on tempo and mode in evolution, and it has since been applied to systems as varied as cichlid fishes with their pharyngeal jaws, phytophagous insects, and mammals that acquired the hypocone, a cusp widely regarded as a key innovation opening herbivorous niches. What distinguishes the new feliform analysis is that it treats the adaptive zone not merely as a descriptive category but as a testable statistical regime, asking whether trait evolution within each zone obeys stabilizing selection around an optimum or drifts idly. This quantitative framing connects the study to a mature comparative-methods literature on Ornstein-Uhlenbeck models, in which the strength of selection and the location of adaptive peaks can be estimated directly from trait data distributed across a phylogeny.

The energetic dimension of carnivore ecology also deserves emphasis. Body mass and diet are tightly coupled in terrestrial carnivores because meat is a patchy, energetically expensive resource, and small predators can subsist on invertebrates and mixed foods that would never sustain a large-bodied hunter. This scaling relationship helps explain why the ancestral cataract of carnivory is populated by small, flexible taxa, while the versatile omnivore zone is defined by large size combined with dietary breadth, a combination that requires generalized teeth capable of processing both flesh and plant material. The carnassial blade, meanwhile, is a structure whose functional performance degrades gracefully across many diets, which is precisely why its length carries little information except at the hypercarnivorous extreme.

There is also a conservation angle worth noting. Living feliforms occupy all three identified zones, and understanding which lineages sit under strong functional constraint may inform expectations about their vulnerability to prey depletion and habitat change, since specialists dependent on intact vertebrate communities face narrower margins than flexible generalists.

Subject of Research: Adaptive zones and evolutionary regimes in the dental and ecological evolution of feliform carnivores

Article Title: Adaptive zones of feliforms and evolutionary regimes within terrestrial mammalian carnivores

Article References: Barrett, P. Z., & Hopkins, S. S. B. (2026). Adaptive zones of feliforms and evolutionary regimes within terrestrial mammalian carnivores. Nature Ecology & Evolution. https://doi.org/10.1038/s41559-026-03176-1

Image Credits: AI Generated

DOI: 10.1038/s41559-026-03176-1

Keywords: feliforms, adaptive zones, hypercarnivory, carnassial, total-evidence phylogeny, macroevolution, ecomorphology, diet prediction, Carnivora, paleontology, mosaic evolution, Ornstein-Uhlenbeck models