**Background:** Environmental controls of species diversity are a central focus in evolutionary biology. Sharks are globally distributed marine predators occupying mainly higher trophic levels with varied dietary preferences. Recent phylogenetic studies revealed uneven diversification across habitats from reefs to deep water. This study tested whether morphological diversification (disparity) in the feeding system (mandibles) follows these habitat-driven patterns and examined links to morphological specialisation.
**Methods:** The authors conducted a 3D geometric morphometric analysis using computed tomography models of 145 specimens representing 90 extant shark species across 9 orders. They defined 6 landmarks, 51 curve sliding landmarks, and 53 surface landmarks to describe jaw shape. A generalized Procrustes alignment was performed, followed by principal components analysis (PCA) and phylogenetically aligned PCA. Phylogenetic comparative methods included phylogenetic MANOVA/MANCOVA, evolutionary rate estimation, morphological disparity analysis (Procrustes variance with 100 bootstraps), disparity-through-time analysis, ancestral state reconstruction, and Bayesian variable rates models in BayesTraits (rjMCMC, 5 independent chains, 200,000,000 iterations). Trophic data were gathered from literature (primarily Cortes 1999), with diet categories clustered using Bray-Curtis index and UPGMA into 8 feeding guilds. Trophic levels were categorized as low-level predators (TR=3.2-3.8), mesopredators (TR=3.81-4.2), and top predators (TR>4.2). Habitats followed Dulvy et al. categories: reef (n=28), shelf (n=24), pelagic (n=17), and deep-sea (n=21).
**Key Results:** PCA revealed PC1 explained 40.9% of total variation (correlated with jaw-closing mechanical advantage, R=0.89, p<0.001) and PC2 explained 15.75% (correlated with symphyseal depth, R=0.41, p<0.001). Phylogenetic signal was significant (Kmult=0.5318, p=0.001). Phylogenetic MANOVA identified habitat as the only significant predictor of shape (p<0.001), but when accounting for body size (log centroid size), significant differences emerged among orders, trophic levels, diet composition, and habitat occupation. Evolutionary rates were highest in orectolobiforms (σ=1.439775e-06), followed by squaliforms (σ=1.408004e-06) and hexanchiforms (σ=1.136257e-06), with lamniforms and carcharhiniforms showing 7.287 and 12.989 fold lower rates respectively. Deep-sea species evolved fastest (σ=9.40033e-07, 1.419 fold vs reef, 5.251 fold vs pelagic, 7.692 fold vs shelf), followed by reef-associated species (σ=6.624241e-07). Morphological disparity (Procrustes variance) was highest in squaliforms (PV=0.036, p<0.001), deep-sea species (PV=0.042), low-level predators (PV=0.044), and cephalopod consumers (PV=0.04). Disparity-through-time analysis showed early high disparity declining through time, with Early Burst as the best-supported evolutionary model (GIC=-770590.6). Ancestral state reconstruction suggested piscivory as the ancestral diet state, with independent evolution towards large predators in carcharhiniforms and lamniforms. Deep-sea habitat was inferred as ancestral for squaliforms, while reef was ancestral for orectolobiforms.
**Clinical Implications:** This study provides fundamental insights into evolutionary biology and marine ecology rather than direct clinical applications. However, understanding the drivers of morphological diversification in sharks has implications for conservation biology, particularly for identifying deep-sea and reef habitats as critical hotspots of evolutionary innovation. The finding that deep-water sharks display highly divergent jaw morphologies and elevated evolutionary rates suggests these environments may be particularly vulnerable to anthropogenic disturbances, as specialized morphologies may limit adaptive capacity. The decoupling of diversity and disparity between deep-sea (coupled) and reef (decoupled) habitats provides a framework for predicting how different marine ecosystems may respond to environmental change. These results also validate the use of jaw morphology as a proxy for ecological function in paleobiological studies, which has implications for understanding ancient marine food webs and ecosystem structure.