Determinism and contingency in the evolution of giant filter-feeding vertebrates.

Giant suspension feeders have evolved repeatedly across vertebrates, but the relative roles of deterministic factors and phylogenetic contingency remain unclear. To test this, we integrate new selachian data with published pachycormiform and cetacean datasets to reconstruct phylomorphospaces from standardized cranial, dental, and jaw metrics (gape, orbit size, mechanical advantage, mandibular aspect ratio), evaluate convergence using quantitative metrics, and fit alternative multivariate evolutionary models. Independent lineages evolve from distinct macrophagous ancestors but follow roughly parallel trajectories into a shared morphospace region linked to efficient capture of small planktonic prey. Overlap is limited: Pachycormiforms and filter-feeding selachians occupy adjoining areas, while mysticetes extend to a distinct endpoint along the same functional continuum. Convergence is strongest across clades, particularly between pachycormiforms and selachians, and comparatively weak within clades. Evolutionary modeling favors multiregime Ornstein-Uhlenbeck processes over Brownian or single-regime alternatives, identifying either three clade-specific optima or a shared optimum for pachycormiforms and selachians distinct from cetaceans. These results reveal repeated convergence on analogous functional designs stabilized around lineage-specific adaptive peaks. Giant filter-feeders thus illustrate how selective pressures can guide evolutionary direction, while historical constraints shape trajectories-showing determinism and contingency as complementary forces.