While most of these hypotheses predict that gigantism should have evolved relatively early in the clade's history, as in terrestrial mammals, the mysticete fossil record suggests a much later origin of exceptionally large size but see. increased extinction risk) associated with large size. Macroevolutionary analyses based on phylogenies of extant cetaceans have suggested a number of possible explanations for the evolution of mysticete gigantism, including diet-related niche partitioning among the earliest representatives of crown cetacean clades, rapid rates of body size evolution along the mysticete stem or as one expected outcome of trade-offs between the short-term fitness benefits of size increases versus long-term costs (i.e. However, the ecological mechanisms and evolutionary processes that promote and maintain gigantism remain poorly understood in general, and we lack a comprehensive understanding of how and when mysticetes, in particular, attained such large sizes. That such large animals should feed on such small prey is not without precedent the fossil record demonstrates that large-bodied suspension feeders have arisen on several occasions in diverse clades. All living baleen whales (Mysticeti), including the blue whale, are obligate suspension feeders and they possess a complex suite of adaptations that enhance the energetic efficiency of foraging on small-bodied, low trophic-level prey. Vertebrates have evolved to gigantic sizes repeatedly over the past 250 Myr, reaching their extreme in today's blue whale, Balaenoptera musculus, which is the largest animal to have ever lived. High prey densities resulting from wind-driven upwelling, rather than abundant resources alone, are the primary determinant of efficient foraging in extant mysticetes and Late Pliocene changes in ocean dynamics may have provided an ecological pathway to gigantism in multiple independent lineages. This transition, from Brownian motion-like dynamics to a trended random walk towards larger size, is temporally linked to the onset of seasonally intensified upwelling along coastal ecosystems. By fitting phylogenetic macroevolutionary models to a dataset consisting of living and extinct species, we show that mysticetes underwent a clade-wide shift in their mode of body size evolution during the Plio-Pleistocene. Hypotheses for the evolution of exceptionally large size in mysticetes range from niche partitioning to predator avoidance, but there has been no quantitative examination of body size evolutionary dynamics in this clade and it remains unclear when, why or how gigantism evolved. Vertebrates have evolved to gigantic sizes repeatedly over the past 250 Myr, reaching their extreme in today's baleen whales (Mysticeti).
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