Extreme body elongation in fishes is a major evolutionary transformation that extends the boundaries of morphological diversity and alters aspects of function, behavior, and ecology. Prior studies have identified features of the cranial and axial skeleton that characterize elongate fishes, but a lack of detailed reconstructions of anatomical evolution has limited inferences about factors that underlie major shifts in body shape. In this study, we fit multi-peak adaptive (Ornstein-Uhlenbeck) evolutionary models to species body shape and anatomical dimensions in Pelagiaria, a radiation of open-ocean fishes whose species span a continuum from deep-bodied to highly elongate. We inferred an ancestral fusiform adaptive peak that is retained by several major pelagiarian lineages (e.g., Scombridae) and found robust support for multiple transitions to deep-bodied (in the families Stromateidae, Bramidae, and Caristiidae) and elongate-bodied optima (within Trichiuroidei), including two instances of sequential shifts toward increasingly elongate optima that followed distinct paths of anatomical evolution. Within Trichiuridae, initial increases in head length and vertebral number were followed by changes in head and vertebral shape. Within an elongate-bodied subclade of taxa traditionally identified as ‘gempylids’, shifts in head and vertebral shape as well as number of precaudal vertebrae preceded an increase in number of caudal vertebrae. Altogether, this mosaic of anatomical peak shifts suggests that body shape transformations were associated with differing selective demands and developmental changes.

Note: Absalomichthys_velifer measurements are from an image in David LR. 1943. Miocene fishes of southern California. Geological Society of America Special Papers 43: 1-186.