Daggers, swords, scythes and sickles: Pachycormid fins as ecological predictors

Pachycormids occupy a key position within Actinopterygii, as part of the Holostei- Teleostei Transition, although their precise position in this hierarchy has been fought over for some years. Discoveries in the last three years have expanded our global knowledge of the diversity, distribution and success of this group, continuing the recent 'Pachycormid Renaissance'. However, clarity over the definitions of pachycormid taxa has been undermined by the number of type specimens destroyed during World War II, introducing a need for neotype material to be identified (e.g. Asthenocormus titanius, Hypsocormus macrodon), and comparative work has revealed how poorly constrained a number of historical genera are, particularly those of the Toarcian (Early Jurassic) Holzmaden shale fauna that were the foundation of Arthur Smith Woodward's family Pachycormidae in 1895. These historical problems with descriptions and material have undermined confidence in recent phylogenetic analyses. The characteristic unusually long pectoral fins appear to have developed in conjunction with otherwise reduced skeletal ossification to counteract buoyancy problems in a group lacking a gas bladder. A sample of over 90 specimens from 16 recognised pachycormid genera was assessed, demonstrating that the ubiquitously stated 'scythe'-like pectoral fin is not a pachycormid synapomorphy: three clear and distinct pectoral fin structural morphotypes emerged, reflecting a diversity of pachycormid lifestyles that changed throughout the Mesozoic, from agile pursuit predator to slow-cruising suspension feeder. Those morphotypes closely mirror modern fuel-saving wingtip designs from today's aerodynamicists, converging on similar solutions to these enigmatic and fascinating fish some 160 million years later.