  We carry out three dimensional smoothed particle hydrodynamics simulations to
study the role of gravitational and drag forces on the concentration of large
dust grains (St > 1) in the spiral arms of gravitationally unstable
protoplanetary discs, and the resulting implications for planet formation. We
find that both drag and gravity play an important role in the evolution of
large dust grains. If we include both, grains that would otherwise be partially
decoupled will become well coupled and trace the spirals. For the dust grains
most influenced by drag (with Stokes numbers near unity), the dust disc quickly
becomes gravitationally unstable and rapidly forms clumps with masses between
0.15 - 6 Earth masses. A large fraction of clumps are below the threshold where
runaway gas accretion can occur. However, if dust self-gravity is neglected,
the dust is unable to form clumps, despite still becoming trapped in the gas
spirals. When large dust grains are unable to feel either gas gravity or drag,
the dust is unable to trace the gas spirals. Hence, full physics is needed to
properly simulate dust in gravitationally unstable discs.
  Dust trapping of large grains in spiral arms of discs stable to gas
fragmentation could explain planet formation in very young discs by a
population of planetesimals formed due to the combined roles of drag and
gravity in the earliest stages of a disc's evolution. Furthermore, it
highlights that gravitationally unstable discs are not just important for
forming gas giants quickly, it can also rapidly form Earth mass bodies.
