  Recent observations of the low-mass rotation distributions of the Pleiades
and Praesepe clusters have revealed a ubiquitous correlation between mass and
rotation, such that late M dwarfs rotate an order-of-magnitude faster than
early M dwarfs. In this paper, we demonstrate that this mass-rotation
correlation is present in the 10 Myr Upper Scorpius association, as revealed by
new ${\it K2}$ rotation measurements. Using rotational evolution models we show
that the low-mass ($0.1-0.6 M_{\odot}$) rotation distribution of the 125 Myr
Pleiades cluster can only be produced if it hosted an equally strong
mass-rotation correlation at 10 Myr. This suggests that physical processes
important in the early pre-main sequence (star formation, accretion,
disk-locking) are primarily responsible for the M dwarf rotation morphology,
and not quirks of later angular momentum evolution. Such early mass trends must
be taken into account when constructing initial conditions for future studies
of stellar rotation. Finally, we show that the average M star loses $\sim
25-40$% of its angular momentum between 10 and 125 Myr, a figure accurately and
generically predicted by modern solar-calibrated wind models. Their success
rules out a lossless pre-main sequence, and validates the extrapolation of
magnetic wind laws designed for solar-type stars to the low-mass regime at
early times.

Solar and Stellar Astrophysics