  We utilize deep JWST NIRCam observations for the first direct constraints on
the Galaxy Stellar Mass Function (GSMF) at z>10. Our EPOCHS v1 sample includes
1120 galaxy candidates at 6.5<z<13.5 taken from a consistent reduction and
analysis of publicly available deep JWST NIRCam data covering the PEARLS,
CEERS, GLASS, JADES GOOD-S, NGDEEP, and SMACS0723 surveys, totalling 187
arcmin2. We investigate the impact of SED fitting methods, assumed star
formation histories (SFH), dust laws, and priors on galaxy masses and the
resultant GSMF. Whilst our fiducial GSMF agrees with the literature at z<13.5,
we find that the assumed SFH model has a large impact on the GSMF and stellar
mass density (SMD), finding a 0.75 dex increase in the SMD at z=10.5 between a
flexible non-parametric and standard parametric SFH. Overall, we find a flatter
SMD evolution at z > 9 than some studies predict, suggesting a rapid buildup of
stellar mass in the early Universe. We find no incompatibility between our
results and those of standard cosmological models, as suggested previously,
although the most massive galaxies may require a high star formation
efficiency. We find that the 'Little Red Dot' galaxies dominate the z=7 GSMF at
high-masses, necessitating a better understanding of the relative contributions
of AGN and stellar emission. We show that assuming a theoretically motivated
top-heavy IMF reduces stellar mass by 0.5 dex without affecting fit quality,
but our results remain consistent with existing cosmological models with a
standard IMF.
