Left panel: distribution of stellar masses of spec-z members of PCl J1001+0220 (red histogram) and coeval field galaxies (blue histogram) as measured by GOSSIP+. The projected radial cut used to define membership is indicated at the top left of the panel. Right: distributions of the formation ages of proto-cluster spec-z members and coeval field galaxies, where age_f is defined as the time elapsed since the galaxy began forming stars (see Thomas et al. 2017b). Colors are identical to the left panel. The age of Universe at the redshift of the proto-cluster (z ~ 4.57) using our adopted cosmology is denoted by the dashed line. Two coeval field galaxies with log(age_f yr⁻¹) ~ 7.7 are not shown for clarity. Average effective 1σ lower and upper uncertainties for age estimates of the galaxies in each sample are shown in the top left. There is an obvious excess of more (stellar) massive and older galaxies within the bounds of the proto-cluster; both sets of distributions are inconsistent with being drawn from the same underlying population at a >2.5σ level and have average quantities which are inconsistent at an even larger significance. Additionally, the majority of proto-cluster members (5/9) appear to have formed within 500 Myr of the Big Bang as compared to only ~10% of coeval field galaxies.