  Observational galaxy cluster studies at z>1.5 probe the formation of the
first massive M>10^14 Msun dark matter halos, the early thermal history of the
hot ICM, and the emergence of the red-sequence population of quenched
early-type galaxies. We present first results for the newly discovered X-ray
luminous galaxy cluster XMMU J1007.4+1237 at z=1.555, detected and confirmed by
the XMM-Newton Distant Cluster Project (XDCP) survey. We selected the system as
a serendipitous weak extended X-ray source in XMM-Newton archival data and
followed it up with two-band near-infrared imaging and deep optical
spectroscopy. We can establish XMMU J1007.4+1237 as a spectroscopically
confirmed, massive, bona fide galaxy cluster with a bolometric X-ray luminosity
of Lx=(2.1+-0.4)\times 10^44 erg/s, a red galaxy population centered on the
X-ray emission, and a central radio-loud brightest cluster galaxy. However, we
see evidence for the first time that the massive end of the galaxy population
and the cluster red-sequence are not yet fully in place. In particular, we find
ongoing starburst activity for the third ranked galaxy close to the center and
another slightly fainter object. At a lookback time of 9.4Gyr, the cluster
galaxy population appears to be caught in an important evolutionary phase,
prior to full star-formation quenching and mass assembly in the core region.
X-ray selection techniques are an efficient means of identifying and probing
the most distant clusters without any prior assumptions about their galaxy
content.
