  We present data for 16 galaxies in the overdensity JKCS 041 at $z \simeq
1.80$ as part of the K-band Multi-Object Spectrograph (KMOS) Cluster Survey
(KCS). With 20-hour integrations, we have obtained deep absorption-line spectra
from which we derived velocity dispersions for seven quiescent galaxies. We
combined photometric parameters derived from Hubble Space Telescope images with
the dispersions to construct a fundamental plane (FP) for quiescent galaxies in
JKCS 041. From the zero-point evolution of the FP, we derived a formation
redshift for the galaxies of $z_{form} = 3.0\pm0.3$, corresponding to a mean
age of $1.4\pm0.2$ Gyrs. We tested the effect of structural and velocity
dispersion evolution on our FP zero point and found a negligible contribution
when using dynamical mass-normalized parameters ($\sim 3\%$), but a significant
contribution from stellar-mass-normalized parameters ($\sim 42 \%$). From the
relative velocities of the galaxies, we probed the three-dimensional structure
of these 16 confirmed members of JKCS 041, and found that a group of galaxies
in the south west of the overdensity had systematically higher velocities. We
derived ages for the galaxies in the different groups from the FP. We found the
east-extending group had typically older galaxies ($2.1\substack{+0.3\\-0.2}$
Gyrs), than those in the south-west group ($0.3\pm0.2$ Gyrs). Although based on
small numbers, the overdensity dynamics, morphology, and age results could
indicate that JKCS 041 is in formation and may comprise two merging groups of
galaxies. The result could link large-scale structure to ages of galaxies for
the first time at this redshift.

Astrophysics of Galaxies