  The change of the total mass density slope, $\gamma$, of early-type galaxies
through cosmic time is a probe of evolutionary pathways. Hydrodynamical
cosmological simulations show that at high redshifts density profiles of
early-type galaxies were on average steep ($\gamma \sim -3$). As redshift
approaches zero, gas-poor mergers progressively cause the total mass density
slope to approach the `isothermal' slope of $\gamma \sim -2$. Simulations
therefore predict steep density slopes at high redshifts, with little to no
evolution in density slopes below $z \sim 1$. Gravitational lensing results in
the same redshift range find the opposite, namely a significant trend of
shallow density slopes at high redshifts, becoming steeper as redshift
approaches zero. Gravitational lensing results indicate a different
evolutionary mechanism for early-type galaxies than dry merging, such as
continued gas accretion or off-axis mergers. At redshift zero, isothermal
solutions are obtained by both simulations and dynamical modelling. This work
applies the Jeans dynamical modelling technique to observations of galaxies at
intermediate redshifts ($0.29 < z < 0.55 $) in order to derive density slopes
to address the tension between observations and simulations. We combine
two-dimensional kinematic fields from MUSE data with Hubble Space Telescope
photometry. The density slopes of 90 early-type galaxies from the Frontier
Fields project are presented. The total sample has a median of ${\gamma = -2.11
\pm 0.03}$ (standard error), in agreement with dynamical modelling studies at
redshift zero. The lack of evolution in total density slopes in the past 4-6
Gyrs supports a dry merging model for early-type galaxy evolution.
