  We present a fundamental plane (FP) analysis of 141 early-type galaxies in
the Shapley supercluster at z=0.049 based on spectroscopy from the AAOmega
spectrograph at the AAT and photometry from the WFI on the ESO/MPI 2.2m
telescope. The key feature of the survey is its coverage of low-mass galaxies
down to sigma_0~50km/s. We obtain a best-fitting FP relation log r_e=1.06 log
sigma_0 - 0.82 log < I >_e + cst in the R band. The shallow exponent of sigma_0
is a result of the extension of our sample to low velocity dispersions. We
investigate the origin of the intrinsic FP scatter, using estimates of age,
metallicity and alpha/Fe. We find that the FP residuals anti-correlate
(>3sigma) with the mean stellar age in agreement with previous work. However, a
stronger (>4sigma) correlation with alpha/Fe is also found. These correlations
indicate that galaxies with effective radii smaller than those predicted by the
FP have stellar populations systematically older and with alpha over-abundances
larger than average, for their sigma_0. Including alpha/Fe as a fourth
parameter in the FP, the total scatter decreases from 0.088 dex to 0.075 dex
and the estimated intrinsic scatter decreases from 0.068 dex to 0.049 dex.
Thus, variations in alpha/Fe account for >> 30% of the total variance around
the FP, and >> 50% of the estimated intrinsic variance. This result indicates
that the distribution of galaxies around the FP are tightly related to the
enrichment, and hence to the timescale of star-formation. Our results appear to
be consistent with the merger hypothesis for the formation of ellipticals which
predicts that a significant fraction of the scatter is due to variations in the
importance of dissipation in forming merger remnants of a given mass.
