  FRB 20121102A is the first known fast radio burst (FRB) from which repeat
bursts were detected, and one of the best-studied FRB sources in the
literature. Here we report on the analysis of 478 bursts (333 previously
unreported) from FRB 20121102A using the 305-m Arecibo telescope - detected
during approximately 59 hours of observations between December 2015 and October
2016. The majority of bursts are from a burst storm around September 2016. This
is the earliest available sample of a large number of FRB 20121102A bursts, and
it thus provides an anchor point for long-term studies of the source's evolving
properties. We observe that the bursts separate into two groups in the
width-bandwidth-energy parameter space, which we refer to as the low-energy
bursts (LEBs) and high-energy bursts (HEBs). The LEBs are typically longer
duration and narrower bandwidth than the HEBs, reminiscent of the
spectro-temporal differences observed between the bursts of repeating and
non-repeating FRBs. We fit the cumulative burst rate-energy distribution with a
broken power-law and find that it flattens out toward higher energies. The
sample shows a diverse zoo of burst morphologies. Notably, burst emission seems
to be more common at the top than the bottom of our 1150 - 1730 MHz observing
band. We also observe that bursts from the same day appear to be more similar
to each other than to those of other days, but this observation requires
confirmation. The wait times and burst rates that we measure are consistent
with previous studies. We discuss these results, primarily in the context of
magnetar models.
