  The first measurements of energy spectra and substructure of anti-$k_{T}$
jets in hadronic $Z^0$ decays in $e^+e^-$ collisions are presented. The
archived $e^+e^-$ annihilation data at a center-of-mass energy of 91.2 GeV were
collected with the ALEPH detector at LEP in 1994. In addition to inclusive jet
and leading dijet energy spectra, various jet substructure observables are
analyzed as a function of jet energy which includes groomed and ungroomed jet
mass to jet energy ratios, groomed momentum sharing, and groomed jet radius.
The results are compared with perturbative QCD calculations and predictions
from the SHERPA, HERWIG v7.1.5, PYTHIA 6, PYTHIA 8, and PYQUEN event
generators. The jet energy spectra agree with perturbative QCD calculations
which include the treatment of logarithms of the jet radius and threshold
logarithms. None of the event generators give a fully satisfactory description
of the data.
