  The evolution of present-day fossil galaxy groups is studied in the
Millennium Simulation. Using the corresponding Millennium gas simulation and
semi-analytic galaxy catalogues, we select fossil groups at redshift zero
according to the conventional observational criteria, and trace the haloes
corresponding to these groups backwards in time, extracting the associated dark
matter, gas and galaxy properties. The space density of the fossils from this
study is remarkably close to the observed estimates and various possibilities
for the remaining discrepancy are discussed. The fraction of X-ray bright
systems which are fossils appears to be in reasonable agreement with
observation, and the simulations predict that fossil systems will be found in
significant numbers (3-4% of the population) even in quite rich clusters. We
find that fossils assemble a higher fraction of their mass at high redshift,
compared to non-fossil groups, with the ratio of the currently assembled halo
mass to final mass, at any epoch, being about 10 to 20% higher for fossils.
This supports the paradigm whereby fossils represent undisturbed, early-forming
systems in which large galaxies have merged to form a single dominant
elliptical.
