We have crown-group anseriforms in the Late Cretaceous,
why not grebes and loons?
Because grebes and loons are nested rather high within Neoaves, whatever
their precise relationships.
Anyway -- we've had the discussion about the isolated foot called
*Neogaeornis* and the heap of fragments of undescribed sizes called
*Polarornis*. http://dml.cmnh.org/2004Dec/msg00048.html