Tim Williams wrote-
"Mayr (2016) notes that the Messel ecosystem (Eocene) is remarkable for
the large and diverse number of flightless birds (_Palaeotis_,
_Gastornis_, _Strigogyps_, _Dynamopterus_). He attributes this to the
absence of large mammalian carnivores - not to the absence of
predators per se. ... But there
is also the possibility that competition with new large mammalian
herbivores (conspicuously absent from Messel) also played a part."
Isn't this likely a depositional issue, much like the Jehol fauna? Messel was continental, right? It's not like continental faunas exist without large mammalian taxa today- something's going to move in.