Pumiliornis means as much as “dwarf bird”, and with that, all has been said.
No, not so fast ….
The genus/species was described in 1999 and is, to my knowledge, so far known from three skeletal finds, of which one even contains the remains of its last meal, namely pollen.
All in all, Pumiliornis tessellatus resembled today’s sunbirds (Nectariniidae) or the sunbird-asitys (Eurylaimidae) in being very small and having an elongated beak. Its beak, however, was quite unlike those of the members of the beforementioned two families, it resembled the beak of a plover (Charadriidae), especially its narial opening (nose hole), which was rather slit-like and not round.
Pumiliornis was apparently a flower-visitor that fed on nectar and pollen (as is known from the content of the gut of one specimen), however, it may not have been specialized to that diet and may also have taken insects and other small invertebrates.
The bird was small, very small, in my reconstruction it reaches a length of only 7,5 cm, this size, however, is of course depending on the length of its tail feathers, which unfortunately are not preserved in any of the known specimens. I’ve reconstructed the bird with a rather short tail, which may some day turn out to be completely wrong, who knows.
The feet corresponded to the typical scheme of recent passerine birds, i.e. they have three toes pointed forward and one towards the back. However, the feet appear to have been facultative or semi-zygodactyl, which in turn means, in simple terms, the first toe usually pointed forwards, but could be held backward when needed.
The genus/species cannot be assigned to any living bird family, not even an order, but is now known to belong to the extended Passeriformes-orbit, which in addition to the passerine birds also includes the falcons (Falconiformes) and the parrots (Psittaciformes). In fact it is now known to have been a member of the extinct family Psittacopedidae, that apparently also contains other unusual genera like Morsoravis, Psittacopes and the recently described, very interesting Eofringillirostrum and probably others too.
Finally, it should be mentioned that this bird was not a dwarf spoonbill, as claimed by a certain person. 😉
 Gerald Mayr: Pumiliornis tessellatus n. gen. n. sp., a new enigmatic bird from the Middle Eocene of Grube Messel (Hessen, Germany). Courier Forschungsinstitut Senckenberg. 216: 75-83. 1999
 Daniel T. Ksepka; Lance Grande; Gerald Mayr: Oldest finch-beaked birds reveal parallel ecological radiations in the earliest evolution of Passerines. Current Biology 29: 1-7. 2019