SDSM 64281, also known as „Ornithurine C“ is mentioned in a study from 2011 that deals with the extinction of several bird clades at the end of the Cretaceous. 
This form is apparently known from at least one fragmented coracoid and comes from a bird that in life must have had a weight of about 3 kg. Unfortunatley the study fails to inform if this form is known from only the aforementioned single coracoid, and if not, if its remains were recovered only from the earliest Paleocene layers or if they were also recovered from the lates Cretaceous layers as it is the case with all other bird remains in the study.:
„One of these species, Ornithurine C, is [also or only?] known from the Paleocene and therefore represents the only Maastrichtian bird known to cross the K–Pg boundary.“
Apparently, this species is known from at least four coracoids or remains of such, and they are named as „SDSM 64281A“, „SDSM 64281B“, „UCMP 175251“, and „MOR 2918“ and most are indeed of Late Cretaceous age, but just not all of them.
According to the authors this species might be identical with a species that was named as Graculavus augustus Hope, a bird that apparently belongs to the Charadriiformes but was very much unlike any of the charadriiform birds living today, in life it may have appeared like some kind of giant stone-curlew aka. thick-knee. 
 Nicholas R. Longrich; Tim Tokaryk; Daniel J. Field: Mass extinction of birds at the Cretaceous-Paleogene (K-Pg) boundary. PNAS 108 (37) 15253-15257. 2011  Nicholas R. Longrich; Tim Tokaryk; Daniel J. Field: Mass extinction of birds at the Cretaceous-Paleogene (K-Pg) boundary. PNAS 108 (37) 15253-15257. 2011. Supplementary Information
You may know that I’m a bit obsessed with the birds of the Paleocene era, partly because we know so much of a nothing about them, especially about those from the Early Paleocene, the beginning of the „T-time“, the time immediately after the K/T extinction event.
Now, there’s now a new paper out that is somewhat a review of the birds that existed at around exactly this time, the K/T boundary … on the continent of Antarctica to be more precicely, but also beyond that time up to the Oligocene. 
I have not yet read it completely, but since nearly all bird fossils from that area are limited to single bones or sometimes partial skeletons, it does not shed so much new light on the previous records.
 Carolina Acosta Hospitaleche; Piotr Jadwiszczak; Julia A. Clarke; Marcos Cenizo: The fossil record of birds from the James Ross Basin, West Antarctica. Advances in Polar science 30(3): 250-272. 2019
MNT-11-7952 is a remarkable fossil of an enigmatic bird with an exceptional preservation; imprints of the tail feathers preserved showing a bluish gray hue, the legs and feet still showing traces of their soft tissue.
This, however, is all that’s known so far, the two slabs contain nothing but the arse, sorry, the rump, the legs and the tail feathers.
The feathers are very long and narrow, reminding on the tail feathers of recent mousebirds (Coliiformes), yet the feet appear to be anisodactyl, unlike in any known mousebird, extinct or extant.
The fossil dates to the Middle Paleocene, thus has an age of 60 to 61 Millions of years, and in my opinion, my indeed be a Coliiform bird.
I’ll try to reconstruct this as much as possible. 🙂
Here is now a little sketchy try to reconstruct that bird, including its nearly 20 cm long tail feathers, it may have reached a total length of about 34 cm, which is very well within the size range of modern mousebirds!
 Gerald Mayr; Sophie Hervet; Eric Buffetaut: On the diverse and widely ignored Paleocene avifauna of Menat (Puy-de-Dôme, France): new taxonomic records and unusual soft tissue preservation. Geological Magazine: 1-13. 2018
There is a new paper out in which several new bird remains are described, however, unfortunately without describing any species because these remains are just too fragmentary. The remains themselves descent from the Middle Paleocene of Belgium and from the Late Paleocene of France.
There’s a very small Gastornis sp., a lithornithid, a ralloid, and a unassignable „higher landbird“ all from Belgium, and there are another lithornithid, an pelagornithid, a possible leptosomatiform and a probable cariamaform all from France.
Well, and that’s almost all.
 Gerald Mayr; Thierry Smith: New Paleocene bird fossils from the North Sea Basin in Belgium and France. Geologica Belgica 22(1-2): 35-46. 2019