Larson , Brown and Evans 2016 conclude:
“To explain this sudden extinction of toothed maniraptorans and the survival of Neornithes, we propose that diet may have been an extinction filter and suggest that granivory associated with an edentulous beak was a key ecological trait in the survival of some lineages.” … like birds (Euornithes).
A few days ago we looked at the most likely candidate at present to nest at the base of all extant birds, and it wasn’t a little seed-eater. Unfortunately, the Larson et al. study was done without a phylogenetic analysis based on morphology. So they don’t know what the basalmost Euornithine was or looked like. Rather they looked at tooth shapes in derived theropods… and threw a Hail Mary pass.
The authors report,
“To date, only one Maastrichtian bird has been assigned to a crown group clade based on a phylogenetic analysis , suggesting that crown group birds were less common than contemporary non-neornithine birds in the Cretaceous. There are also no Late Cretaceous neornithines or advanced ornithuromorphs with known cranial remains.”
as basalmost Euornithine birds appears unlikely given that basalmost Euornithine birds resemble cranes and ratites. Moreover, the crown group Maastrichtian bird isn’t part of the crown group according to the LRT.
Larson DW, Brown CM and Evans DC 2016. Dental Disparity and Ecological Stability in Bird-like Dinosaurs prior to the End-Cretaceous Mass Extinction. Current Biology 26(10):1325–1333.