The exact relationship between ardeids, threskiornithids and suborder Pelcani is still being investigated with two hypotheses. One hypothesis is ardeids and threskiornithids are each other's sister taxon in suborder Ardei (as presented here).[3] The other hypothesis is the ardeids are the sister taxon to the pelecanid-related clade, with threskiornithids the basal most group. This hypothesis is supported from some morphological and the earliest representations of non-threskiornithid pelecaniforms come from the Oligocene deposits in Africa, while threskiornithids have been found in Eocene deposits of North America and Europe.[8]
^ abPrum, Richard O.; Berv, Jacob S.; Dornburg, Alex; Field, Daniel J.; Townsend, Jeffrey P.; Lemmon, Emily Moriarty; Lemmon, Alan R. (2015). "A comprehensive phylogeny of birds (Aves) using targeted next-generation DNA sequencing". Nature. 526 (7574): 569–573. Bibcode:2015Natur.526..569P. doi:10.1038/nature15697. ISSN0028-0836. PMID26444237. S2CID205246158.