Harrisonavis croizeti (Torres et al. 2015, Fig. 1) is very clearly an Oligocene flamingo. Other than the angle of its beak, it is a close match to the living flamingo, Phoenicopterus (Fig. 1).
Figure 1. Click to enlarge. Candidate taxa in the ancestry of flamingos. Most bird experts like long-necked Paleolodus, a tinamou in the LRT. Ignored is Cariama, which shares more traits with flamingos in the LRT.
The trouble comes when you try to delve deeper into flamingo ancestry.
Bird experts, like Mayr 2004, say Palaelodus (Fig. 2 ) is the next outgroup to flamingos. Well, in a way, it is, but the large reptile tree (LRT, 1051 taxa, Fig. 2) nests Palaeodus with Struthio, the ostrich, just outside of the clade that starts with Phoenicopterus, the flamingo, and Cariama, the seriema. So, it’s a close one! Palaelodus was the last of the flying ostriches. All of the basalmost neognaths had long, stilt-like legs.
Figure 2. Subset of the LRT focusing on birds. Here various aspects of birds are shown, including age, teeth, feeding behavior and basic clades.
Figure 3. Nearly proportioned like its giant descendant, Palaeotis was an Eocene ostrich less than 1/3 as tall. Compare to Palaelodus.
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Torres CR, De Pietri VL, Louchart A and van Tuinen M 2015. New cranial material of the earliest filter feeding flamingo Harrisonavis croizeti (Aves, Phoenicopteridae) informs the evolution of the highly specialized filter feeding apparatus. Organisms, Diversity & Evolution. DOI: 10.1007/s13127-015-0209-7