From the Degrange 2021 abstract
“Phorusrhacidae, popularly known as ‘terror birds’, are the most speciose clade within the avian order Cariamiformes, with a fossil record that ranges from the Eocene to the Pleistocene. Although several species have preserved skulls, our understanding of their cranial morphology remains incomplete. Here, a comprehensive overview of the current knowledge of phorusrhacid skull anatomy is presented.”
Some phylogenetic issues here due to taxon exclusion.
Degrange 2021 nests terror birds within the clade Cariamiformes, which include Cariama (Fig. 1) , the sister to Phoenicopterus, the flamingo in the large reptile tree (LRT, 1793+ taxa). By adding taxa, rather than relying on outdated tradition, Degrange should have nested phorusrhacids withe the secretary bird, Sagittarius (Fig. 1), the sister to phorusrhachids in the LRT. We looked at this taxonomic problem in September 2017. and again in November 2017.
According to Wikipedia,
“Molecular phylogenetic studies have shown that Cariamiformes is basal to the Falconiformes, Psittaciformes and Passeriformes”
adding taxa while avoiding molecular studies and outdated traditions recovers a cladogram in which gradual evolution is demonstrated at every node.
Degrange FJ 2021. A Revision of Skull Morphology In Phorusrhacidae (Aves, Cariamiformes) Journal of Vertebrate Paleontology Article: e1848855