Flightless Tradition

The ancestors of Phorusrhacos were mainly flightless animals. Studies like Alvarenga et al. (2003) placed these giant birds in the family of rails (Ralliformes), indicating that their closest living relatives are flighted birds. Garcia et al. (2020) published a tree of the rail family that did not include Phorusrhacos; their data was genetic though, and (to my knowledge) no genetic studies have been possible within the Phorusrhacidae. 

Ignoring rails for the moment, we turn to the immense height of Phorusrhacos. At 2.4 m (7.9 ft) tall, Phorusrhacos would have towered over modern human beings. These birds were barely above the middle height of their family though. Other members of the family Phorusrhacidae were over 3.2 m (10 ft) tall. Phorusrhacos was not the smallest of these birds either though; that honor currently belongs to two genera currently: Procariama simplex (Rovereto, 1914) and Psilopterus bachmanni (Moreno and Mercerat 1891) measuring in at 70 cm (2.3 ft) tall. Psilopterus genera are often estimated to be slightly taller than this, but the lower end estimate is the same as for Proceriama. A nearly 1 m or 2.5 foot tall terror bird is no laughing matter of course, but we can all agree that a bird the size of Phorusrhacos is very awe inspiring bird. I think we should appreciate that these birds existed and that they were all very large and dominant birds. I also think we might want to heave a sigh of relief that they have not persisted in history to the present day.

Though, I admit I would love to keep one as a pet, maybe get a saddle...

Sources to Consider

Alvarenga, H. M.F.; Höfling, E. (2003). "Systematic revision of the Phorusrhacidae (Aves: Ralliformes)". Papéis Avulsos de Zoologia. 43 (4): 55–91. doi:10.1590/S0031-10492003000400001

Degrange, F. J.; Tambussi, C. P.; Taglioretti, M. L.; Dondas, A.; Scaglia, F. (2015). "A new Mesembriornithinae (Aves, Phorusrhacidae) provides new insights into the phylogeny and sensory capabilities of terror birds". Journal of Vertebrate Paleontology. 35 (2): e912656. doi:10.1080/02724634.2014.912656

Garcia-R, J.C.; Lemmon, E.M.; Lemmon, A.R.; French, N. (2020). "Phylogenomic reconstruction sheds light on new relationships and timescale of rails (Aves: Rallidae) evolution". Diversity. 12 (2): 70. doi:10.3390/d12020070.