Paracrax gigantea/antiqua by Ornito Frenia
Bathornithidae was a lineage of flightless birds closely related to the more famous terror birds and modern seriemas that lived in North America from the Eocene to Miocene.
Basal forms like Paracatharthes probably could still fly, but as early as the late Eocene we see flightless forms, culminating in Oligocene forms like these, tur titans by bird standards.
Bathornithid diet is still somewhat uncertain, but based on their close relations, both alive and extinct, it can be assertained that they were carnivores.
So there you have it, North America’s most obscure theropod lineage.