Fossil Vertebrate of the Month - Terror Bird

This is the largest group of organisms I've ever featured as a FVOTM, but given that we're coming up on Halloween, it seemed appropriate to spotlight a family whose common name is based on how terrifying they were.  Terror birds (or, more correctly, phorusrhacids), represented by the LA County Museum's mount of Paraphysornis in the picture at left, were a group of giant, flightless birds related to living seriemas, most of which have been uncovered in South America.  Flightless birds are not unusual, as anyone who's seen an ostrich, emu, or rhea (or fossils of elephant birds, moas, or mihirungs) can attest.  However, phorusrhacids were different in one key respect: they were carnivorous.  Carnivory has been suggested for some other land birds - chief among them the Eocene Gastornis, itself a possible terror bird ancestor - but the huge size, robust build, and raptor-like beaks of phorusrhacids leave no doubt.  In fact, the near absence of large mammalian carnivores in South America for most of the Cenozoic indicates that the top predator niche on that continent was occupied by terror birds (they would have preyed upon one of the strangest herbivore faunas in the world, composed of, among other things, meridiungulates, xenarthrans, and - somewhat inexplicably - platyrrhine primates and hystricomorph rodents).  Phorusrhacids were key players in the American Biotic Interchange; once thought to have gone extinct when mammalian carnivores (including the iconic Smilodon) moved in from the north, it is now known that terror birds actually expanded onto the Gulf Coastal Plain in North America, where they were represented by Titanis, one of the largest birds ever to have lived (though it was not the largest phorusrhacid - that honor is currently bestowed on the recently-described Kelenken from Argentina).