Brontosaurus
"Brontosaurus" was a large, long-necked quadrupedal animal with a long, whip-like tail, and forelimbs that were slightly shorter than its hindlimbs. The largest species, "B. excelsus", weighed up to and measured up to long from head to tail. For comparison, the current largest land animal, the average male African bush elephant is 3.20 m (10.5 ft) tall at the shoulder and has a body mass of 6,000 kg (13,228 lb) or 6.6 short tons.
Brontornis
Possibly, the fossils described as "B. platyonyx" represent another species; they are about one-third smaller than the largest "Brontornis" bones. It is much more likely, however, that they represent sexual dimorphism. In today's hawks and owls, females are usually considerably larger than the males; this prevents overutilization of one size class of prey. It is not known whether phorusrhacid males or females would have been larger, but the fossils of the North American phorusrhacid "Titanis" also show considerable variation in size, suggesting that there was indeed at least a tendency for differently sized sexes.
Recent work has cast doubt on the hypothesis that "Brontornis" is a phorusrhacid. "Brontornis" may actually represent an anseriform, with other genera traditionally assigned to Brontornithinae ("Physornis" and "Paraphysornis") being true phorusrhacids. The subfamily containing the latter two had been proposed to be renamed to Physornithinae, with "Physornis fortis" as the type. If these conclusions are valid, this would mean that there are three groups of giant basal anseriformes, in chronological order of divergence: the gastornithids ("Gastornis" and kin), "Brontornis", and finally the mihirungs of Australia. However, other analyses have also argued that "Brontornis" exhibits thoracic vertebrae diagnostic of phorusrhacids, supporting its placement within that group.