The Palaeocene Epoch lasted from 65 to 55.8 million years ago. It is during this time that terrestrial faunas recovered from the mass-extinction at the end of the Cretaceous that wiped out the non-avian dinosaurs and other large vertebrates of the Mesozoic. The Palaeocene fossil record shows the radiation of many modern mammal groups, and one or two others that have gone extinct in the intervening time. The epoch almost certainly saw the radiation of many modern avian groups as well, but the fossil record for birds is notoriously poor, largely due to their hollow lightweight bones and their lack of teeth. The avian fossil record is particularly poor in the southern hemisphere, where almost all bird fossils are large (or giant) flightless terrestrial birds or aquatic birds such as penguins.
On 13 December 2011 a paper appeared in the journal Acta Palaeontologica Polonica in which a team lead by Gerald Mayr of the Sektion Ornithologie at the Forshungsinstitut Senckenberg in Frankfurt in which they describe a new fossil bird from a Late Palaeocene fissure filling in São José de Itaboraí in Brazil (the only location in the southern hemisphere from which small terrestrial birds have previously been discovered.
The bird is described from its leg-bones only, which is not uncommon with bird fossils, since the leg-bones tend to be the most robust, and therefore most easily preserved, part of the skeleton. The bird is named as Itaboravis elaphrocnemoides (the bird from Itaboraí that resembles Elaphrocnemus) and placed in the order Cariamae. The Cariamae are a little understood group of birds with a single living species (the Seriema) and fossil representatives known from the Cretaceous of Antarctica, the Eocene-to Oligocene of Europe and the Miocene of South America.
The name 'elephrocnemoides' meaning Elaphrocnemus-like refers to Elaphrocnemus phasianus, an Eocene Cariamaeid from Quercy in France, which Elaphrocnemus most closely resembles, though Mayr et al. also note a resemblance to the living Tinamus, primitive flying birds from South America more closely related to the Rheas, Ostriches and Emus than to modern flying birds.
(A₁) Left coracoid of Itaboravis in dorsal view. (A₂) The same, in medial view. (A₃) The same, in ventral view. (B) Left coracoid of Elaphrocnemus in dorsal view. (C) Left coracoid of the extant Tinamu Crypturellus parvirostris in dorsal view. (G₁) Right humerus of Itaboravis in cranial view. (G₂) The same in ventral view. (G₃) The same in caudal view. (H₁) Distal end of the left humerus of Itaboravis in cranial view. (H₂) The same in caudal view. From Mayr et al. (2009).
The extant Seriemas (Cariama cristata and Chunga burmeisteri) are long legged terrestrial ground-birds that were thought to be related to Cranes until recent genetic studies proved otherwise; they are now placed in a group with songbirds, birds of prey and parrots. They are omnivores eating insects, small birds and mammals, frogs, snakes, lizards and some plant seeds. Like the Secretary Bird of Africa they will stamp potentially harmful prey (particularly snakes) to death. They can fly, but not well.
The extinct Phosrusrhacids, or Terror Birds, of South (and in one case North) America were also Cariamaeids. They were large, flightless carnivores that prayed on the grazing mammals of the American grasslands. The earliest known example of these is Paleopsilopterus itaboraiensis, which comes from the same São José de Itaboraí location as Itaboravis elaphrocnemoides. By the Miocene these birds had become the top predators across much of South America, an grew to be some of the largest birds ever to live. One species, Titanis walleri, crossed into North America an survived into the Pleistocene.
Reconstruction of the Miocene Terror Bird Kelenken guillermoi, by Natural Science Illustrator Stephanie Abramowicz. K. guillermoi had the largest skull of any bird ever discovered, and was presumably a fearsome predator.
A variety of Cariamaeids seem to have lived in the Cenozoic of Europe. They do not appear to have been closely related to the South American members of the group, having split off early in the group's history, probably within the Cretaceous. They were mainly chicken-sized birds and those for which the wings can be reconstructed seem to have been flightless. Since they do not seem well adapted for running either, they are unlikely to have chased their prey, so (given the largely carnivorous nature of the group) they probably lived by foraging for insects.
The sole Cretaceous specimen (slightly dubious) is an isolated femur from Vega Island at the tip of the Antarctic Peninsula. This offers no real clue as to how the creature that left it lived.
See also How did raptors use their claws? (and did it help them learn to fly?), Giant Bird from the Cretaceous of Kazakhstan, New 'oldest bird' found in China and Birds on Sciency Thoughts YouTube.