The dino to bird folks incorrectly consider creatures like Alvarezsaurids to be dinosaurs. However, Alvarezsaurids were secondarily flightless primitive birds.
The dino to bird folk make that mistake because the Alvarezsaurids were land-based.
"Alvarezsauridae is an enigmatic family of small, long-legged running dinosaurs. Although originally thought to represent the earliest known flightless birds, a consensus of recent work suggests that they are primitive members of the Maniraptora."
"Alvarezsauroids were originally considered to be a group of flightless birds, but it is now widely accepted that they are not nested within Aves (1–3) and instead represent a basal maniraptoran lineage."As part of this, it would follow that alvaresaurids had backward pointing pubic bones. There does not seem to be much material on this, but see the following:
"Patagonykus puertai Novas, 1997 Coniacian (LK) of Argentina. This animal is apparently transitional between Alvarezsaurus and the rest of Parvicursorinae. It is known from vertebrae, coracoids, a forelimb, partial hips (weakly opisthopubic [pointing backwards, like in dromaeosaurids and birds]), and hindlimbs."
"Mononykus [Alvarezsauridae] is currently represented by a single holotype specimen, catalog number IGM N107/6. This specimen consists of a partial skeleton lacking a tail, and only small fragments of skull bones, including a complete braincase."
In the Cretaceous Alvarezsauridae, the forelimbs are further transformed into bizarrely poweful arms with a huge thumb claw and exceedingly small digits II and III. The alvarezsaurids have a backwards pointing pubis.
Perle et al. (1993) described the next alvarezsaur to be discovered, naming it Mononychus olecranus (meaning "one claw"). A month later they changed the genus name to Mononykus, because the earlier spelling was already the genus name of an extant butterfly. Perle et al. (1993)
mistakenlydescribed Mononykus as a member of Avialae, and one more advanced than Archaeopteryx. They argued that the family Alvarezsauridae was actually a group of Mesozoic flightless birds on the basis of derived features that were unique to birds. Novas (1996) described another member of the class called Patagonykus puertai. Karhu and Rautian (1996) described a Mongolian member of the family;Parvicursor remotus. Chiappe et al.(1998) described another Mongolian member, Shuvuuia mongoliensis and mistakenlyfound it to be even more derived, concluding that the alvarezsaurs were actually crown-group (that is, modern) birds.
mistakenassignments of alvarezsaurs to birds were caused primarily by features that are strikingly, or even uniquely, avian. The sternum, for example, is elongated and deeply keeled for an enlarged pectoralis muscle, as it is in neognathous birds and volant ratites. One bone in the skull of Shuvuuia appeared to be an ectethmoid fused to a prefrontal. The ectethmoid is an ossification known only in Neornithes. Other birdlike characters included the palatine, foramen magnum, cervical and caudal vertebrae, and many others.
Although these animals are extremely bird-like, no fossilized feathers have been found. If it had feathers, then they were not preserved. Mary Schweitzer has made an important study of the "dino fuzz" fossilized integument found with Shuvuuia desertit [Alvarezsauridae] and arrived to the conclusion that the main chemical component is Beta-Keratin: Important demonstration that "dino fuzz" is in reality feathers.http://en.wikipedia.org/wiki/Alvarezsaur
AlvarezsaursTemporal range: Late Jurassic-Late Cretaceous,160–65.5 Ma
Although very little fossil material was known from the head of Mononykus (and little is still known about its cranium), the postcranial anatomy of this new species strongly hinted that it belonged somewhere within birds. Like birds, Mononykus has a wrist that is fused to the metacarpals, a keeled sternum, a thin, posteriorly-projecting pubis, an extra ridge medial to the cnemial crest, and unserrated teeth with constricted roots. However, the very short, powerful arm of Mononykus could not have been used in flight. Scientists at the time hypothesized that Mononykus was a member of a flightless lineage of early birds, and that it might have used the robust forelimb to dig out insects.
Phorusrhacids, colloquially known as "terror birds", were a clade of large carnivorous flightless birds that were the largest species of apex predators in South America during the Cenozoic, 62–2 million years (Ma) ago.They were roughly 1–3 metres (3.3–9.8 ft) tall. Their closest modern-day relatives are believed to be the 80 cm-tall seriemas. Titanis walleri, one of the larger species, is known from Texas and Florida in North America.
South America had birds of this type, but they are thought to have become extinct when the land bridge formed to North America and predators such as the cougar and coyote gradually replaced them. However, this assumption was recently overturned. Fossils from Texas and Florida show that at least one species of South American terror bird managed to establish itself in North America, and was still alive two million years ago. This species had wings that had evolved back into arms, with clawed "hands" on them. It may have looked remarkably like a small, predatory
dinosaur[Alvarezsauridae?]. Estimates of its height range from a modest six feet to possibly as big as twelve feet for adult birds. The two-million-year-old fossils have been dated accurately without a doubt. Other North American terror bird fossils might date to as recently as 15,000 years ago, a time when people could have seen them. In fact, there is North American Indian folklore that is rather suggestive of the terror bird, even getting the details right, such as clawed arms instead of wings.
The seriemas are the sole extant members of the small and ancient clade Cariamidae, which is also the sole surviving lineage of the Cariamae group. Once believed to be related to cranes, they have been placed by one recent study near the falcons, parrots and passerines, as well as the extinct terror birds. The seriemas are large, long-legged terrestrial birds that range from 70 to 90 cm. They live in grasslands, savanna, dry woodland and open forests of Brazil, Bolivia, Argentina, Paraguay and Uruguay.