A field guide to mesozoic birds and other winged dinosaurs

Basal Caenagnathiformes
The first highly diverse offshoot of the early Mesozoic bird lineage
are a group of bizarre, omnivorous ground birds called the carnagnathiformes
(“recent jaws”, alternately oviraptorosaurs). The most primitive
types, like Caudipteryx, were long-legged and fairly small-winged, with
only a small number of teeth, or beaks. More advanced carnagnathiformes
became larger, and some had elaborate casques similar to modern hornbills
or cassowaries. At least one grew to enormous sizes: Gigantoraptor erlianensis,
at up to 1.4 tons, are the largest birds of all time.
Most researchers, based on cladistic analysis, find the caenagnathiformes
to be more distantly related to modern birds than is Archaeopteryx
lithographica. Despite this, they share some strikingly bird-like features that
must otherwise be explained by convergent evolution, including nearly
toothless jaws and shortened tails with fused vertebrae at the tips.
Famously, several specimens have been found brooding their nests
as modern birds do, indicating that some bird behaviors likely evolved before
or concurrently with the advent of true feathers. In part due to the
characteristics of the most primitive known species Protarchaeopteryx robusta,
many researchers had speculated that carnagnathiformes were close
relatives of the segnosaurs (also known as therizinosaurs), bizarre bird-like
herbivorous dinosaurs with distinctively huge, scythe-shaped claws on
their hands. However, contrary to what would be expected, impressions of
feathers from segnosaurs showed only down feathers and simpler quill-like
filaments, unlike the true vaned feathers of caenagnathiformes. This, as well
as some more detailed phylogenetic analyses, has shown segnosaurs to be
more primitive than true, wing-bearing birds.
The caenagnathiform diet has remained largely mysterious. Only
the most primitive species had teeth, and most later groups were beaked.
Some fossils preserve gastroliths in the stomach, suggesting at least partial
herbivory, while others have preserved the remains of small lizards in the
stomach contents. The beaks of most species were stout and strong, resembling
those of parrots or turtles.
The wings were in many cases able to fold more tightly against the
body than those of avialans. Despite this, the wings were generally small
and all known species were flightless and probably primarily terrestrial. The
tails, while short, were extremely strong and flexible, and this unusual range
of motion was probably employed in mating displays.
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