Basal Caenagnathiformes The first highly diverse offshoot of the early Mesozoic bird lineage are a group of bizarre, omnivorous ground <strong>birds</strong> called the carnagnathiformes (“recent jaws”, alternately ovirap<strong>to</strong>rosaurs). The most primitive types, like Caudipteryx, were long-legged <strong>and</strong> fairly small-<strong>winged</strong>, with only a small number of teeth, or beaks. More advanced carnagnathiformes became larger, <strong>and</strong> some had elaborate casques similar <strong>to</strong> modern hornbills or cassowaries. At least one grew <strong>to</strong> enormous sizes: Gigan<strong>to</strong>rap<strong>to</strong>r erlianensis, at up <strong>to</strong> 1.4 <strong>to</strong>ns, are the largest <strong>birds</strong> of all time. Most researchers, based on cladistic analysis, find the caenagnathiformes <strong>to</strong> be more distantly related <strong>to</strong> modern <strong>birds</strong> than is Archaeopteryx lithographica. Despite this, they share some strikingly bird-like features that must <strong>other</strong>wise be explained by convergent evolution, including nearly <strong>to</strong>othless jaws <strong>and</strong> shortened tails with fused vertebrae at the tips. Famously, several specimens have been found brooding their nests as modern <strong>birds</strong> do, indicating that some bird behaviors likely evolved before or concurrently with the advent of true feathers. In part due <strong>to</strong> 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 <strong>dinosaurs</strong> with distinctively huge, scythe-shaped claws on their h<strong>and</strong>s. However, contrary <strong>to</strong> what would be expected, impressions of feathers from segnosaurs showed only down feathers <strong>and</strong> simpler quill-like filaments, unlike the true vaned feathers of caenagnathiformes. This, as well as some more detailed phylogenetic analyses, has shown segnosaurs <strong>to</strong> be more primitive than true, wing-bearing <strong>birds</strong>. The caenagnathiform diet has remained largely mysterious. Only the most primitive species had teeth, <strong>and</strong> most later groups were beaked. Some fossils preserve gastroliths in the s<strong>to</strong>mach, suggesting at least partial herbivory, while <strong>other</strong>s have preserved the remains of small lizards in the s<strong>to</strong>mach contents. The beaks of most species were s<strong>to</strong>ut <strong>and</strong> strong, resembling those of parrots or turtles. The wings were in many cases able <strong>to</strong> fold more tightly against the body than those of avialans. Despite this, the wings were generally small <strong>and</strong> all known species were flightless <strong>and</strong> probably primarily terrestrial. The tails, while short, were extremely strong <strong>and</strong> flexible, <strong>and</strong> this unusual range of motion was probably employed in mating displays. 54
Above: Relationships of basal caenagnathiforms over time. Phylogeny approximated based on Senter 2007 <strong>and</strong> <strong>other</strong> sources. 55
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