Temporal range: Jurassic – Recent
|The hands of Deinonychus (left) and Archaeopteryx (right) are typical of maniraptorans.|
Many paleontologists believe birds evolved from them some 150 or so million years ago. Hence, according to phylogenetic taxonomy, birds are by definition maniraptors, and the other maniraptors are their closest relatives. It is now known, from the Jehol biota, that many or all of this clade had feathers. They are often called dinobirds.
Maniraptorans are characterized by long arms and three-fingered hands (though reduced or fused in some lineages), as well as a "half-moon shaped" (semi-lunate) bone in the wrist (carpus). Maniraptorans are the only dinosaurs known to have breast bones (ossified sternal plates). In 2004, Tom Holtz and Halszka Osmólska pointed out six other maniraptoran characters relating to specific details of the skeleton. Unlike most other saurischian dinosaurs, which have pubic bones that point forward, several groups of maniraptorans have an ornithischian-like backwards-pointing hip bone. A backward-pointing hip characterizes the therizinosaurs, dromaeosaurids, avialans, and some primitive troodontids. The fact that the backward-pointing hip is present in so many diverse maniraptoran groups has led most scientists to conclude that the "primitive" forward-pointing hip seen in advanced troodontids and oviraptorosaurs is an evolutionary reversal, and that these groups evolved from ancestors with backward-pointing hips.
Feathers and flight
Modern pennaceous feathers and remiges are known in the advanced maniraptoran group Aviremigia. More primitive maniraptorans, such as therizinosaurs (specifically Beipiaosaurus), preserve a combination of simple downy filaments and unique elongated quills. Simple feathers are known from more primitive coelurosaurs such as Sinosauropteryx prima, and possibly from even more distantly related species such as the ornithischian Tianyulong confuciusi and the flying pterosaurs. Thus it appears as if some form of feathers or down-like integument would have been present in all maniraptorans, at least when they were young.
Maniraptora is the only dinosaur group known to include flying members, though how far back in this lineage flight extends is controversial. Powered and/or gliding flight is believed to have been present in some types of dromaeosaurid, such as Rahonavis and Microraptor. Zhenyuanlong suni, a dromaeosaurid, was too heavy to fly but still had wings with feathers required for flying, which suggests its ancestors had the ability for aerial locomotion. Other groups, like the Oviraptorosauria who had a tail with pygostyle like features, are not known to have been capable of flight, but some scientists have suggested that they could be descended from ancestors which flew. Gregory S. Paul has suggested that this might be the case. Paul has gone as far as to propose that Therizinosauria, Alvarezsauroidea, and the non-maniraptoran group Ornithomimosauria descended from flying ancestors as well.
Scientists have traditionally assumed that maniraptorans were ancestrally hypercarnivorous, that is, that most non-avialan species primarily ate and hunted only other vertebrates. However, a number of discoveries made during the first decade of the 21st century, as well as re-evaluation of older evidence, began to suggest that maniraptorans were a primarily omnivorous group, including a number of sub-groups that ate mainly plants, insects, or other food sources besides meat. Additionally, phylogenetic studies of maniraptoran relationships began to more consistently show that herbivorous or omnivorous groups were spread throughout the Maniraptora, rather than representing a single side-branch as previously thought. This led scientists such as Lindsay Zanno to conclude that the ancestral maniraptoran must have been omnivorous, giving rise to several purely herbivorous groups (such as the therizinosaurs, primitive oviraptorosaurs, and some avialans) and that, among non-avians, only one group reverted to pure carnivores (the dromaeosaurids). Most other groups fell somewhere in between the two extremes, with alvarezsaurids and some avialans being insectivorous, and with advanced oviraptorosaurs and troodontids being omnivorous.
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