Maniraptora

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Maniraptora ("hand snatchers") is a clade of coelurosaurian dinosaurs which includes the birds and the dinosaurs that were more closely related to them than to Ornithomimus velox. It contains the major subgroups Aves, Deinonychosauria, Oviraptorosauria and Therizinosauria. Ornitholestes and the Alvarezsauridae are also often included. Together with the next closest sister group, the Ornithomimosauria, Maniraptora comprises the more inclusive clade Maniraptoriformes. Maniraptors first appear in the fossil record during the Jurassic Period (see Eshanosaurus), and survive today as over 9,000 species of living birds.

[edit] Characteristics

Maniraptors are characterized by elongated arms and tridactyl hands, as well as a "half-moon shaped" (semi-lunate) bone in the wrist (carpus). Maniraptors are the only dinosaurs known to have ossified sternal plates. Holtz and Osmolska (2004) name six other maniraptoran characters, including a reduced or absent olecranon process of the ulna, and the greater trochanter and cranial trochanter of the femur fused into a trochanteric crest. An elongated, backwards-pointing pubic bone is present in therizinosauroids, dromaeosaurids, avialans, and the basal troodontid Sinovenator, which suggests that the propubic condition in advanced troodontids and oviraptorosaurs is a reversal. ^[1] Turner et al. (2007) name seven synapomorphies that diagnose Maniraptora.^[2]

Modern pennaceous feathers and remiges are known from all maniraptoran groups except therizinosaurs, but they are inferred there as well. Fossils of Beipiaosaurus preserve some sort of feathers but their structure is not clear.^[3] Powered flight is present in members of Aves (or Avialae), and possibly in some dromaeosaurids such as Rahonavis.^[4] Also, simple feathers are known from more primitive dinosaurs such as Sinosauropteryx. Thus it appears as if some form of feathers or down-like integument may have been present in all maniraptorans at least when they were young. Powered flight on the other hand is probably an autapomorphy of only avialan lineages.^[2]

[edit] Phylogeny

The Maniraptora was originally named by Jacques Gauthier in 1986, for a branch-based clade defined as all dinosaurs closer to modern birds than to the ornithomimids. Gauthier noted that this group could be easily characterized by their long forelimbs and hands, which he interpreted as adaptations for grasping (hence the name Maniraptora, which means "hand snatchers" in relation to their 'seizing hands'). In 1994, Thomas R. Holtz attempted to define the group based on the characteristics of the hand and wrist alone (an apomorphy-based definition), and included the long, thin fingers, bowed, wing-like forearm bones, and half-moon shaped wrist bone as key characters. Most subsequent studies have not followed this definition, however, preferring the earlier branch-based definition.

The branch-based definition usually includes the major groups Deinonychosauria, Oviraptorosauria, Therizinosauria, and Aves. Other taxa often found to be maniraptorans include the alvarezsaurids, Ornitholestes^[2] and, less frequently, the compsognathids. Several taxa have been assigned to the maniraptora more definitively, though their exact placement within the group remains uncertain. These forms include the scansoriopterygids, Pedopenna, and Yixianosaurus, and the dubious Bradycneme.

The following cladogram follows Norell et al. 2006, with clade names after the definitions by Sereno, 2005.^[5][6]

[edit] Alternative theories

Czerkas and Yuan (2002) reported that some maniraptoran traits, such as a long, backwards-pointed, pubis, short ischia, as well as a perforated acetabulum (a hip socket that is a hole) are apparently absent in the Scansoriopterygidae (see Epidendrosaurus and Scansoriopteryx). However, other maniraptorans are propubic (see Characteristics, above) and all known scansoriopterygid specimens are hatchlings, so that their odd mix of "primitive" and "advanced" features could be due to ontogeny (i.e., their early stage of physical development). The authors consider them truly primitive, and hypothesize that maniraptorans may have branched off from theropods at a very early point, or may even have descended from pre-theropod dinosaurs.^[7] Zhang et al. (2002), in describing the closely related or conspecific specimen Epidendrosaurus, do not confirm any of the primitive traits mentioned by Czerkas, but do mention a primitive distal scapula. They place Epidendrosaurus firmly within Maniraptora.^[8]

[edit] References

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