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Yixianosaurus (Theropoda) restudied



From: Ben Creisler
bcreisler@gmail.com


Another new paper in Vertebrata PalAsiatica:

T. Alexander Dececchi, Hans C. E. Larsson & David W. E. Hone (2012)
Yixianosaurus longimanus (Theropoda: Dinosauria) and its bearing on
the evolution of Maniraptora and ecology of the Jehol fauna.
Vertebrata PalAsiatica 50(2):111-139
http://www.ivpp.cas.cn/cbw/gjzdwxb/xbwzxz/201204/P020120423370142063957.pdf


We re-examine the partial skeleton of the enigmatic small feathered
theropod Yixianosaurus longimanus from the Lower Yixian Formation,
China. A phylogenetic anqlysis recovers Yixianosaurus as a basal
maniraptoran, in a polytomy with Alverzsaurus, Therizinosauria,
Alvarezsauroidea excluding Alvarezsaurus, and a well-resolved clade of
derived maniraptorans including Oviraptorosauria and Paraves. The
blend in Yixianosaurus of primitive traits, such as a low branchial
index and robust digit III, and derived traits such as subrectangular
coracoid, suggests a more complicated mosaic of forelimb evolution
within Maniraptora than previously proposed. The robust forelimb bones
and massive, recurved, and pointed manual unguals suggest
Yixianosaurus was predatory, although this assessment remains
tentative until more complete fossils of this taxon are found. The
basal maniraptoran phylogenetic position implies a larger range of
forelimb morphologies at this point in coelurosaurian evolution. In
many ways, the robust forelimb and hypertrophied recurved unguals are
similar to those of Tanycolagreus and Coelurus, and may be
morphologically transitional between these taxa and basal
therizinosaurs and oviraptorosaurs. The presence of large, veined,
pennaceous contour feathers on Yixianosaurus indicates the origin of
these integumentary structures may be deeper in coelurosaur phylogeny
than previously reported. Finally, the robust, elongate forelimbs
suggest a different ecological role than inferred for other sympatric
small theropods, and lend support to the idea of niche partitioning
among the small non-avian theropods of the Jehol fauna.