Tuesday, September 8, 2015

[Paleontology • 2015] The First Dromaeosaurid (Dinosauria: Theropoda) from the Early Cretaceous Bayan Gobi Formation of Nei Mongol, China


Figure 6. A palaeoreconstruction of IVPP V22530 next to its inferred depositional setting, a muddy lake environment.
Illustration: Julius T. Csotonyi || DOI: 10.7287/PeerJ.preprints.1340v1


Abstract

The first dromaeosaurid theropod from the Early Cretaceous Bayan Gobi Formation is identified based on an incompletely preserved partially-articulated left leg, increasing the known diversity of its understudied ecosystem. The leg belongs to specimen IVPP V22530 and includes a typical deinonychosaurian pedal phalanx II-2 with a distinct constriction between the enlarged proximal end and the distal condyle as well as a typical deinonychosaurian enlarged pedal phalanx II-3. It possesses a symmetric metatarsus and a slender and long MT V that together suggest it is a dromaeosaurid. Two anatomical traits suggest the leg is microraptorine-like, but a more precise taxonomic referral was not possible: metatarsals II, III and IV are closely appressed distally and the ventral margin of the medial ligament pit of phalanx II-2 is close to the centre of the rounded distal condyle. This taxonomic status invites future efforts to discover additional specimens at the study locality because - whether it is a microraptorine or a close relative - this animal is expected to make important contributions to our understanding of dromaeosaurid evolution and biology. IVPP V22530 also comprises of an isolated manual ungual, a proximal portion of a right dorsal rib and an indeterminate bone mass that includes a collection of ribs. However, these specimens cannot be confidently referred to Dromaeosauridae, although they may very well belong to the same individual from whom the left leg belongs.

Keywords: dromaeosaurid, microraptorine, paravian, deinonychosaur, theropod, Elesitai, Bayan Gobi Formation, Nei Mongol, Early Cretaceous


Conclusions
IVPP V22530 comprises of an incompletely preserved partially-articulated left dromaeosaurid leg, an isolated pennaraptorans manual ungual, a proximal portion of a right theropod dorsal rib and an indeterminate bone mass that includes a collection of ribs. Two anatomical traits suggest that the left leg belongs to a microraptorine or a close relative: metatarsals II, III and IV are closely appressed distally and the ventral margin of the medial ligament pit of phalanx II-2 is close to the centre of the rounded distal condyle. This referral means that IVPP V22530 is the first described dromaeosaurid - and small-sized theropod (<1kg) - from the Bayan Gobi Formation, helping to expand our understanding of this understudied Early Cretaceous ecosystem. Aptian to Albian ages have been specifically suggested for the Formation, but constraining them further would be invaluable as a well supported Albian age could make IVPP V22530 the first-known Albian microraptorine-like dromaeosaurid. As a microraptorine IVPP V22530 would extend the geographical range of this clade because the study site is ~500km northwest of Liaoning Province, which is the only area that Early Cretaceous microraptorines are known from. As a close microraptorine relative IVPP V22530 would be the first non-North American example. Thus, further discoveries at the study site will help fill important gaps in our knowledge of dromaeosaurid evolution and biology between the Aptian/Albian and Campanian stages of the Cretaceous.


Michael Pittman​, Rui Pei and Xing Xu. 2015. The First Dromaeosaurid (Dinosauria: Theropoda) from the Early Cretaceous Bayan Gobi Formation of Nei Mongol, China. PeerJ. PrePrints 3:e1654 DOI: 10.7287/PeerJ.preprints.1340v1

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