Similicaudipteryx

Similicaudipteryx is a genus of theropod dinosaur of the family Caudipteridae. Its fossil remains were recovered from the Jiufotang Formation of northeastern China, dating to the early Cretaceous Period (Aptian stage) about 120 million years ago. The genus contains one described species, Similicaudipteryx yixianensis, which was named in 2008 by He, Wang, and Zhou. The name refers to its similarity to the closely related Caudipteryx and its location near the Chinese Yixian beds.[1]

He and colleagues assigned Similicaudipteryx to the Caudipteridae based on its similarity to Caudipteryx (from the older Yixian Formation, 125 million years ago), especially in the shape of the pubic bones, though these bones are differently proportioned than in Caudipteryx. Similicaudipteryx also differed from its relatives by possessing a dagger-shaped pygostyle (the bone at the end of the tail to which feathers anchor in birds) and several unique features of the back vertebrae. The only other oviraptorosaur reported to have a pygostyle is Nomingia, though the feature is widespread in more advanced birds and appears to have evolved at least twice.

Discovery and naming
In 2007 the find of a new caudipterid was reported.[1] In 2008 the new genus Similicaudipteryx, with as type species Similicaudipteryx yixianensis, was named and descrived by He Tao, Wang Xiaolin and Zhou Zonghe. The generic name, derived from Latin similis, "resembling", refers to its similarity to the closely related Caudipteryx. The specific name refers to its provenance from Yixian County.[2] Presently, S. yixianensis is the only species named in the genus.

The holotype specimen, IVPP V 12556, was recovered from the Jiufotang Formation of northeastern China, dating to the early Cretaceous Period middle Aptian stage, about 120 million years ago or perhaps early Albian stage, about 112 million years old. It consists of a nearly complete skeleton of an adult individual compressed on a plate, that however lacks the skull, the front neck and the hands.[2]

In 2010 two additional specimens were referred to Similicaudipteryx, one of a juvenile, STM4-1, and the other of a much larger juvenile, STM22-6. Both were reported as found in the Yixian Formation, dating to the early Aptian stage, about 124 million years ago.[3]

In 2010 Gregory S. Paul renamed the species to Caudipteryx yixianensis,[4] but this has found no acceptance.

Description
Similicaudipteryx is known from three specimens ranging from young juvenile to adult. It was a small oviraptorosaur similar to, but larger than, the perhaps closely related Caudipteryx. Paul in 2010 estimated the length at one metre, the weight at seven kilogrammes.[4] Like Caudipteryx, it had a short snout with a downcurved lower jaw. It had relatively short arms and long legs. Similicaudipteryx also differed from its relatives by possessing a dagger-shaped pygostyle (the bone at the end of the tail to which feathers anchor in birds) consisting of the two most extreme tail vertebrae and several unique features of the back vertebrae. The pubic bone was exceptionally long relative to the ilium. The only other oviraptorosaur reported to have a pygostyle is Nomingia, though the feature is widespread in more advanced birds and appears to have evolved at least twice.

Feathers
The first Similicaudipteryx specimen lacked traces of feathers, but He and colleagues speculated that they were probably present based on its pygostyle, the anchor point of long tail feathers (rectrices) in modern birds.[2] In 2010, two new specimens were described, both of which preserved feather traces. These specimens (both juveniles, though one closer to maturity than the other) showed that the feathers were similar to the related Caudipteryx, with long (symmetrical) vaned feathers on the hand and tail, and the rest of the body covered in simpler, downy feathers.