Sinornithosaurus - Classification

Classification

Sinornithosaurus was a member of the family Dromaeosauridae, a group of agile, predatory dinosaurs with a distinctive sickle-shaped toe claw, which also includes Deinonychus and Utahraptor. S. millenii lived about 125 million years ago in the Aptian age of the Early Cretaceous period, which makes it among the earliest and most primitive dromaeosaurids yet discovered. Fossils of S. haoiana and the possible Sinornithosaurus specimen NGMC 91 were found in younger strata dating to about 122 million years ago. The presence of vaned feathers on Sinornithosaurus is consistent with feather evidence from other dromaeosaurs.

Two species of Sinornithosaurus have been described. S. millenii ("millennium Chinese bird-lizard") is the type species, described in 1999. A second species, S. haoiana ("Hao's Chinese bird-lizard") was described by Liu et al. in 2004 based on a new specimen, D2140, which differed from S. millenii in features of the skull and hips. However, according to Turner, Makovicky and Norell (2012) the supposed distinguishing features of S. haoiana "are either present in Sinornithosaurus millenii or variable among the number of Sinornithosaurus specimens"; the authors considered S. haoiana to be a junior synonym of S. millenii.

An incredibly well-preserved microraptorian nicknamed "Dave" (specimen NGMC 91), was first described in a paper published in the journal Nature by Ji Qiang and colleagues in 2001. They declined to name the specimen because, although the specimen is completely articulated, almost all of the bones shattered when the fossil slabs were split, so that only the silhouettes of theses bones are clear in most of the part and counterpart. This obscured diagnostic skeletal features, which made the specimen's genus uncertain. They noted that it was similar in some respects to Sinornithosaurus millenii, and they suggested that the differences between the two could have been due to age. Ji, along with another team of scientists, further emphasized this similarity in a 2002 paper, in which they formally referred the specimen to Sinornithosaurus, though they considered the exact species questionable. Meanwhile, Stephen Czerkas and colleagues considered the specimen to represent an example of their newly described species Cryptovolans pauli (now usually considered a synonym of Microraptor gui), based on supposed wing proportions.

Phylogenetic studies did not support the idea that NGMC 91 was a close relative of S. millenii. In a 2004 analysis, Phil Senter and colleagues found that it was, in fact, more closely related to Microraptor. Subsequent studies, also by Senter, have continued to show support for this finding despite the fact that some data used in the original study was later found to be flawed.

However, in a 2011 publication Senter stated that a personal examination of the holotype of S. millenii led him to conclude that his earlier separation of NGMC 91 from S. millenii was based on anatomical misinterpretations. As the two specimens were "identical in all character states" used in his phylogenetic analysis, were from similar stratigraphic levels and "uniquely share the presence of a triangular coracoid with a large, oval foramen", Senter concluded that NGMC 91 does belong to the species Sinornithosaurus millenii.

The same conclusion was also reached independently from Senter by Turner, Makovicky and Norell (2012). According to these authors, NGMC 91 shares several apomorphies with both Microraptor zhaoianus and Sinornithosaurus millenii; however, as it "lacks elongate middle caudals that are three to four times the length of the dorsal vertebrae", it cannot be referred to M. zhaoianus. On the other hand, it does possess a posteriorly bifurcated dentary, which is an apomorphy of Sinornithosaurus. The authors concluded that NGMC 91 was a subadult specimen of S. millenii.