Sinornithosaurus (derived from a combination of Latin and Greek, meaning 'Chinese bird-lizard') is a genus of feathered dromaeosaurid dinosaur from the early Cretaceous Period (late Barremian) of the Yixian Formation in what is now China. It was the fifth non–avian feathered dinosaur genus discovered by 1999. The original specimen was collected from the Sihetun locality of western Liaoning. It was found in the Jianshangou beds of the Yixian Formation, dated to 124.5 million years ago. Additional specimens have been found in the younger Dawangzhangzi bed, dating to around 122 million years ago. Xu Xing described Sinornithosaurus and performed a phylogenetic analysis which demonstrated that it is basal, or primitive, among the dromaeosaurs. He has also demonstrated that features of the skull and shoulder are very similar to Archaeopteryx and other avialans. Together these two facts demonstrate that the earliest dromaeosaurs were more like birds than the later dromaeosaurs were. Sinornithosaurus was among the smallest dromaeosaurids, with the holotype measuring 1.2 metres (3.9 ft) long and weighing 3–5 kilograms (6.6–11.0 lb).
Description
Specimens of Sinornithosaurus have preserved impressions of feathers both covering the entirety of the body and forming the wings. These feathers were indistinguishable in form from those found on birds from the same geological deposits. The body (contour) feathers were generally between 3–4.5 cm long and included two types: the first type are formed of several filaments joined into "tufts", similar to modern down feathers. The second type, including those on the arms, were composed of rows of filaments (barbs) joined along a main shaft (rachis), making them similar in structure to modern bird feathers. However, unlike the wing feathers of flying birds, they did not have the secondary branches with tiny little hooks (barbules) that flight feathers have, which allow the flight feathers to form a continuous vane. Some scientists have suggested that the feathers might have allowed Sinornithosaurus to glide for short distances after leaping from trees. A 2010 study indicated that Sinornithosaurus may have had feathers which varied in color significantly across different regions of the body, based on analysis of microscopic cell structures in preserved fossils. Based on the rod and spherical melanosomes found in some samples, scientists have interpreted that Sinornithosaurus had black and rufous feathers.
Paleobiology
Possible venomous bite
In 2009, a team of scientists led by Empu Gong examined a well-preserved Sinornithosaurus skull, and noted several features suggesting it was the first-identified venomous dinosaur. Gong and colleagues noted that the unusually long and fang-like mid-jaw (maxillary) teeth had prominent grooves running down the outer surface, towards the rear of the tooth, a feature seen only in venomous animals. They also interpreted a cavity in the jaw bone just above these teeth as the possible site for the soft-tissue venom gland. Gong and colleagues suggested that these unique features indicated that Sinornithosaurus may have specialized in hunting small prey such as birds, using its long fangs to penetrate feathers and envenomate and stun the prey, like a modern snake. They also suggested that the short, slightly forward-pointing teeth at the tip of the jaw could have been used to strip feathers from birds.
However, in 2010, another team of scientists led by Federico Gianechini published a paper casting doubts on the claim that Sinornithosaurus was venomous. They noted that grooved teeth are not unique to this genus, and in fact grooved teeth are found in many other theropods, including other dromaeosaurids. They also demonstrated that the teeth were not abnormally long as Gong and his team claimed, but rather had come out of their sockets, a preservational artifact common in crushed and flattened fossils. Finally, they could not independently verify the presence of supposed chambers for venom glands cited by Gong's team, finding only the normal sinuses of the skull. In the same journal issue, Gong and his team submitted a reassessment of the 2009 study, casting doubt on their findings. They admitted that grooved teeth were common among theropods (though they suggested they were really only prevalent among feathered maniraptorans), and hypothesized that venom may have been a primitive trait for all archosaurs if not all reptiles, which was retained in certain lineages. They also disputed the claim that the teeth were significantly out of their sockets in the holotype specimen of Sinornithosaurus, though they admitted that they were not in a completely natural position. Gong's reassessment also claimed that certain undescribed specimens had fully articulated teeth showing a similar length. However, these grooved teeth are not direct evidence of venom, as non-venomous species of animals (such as baboons) have similarly grooved teeth.
Circadian rhythm Comparisons between the scleral rings of Sinornithosaurus and modern birds and reptiles indicate that it may have been cathemeral, active throughout the day and night at short intervals.
Flight/gliding The possibility that Sinornithosaurus was capable of gliding has been presented several times, due to its close relation to flying or gliding dromaeosaurs like Microraptor. Chatterjee and Templin 2004 found S. millenii as grouping within dinosaurs with aerodynamic potential for aerial locomotion, an opinion latter also expressed by Darren Naish, while Longrich & Currie 2009 have expressed that it was probably too heavy to fly, though it is worth to note that this latter study was published before the formal description of Changyuraptor, a larger microraptorine with evident flight capacities.
Classification
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