Sinosauropteryx

Sinosauropteryx Temporal range: Early Cretaceous,
Type specimen with filament impressions, Inner Mongolia Museum
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Superorder:	Dinosauria
Order:	Saurischia
Suborder:	Theropoda
(unranked):	Coelurosauria
Family:	†Compsognathidae
Genus:	†Sinosauropteryx Ji & Ji, 1996
Species
S. prima Ji & Ji, 1996 (type)

Sinosauropteryx (meaning "Chinese reptilian wing", in Chinese 中华龙鸟: zhonghua longniao) is the first genus of non-avian dinosaur found with the fossilized impressions of feathers, as well as the first non-avian dinosaur where coloration has been determined. It lived in China during the early Cretaceous period and was a close relative of Compsognathus. It was the first non-avialan dinosaur genus discovered from the famous Jehol Biota of Liaoning Province. The remarkably well-preserved fossils show that Sinosauropteryx was covered with a furry down of very simple feathers — though some contention arose with an alternative interpretation of the filamentous impression as collagen fiber remains. These filaments consisted of a simple two-branched structure, roughly similar to the secondarily primitive feathers of the modern kiwi.

The type and only named species is S. prima (meaning "first"), named in reference to its status as the first feathered non-avialan dinosaur ever discovered. Three specimens have been assigned to S. prima. A larger fossilized skeleton may belong to an unnamed species of Sinosauropteryx.

Description

Size of adult and sub-adult specimens, compared with a human

Sinosauropteryx prima is among the smallest known non-avian theropods, with the type specimen (a nearly mature individual) measuring only 68 cm (27 in) in length, including a very long tail. The largest known specimens reach up to 98 cm (39 in) in length. Sinosauropteryx is distinguished from other small dinosaurs by several features, including having a skull longer than its upper leg bone (femur) and very short, stout forelimbs, with the arms being only 30% the length of the legs. Overall, Sinosauropteryx had proportionately shorter limbs than its close relative Compsognathus.

In addition, Sinosauropteryx had several features unique among all other theropods (bipedal, mainly carnivorous dinosaurs). S. prima had 64 vertebrae in its tail, giving it the longest tail relative to body length of any theropod. It also had very large fingers for its small arms, with the second finger and claw being longer than the entire lower arm (radius).

Three specimens have been assigned to Sinosauropteryx prima: the holotype GMV 2123 (NIGP 127586), and two referred specimens: NIGP 127587 and D 2141. The assignment of a fourth, larger specimen to S. prima was later found to be in error. All of the fossils were found in the Jianshangou and Dawangzhangzi Beds of the Yixian Formation in the Beipiao and Lingyuan regions of Liaoning, China. These fossil beds have been dated to 124.6-122 million years ago, during the late Barremian to early Aptian stages of the Early Cretaceous.

Classification

Cast of the skeleton belonging to GMV 2124, formerly classified as Sinosauropteryx

There is only one named species of Sinosauropteryx, S. prima. A possible second species is represented by the specimen GMV 2124 (aka NGMC 2124), which was described as another, larger specimen of S. prima by Ji and Ji in 1997. However, in a 2002 presentation for the Society of Vertebrate Paleontology, Nick Longrich showed that this specimen differs in several anatomical aspects from the others, including its relatively large size, proportionally longer tibiae, and shorter tail. Longrich suggested that the GMV 2124 specimen was a compsognathid coelurosaur, while Sinosauropteryx proper was a more primitive kind of coeurosaur or even a primitive carnosaur. In 2007, Gishlick and Gauthier also concluded that this specimen was probably a new taxon, and tentatively re-classified it as Sinosauropteryx? sp., though they suggested it may belong in a new genus. In the same year, Ji, Ji and colleagues changed their opinion and also suggested GMV 2124 is probably a new genus.

Sinosauropteryx is important because it had feather-like structures, yet was not very closely related to the previous "first bird" Archaeopteryx. There are many dinosaur families that were more closely related to Archaeopteryx than Sinosauropteryx was, including the deinonychosaurians, the oviraptorosaurians, and the therizinosauroids. This indicates that feathers may have been a characteristic of many theropod dinosaurs, not just the obviously bird-like ones, making it possible that equally distant animals such as Ornitholestes, Coelurus, and Compsognathus had feathers as well, although their close proximity to the origin of feathers and the presence of scales on Juravenator and Tyrannosaurus make the distribution of feathers in primitive coelurosaurs extremely difficult to estimate accurately.

Most paleontologists do not consider Sinosauropteryx to be a bird, because phylogenetically, it lies far from the clade Aves, usually defined as Archaeopteryx plus modern birds. The scientists who discovered and described Sinosauropteryx, however, used a character-based, or apomorphic, definition of the Class Aves, that is, a definition in which any animal with feathers is considered to be a bird. They argued that the filamentous plumes of Sinosauropteryx represent true feathers with a rachis and barbs, and thus that Sinosauropteryx should be considered a true bird. They classified it as belonging to a new biological order, Sinosauropterygiformes, family Sinosauropterygidae, within the subclass Sauriurae.

Paleobiology

Diet

Restoration of specimen in habitat

The specimen NIGP 127587 was preserved with the remains of a lizard (complete with skull) in its stomach, indicating that small, fast-moving animals made up part of the diet of Sinosauropteryx prima. Numerous lizards of this type had previously been found in the same rocks as Sinosauropteryx but have yet to be described.

Another possible Sinosauropteryx specimen, GMV 2124 (Sinosauropteryx? sp.), was found with three mammal jaws in its stomach region. Hurum, Luo & Kielan-Jaworowska identified two of these jaws as belonging to Zhangheotherium and the third to Sinobaatar, showing that these two mammals were part of the animal's diet. Interestingly, Zhangheotherium is known to have had a venom-secreting spur, like the modern platypus, showing that Sinosauropteryx fed on possibly venomous mammals.

Reproduction

In the same specimen of S. prima that preserved the complete stomach contents including a lizard (NIGP 127587), several small eggs were also discovered in the abdomen. Two eggs were preserved just in front of and above the pubic boot, and several more may lie underneath them on the slab. It is unlikely that these were eaten by the animal, as they were in the wrong part of the body cavity for the egg shells to have remained intact. It is more likely that these are unlaid eggs produced by the animal itself.

Each egg measured 36 millimeters (1.4 inch) long by 26 mm (1 in) wide. The total length of this individual was 1.07 m (3.5 ft).

Feathers

All descrbed specimens of Sinosauropteryx preserve integumentary structures (filaments arising from the skin) which most paleontologists interpret as very primitive feathers. These short, down-like filaments are preserved all along the back half of the skull, arms, neck, back, and top and bottom of the tail. Additional patches of feathers have been identified on the sides of the body, and paleontologist Chen, Dong and Zheng proposed that the density of the feathers on the back and the randomness of the patches elsewhere on the body indicated the animals would have been fully feathered in life, with the ventral feathers having been removed by decomposition.

Image showing the entire holotype specimen, on exhibit in Japan

The filaments are preserved with a gap between the bones, which several authors have noted corresponds closely to the expected amount of skin and muscle tissue that would have been present in life. The feathers adhere close to the bone on the skull and end of the tail, where little to no muscle was present, and the gap increases over the back vertebrae, where more musculature would be expected, indicating that the filaments were external to the skin and do not correspond with sub-cutaneous structures.

The random positioning of the filaments and often "wavy" lines of preservation indicate that they were soft and pliable in life. Examination with microscopes shows that each individual filament appears dark along the edges and light internally, suggesting that they were hollow, like modern feathers. Compared to modern mammals the filaments were quite coarse, with each individual strand much larger and thicker than the corresponding hairs of similarly sized mammals.

The length of the filaments varied across the body. They were shortest just in front of the eyes, with a length of 13 mm. Going further down the body, the filaments rapidly increase in length until reaching 35 mm long over the shoulder blades. The length remains uniform over the back, until beyond the hips, when the filaments lengthen again and reach their maximum length midway down the tail at 40 mm. The filaments on the underside of the tail are shorter overall and decrease in length more rapidly than those on the dorsal surface. By the 25th tail vertebrae, the filaments on the underside reach a length of only 35 mm. The longest feathers present on the forearm measured 14 mm.

Though the feathers are too dense to isolate a single structure for examination, several studies have suggested that the presence of two distinct types of filament (thick and thin) interspersed with each other, the tendency for the thick filaments to appear 'stiffer' than thin filaments, and the tendency of thin filaments to lie parallel to each other but at angles to nearby thick filaments, suggests that the individual feathers consisted of a central quill (rachis) with thinner barbs branching off from it, similar to but more primitive in structure than modern bird feathers. Overall, the filaments most closely resemble the "plumules" or down-like feathers of some modern birds, with a thick central quill and long, thin barbs. The same structures are seen in other fossils from the Yixian Formation, including Confuciusornis.

Colouration

See also: Animal colouration

Reconstruction illustrating colouration as suggested by the study of preserved melanosomes

The fossils of Sinosauropteryx have shown an alternation of lighter and darker bands preserved on the tail, which may give an indication of what the animal looked like in life. Chen and colleagues initially interpreted this banding pattern as an artifact of the splitting between the main slab and counter-slab in which the original specimen was preserved. However, in a 2002 lecture and corresponding abstract for the Society of Vertebrate Paleontology, paleontologist Nick Longrich suggested that these specimens actually preserve remnants of the original colouration pattern the animal would have exhibited in life. He argued that the dark, banded stains on the tail were too evenly spaced to have been caused by random separation of the fossil slabs, and that they represent fossilized pigments present in the feathers. Additionally, rather than an artifact of preservation or decomposition, the presence of dark feathers along only the top of the body may also reflect the colour pattern in life, indicating that Sinosauropteryx prima was countershaded with dark colouration on its back and lighter colouration on its underside, with bands or stripes on the tail for camouflage.

Longrich's conclusions were supported in a paper first published online in the journal Nature in January 2010. Fucheng Zhang and colleagues examined the fossilized feathers of several dinosaurs and early birds, and found evidence that they preserved melanosomes, the cells that give the feathers of modern birds their colour. By examining melanosome structure and distribution, Zhang and colleagues were able to confirm the presence of light and dark bands of colour in the tail of Sinosauropteryx. Furthermore, the team was able to compare melanosome types to those of modern birds to determine a general range of colour. From the presence of phaeomelanosomes, spherical melanosomes that make and store red pigment, they concluded that the darker feathers of Sinosauropteryx were chestnut or reddish brown in colour.

Controversy

See also: Feathered dinosaurs

Some researchers have interpreted the filamentous impressions around Sinosauropteryx fossils as remains of collagen fibers, rather than primitive feathers. Since they are clearly external to the body, these researchers have proposed that the fibers formed a frill on the back of the animal and underside of its tail, similar to some modern aquatic lizards. This would refute the proposal that Sinosauropteryx is the most basal known theropod genus with feathers, and also questions the current theory of feather origins itself. It calls into question the idea that the first feathers evolved not for flight but for insulation, and that they made their first appearance in relatively basal dinosaur lineages that later evolved into modern birds.

Most researchers have disagreed with the identification of the structures as collagen or other structural fibers. Notably, the team of scientists that reported the presence of pigmentation cells in the structures argued that their presence proved the structures were feathers, not collagen, as collagen does not contain pigment.

History of discovery

Undescribed fossil specimen at the Hong Kong Science Museum

The first fossil specimen of the dinosaur later named Sinosauropteryx prima was uncovered in August 1996 by Li Yumin. Yumin was a farmer and part-time fossil hunter who often prospected around Liaoning province in order to acquire fossils to sell to individuals and museums. Yumin recognized the unique quality of the specimen, which was separated into two slabs, and sold the slabs to two separate museums in China: the National Geological Museum in Beijing, and the Nanjing Institute of Geology and Paleontology. The director of the Beijing museum, Ji Qiang, recognized the importance of the find, as did visiting Canadian paleontologist Phil Currie and artist Michael Skrepnick, who became aware of the fossil by chance as they explored the Beijing museum's collections after leading a fossil tour of the area during the first week of October, 1996. Currie recognized the significance of the fossil immediately. As The New York Times quoted him, "When I saw this slab of silt stone mixed with volcanic ash in which the creature is embedded, I was bowled over."

Chinese authorities initially barred photographs of the specimen from publication. However, Currie brought a photograph to the 1996 meeting of the Society of Vertebrate Paleontology at the American Museum of Natural History in New York, causing crowds of paleontologists to gather and discuss the new discovery. The news reportedly left paleontologist John Ostrom, who in the 1970s had pioneered the theory that birds evolved from dinosaurs, "in a state of shock." Ostrom later joined an international team of researchers who gathered in Beijing to examine the fossils other team members included feather expert Alan Brush, fossil bird expert Larry Martin, and Peter Wellnhofer, an expert on the early bird Archaeopteryx.

Controversy regarding the identity of the filaments preserved in the Sinosauropteryx specimen began almost immediately, as the team of scientists spent three days in Beijing examining the specimen under a microscope. The results of their studies (reported during a press conference at the Philadelphia Academy of Natural Science on Thursday, April 24, 1997) were inconclusive; the team agreed that the structures preserved on Sinosauropteryx were not modern feathers, but suggested further research was required to discover their exact nature. Paleontologist Alan Feduccia, who had not yet examined the specimen, wrote in Audubon Magazine that the structures on Sinosauropteryx (which he considered at the time to be a synonym of Compsognathus, as Compsognathus prima) were stiffening structures from a frill running along the back, and that dinosaur paleontologists were engaging in wishful thinking when equating the structures with feathers. Subsequent publications saw some of the same team members disagreeing over the identity of the structures, though a majority of scientists have argued for their relationship to feathers as opposed to internal or structural fibers of the skin.