Sinosauropteryx

Sinosauropteryx (meaning "Chinese lizard-wing" ) is the first and most primitive genus of dinosaur found with the fossilized impressions of feathers. It lived in China during the Early Cretaceous period and may have been a close relative of Compsognathus. It was the first dinosaur genus discovered in the famous Liaoning Province. The largest known specimens are 1-1.20 meters (3 ft) in length, most of which was taken up by its extremely long tail. 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.

Three specimens of Sinosauropteryx prima are known: the holotype GMV 2123 (NIGP 127586), NIGP 127587, D 2141. The assignment of a fourth, larger specimen to S. prima was later found to be in error.

Description
Sinosauropteryx prima 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).

Classification
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, this specimen differs in several anatomical aspects from the others, including its relatively large size, proportionally longer tibiae, and shorter tail. In a later paper, Ji, Ji and colleagues changed their opinion and suggested GMV 2124 is probably a new taxon. In 2007, Gishlick and Gauthier supported their conclusion and tentatively re-classified this specimen as ''Sinosauropteryx? sp.'', though it may belong in 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 + modern birds. The scientists who discovered and described Sinosauropteryx, however, used a character-based (apomorphy) definition of the Class Aves, that is, any animal with feathers is a bird. They argued that the filamentous plumes of Sinosauropteryx represent true feathers with a rachis and barbs, and therefore 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.

Diet
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 they 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 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.

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 beyong 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.

Overall, the filaments most closely resemble the "plumules" or down-like feathers of some modern birds, with a very short quill and long, thin barbs. The same structures are seen in other fossils from the Yixian Formation, including Confuciusornis.

Controversy
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.