page_title: Sinosauropteryx
section_title: Identity of filaments
hierarchical_section_title: Sinosauropteryx / History of discovery / Identity of filaments
caption_attribution_description: English: Counterslab of the holotype of Sinosauropteryx prima on display at the Geological Museum of China.
context_page_description: Sinosauropteryx is a compsognathid dinosaur. Described in 1996, it was the first dinosaur taxon outside of Avialae to be found with evidence of feathers. It was covered with a coat of very simple filament-like feathers. Structures that indicate colouration have also been preserved in some of its feathers, which makes Sinosauropteryx the first non-avialian dinosaurs where colouration has been determined. The colouration includes a reddish and light banded tail. Some contention has arisen with an alternative interpretation of the filamentous impression as remains of collagen fibres, but this has not been widely accepted.
  Sinosauropteryx was a small theropod with an unusually long tail and short arms. The longest known specimen reaches up to 1.07 metres in length, with an estimated weight of 0.55 kilograms It was a close relative of the similar but older genus Compsognathus, both genera belonging to the family Compsognathidae. Only one species of Sinosauropteryx has been named: S. prima, meaning "first" in reference to its status as the first feathered non-avialian dinosaur species discovered. Three specimens have been described.
context_section_description: Controversy regarding the identity of the filaments preserved in the first 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 Sciences 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. Palaeontologist Alan Feduccia, who had not yet examined the specimen, wrote in Audubon Magazine that the structures of 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 palaeontologists were engaging in wishful thinking when equating the structures with feathers. Subsequent publications saw some of the team members disagreeing over the identity of the structures.
  Feduccia's frill argument was followed up in several other publications, in which researchers interpreted the filamentous impressions around Sinosauropteryx fossils as remains of collagen fibres rather than primitive feathers. Since the structures are clearly external to the body, these researchers have proposed that the fibres formed a frill on the back of the animal and underside of its tail, similar to some modern aquatic lizards. The absence of feathers would refute the proposal that Sinosauropteryx is the most basal known theropod genus with feathers, and also raise questions about 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 fibres. 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, because collagen does not contain pigment. Gregory S. Paul reidentified what the collagen hypothesis's proponents consider a body outline outside of the fibres as an artefact of preparation: breakage and brushed-on sealant have been misidentified as the outline of the body.
  The hypothesis that the structures were collagen fibers was closely analyzed and disproven by a 2017 paper published by Smithwick et al. The integument of Sinosauropteryx was closely compared to less controversial evidence of collagen fibers preserved in the ichthyosaur Stenopterygius. Although the collagen hypothesis claimed that the central shafts (rachises) of purported theropod feathers were actually misidentified examples of shaft-like collagen fibers, higher quality imagery showed that these similarities were artificial. The supposed shafts in ichthyosaur collagen were actually scratch marks, cracks, and crevasses created during preparation of one of the ichthyosaur specimens. On the other hand, the shafts in the Sinosauropteryx specimens were legitimate examples of fossilized structures. The collagen hypothesis also claims that Sinosauropteryx integument includes beaded structures similar to structures occasionally found in decaying collagen of modern sea mammals. However, this claim was also unsupported, with Smithwick et al. finding no evidence of the beaded structures which collagen hypothesis proponents identified on the specimens. The study proposes that some areas of the fossil preserved in three dimensions cast shadows which would have resembled beaded structures in low quality photographs.
  Other examples of purported collagen fibers in the tail area were revealed to be scratches, similar to those on the rest of the specimen. An area of the bone with an irregular surface was considered evidence that some collagen fibers were less decayed than others. However, Smithwick et al.'
caption: Counter slab of the holotype, on display at the Geological Museum of China
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