Similicaudipteryx
Encyclopedia
Similicaudipteryx, meaning "similar to Caudipteryx
" (which means "tail feather"), is a genus
of theropod dinosaur
of the family
Caudipteridae
. Its fossil
remains were recovered from the Jiufotang
and Yixian Formation
s of northeastern China
, dating to the early Cretaceous Period (Aptian
stage) between 124 and 120 million years ago. The genus contains one described species, Similicaudipteryx yixianensis, which was named in 2008 by He, Wang, and Zhou. The name refers to its similarity to the closely related Caudipteryx
and its location in the Chinese Yixian beds.
(the bone at the end of the tail to which feathers anchor in birds) and several unique features of the back vertebrae. The only other oviraptorosaur reported to have a pygostyle is Nomingia
, though the feature is widespread in more advanced birds and appears to have evolved at least twice.
Additionally, the youngest specimen's vaned feathers appeared to lack barbs except at the tip, instead consisting of a solid sheet. Xu and colleagues interpreted the stark differences in the feathers of the two specimens as primarily age-related. They speculated that hatchling Similicaudipteryx would have been covered in natal down like modern birds. As the animal aged, the down would be replaced by vaned pennaceous feathers on the hands and tail, but ribbon-like and primitive in form, similar to the tail feathers of Confuciusornis
, Epidexipteryx
, and some enantiornithines
. These feathers would be lost through moulting as the animal aged, and replaced with more modern-style barbed feathers. The primary feathers grew more slowly than the tail feathers, not reaching equal size until the animal was close to maturity, and the secondary feathers would not appear at all until this more mature stage. This suggests that the wing feathers had little use at a young age, only becoming fully developed with maturity.
However, feather development specialist Richard Prum disputed the above interpretation of the feathers in a November 2010 letter to the journal Nature. Prum noted that the apparently ribbon-like structure of the juvenile's feathers were consistent with pennaceous feathers in the midst of moulting. In modern birds, new vaned feathers emerge from the feather follicle enclosed in a "pin feather", a solid tube covered in keratin. Usually, the tip of this tube will fall away first, leaving a structure identical to that seen in the Similicaudipteryx fossil. Later, the rest of the sheath falls away when the entire feather has fully developed. Prum also noted, as did Xu and his team, that the structure of the Similicaudipteryx feathers is fundamentally different from other prehistoric birds with ribbon-like tail feathers. In those other species, the ribbon portion is formed from a flattened and expanded rachis, or central quill, of the feather, with the feather barbs expanding out at the tip. In Similicaudipteryx, however, the "ribbon" like portion is the same width as the vaned tip. This is consistent with what is seen in feathers in the process of moulting. Prum concluded that rather than representing an instance of feathers changing in form as the animal aged, this specimen represents the first known fossil evidence of feather moulting.
Prum also noted that in modern birds, tail feathers moult sequentially, not simultaneously as in Similicauipteryx. However, the sequential moulting of modern birds is because the birds need to retain their ability to fly during the moult. For lineages more primitive than the advent of flight, like Similicaudipteryx, this would not have been an issue, and all the wing and tail feathers of primitive feathered theriopods may have moulted simultaneously.
Caudipteryx
Caudipteryx is a genus of peacock-sized theropod dinosaurs that lived in the Aptian age of the early Cretaceous Period . They were feathered and remarkably birdlike in their overall appearance....
" (which means "tail feather"), is a genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of theropod dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
of the family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
Caudipteridae
Caudipteridae
Caudipteridae is a family of oviraptorosaurian dinosaurs known from the Early Cretaceous of China. Found in the Yixian and Jiufotang Formations, the group existed between 125-120 million years ago. Distinguishing characteristics of this group have been indicated as including a unique dagger-shaped...
. Its fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
remains were recovered from the Jiufotang
Jiufotang Formation
The Jiufotang Formation is an Early Cretaceous geological formation in Chaoyang, Liaoning which has yielded fossils of feathered dinosaurs, primitive birds, pterosaurs, and other organisms. . It is a member of the Jehol group. The exact age of the Jiufotang has been debated for years, with...
and Yixian Formation
Yixian Formation
The Yixian Formation is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans 11 million years during the early Cretaceous period...
s of northeastern China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
, dating to the early Cretaceous Period (Aptian
Aptian
The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous epoch or series and encompasses the time from 125.0 ± 1.0 Ma to 112.0 ± 1.0 Ma , approximately...
stage) between 124 and 120 million years ago. The genus contains one described species, Similicaudipteryx yixianensis, which was named in 2008 by He, Wang, and Zhou. The name refers to its similarity to the closely related Caudipteryx
Caudipteryx
Caudipteryx is a genus of peacock-sized theropod dinosaurs that lived in the Aptian age of the early Cretaceous Period . They were feathered and remarkably birdlike in their overall appearance....
and its location in the Chinese Yixian beds.
Description
Similicaudipteryx is known from three specimens ranging from young juvenile to adult. It was a small oviraptorosaur similar to the closely related Caudipteryx. Like Caudipteryx, it had a short snout with a downcurved lower jaw. It had relatively short arms and long legs. Similicaudipteryx also differed from its relatives by possessing a dagger-shaped pygostylePygostyle
Pygostyle refers to a number of the final few caudal vertebrae fused into a single ossification, supporting the tail feathers and musculature. In modern birds, the rectrices attach to these....
(the bone at the end of the tail to which feathers anchor in birds) and several unique features of the back vertebrae. The only other oviraptorosaur reported to have a pygostyle is Nomingia
Nomingia
Nomingia is a genus of oviraptorid theropod dinosaur hailing from the Late Cretaceous Bugin Tsav Beds of Mongolia.-Discovery and naming:...
, though the feature is widespread in more advanced birds and appears to have evolved at least twice.
Feathers
The first Similicaudipteryx specimen lacked traces of feathers, but He and colleagues speculated that they were probably present based on its pygostyle, the anchor point of long tail feathers (rectrices) in modern birds. In 2010, two new specimens were described, both of which preserved feather traces. These specimens (both juveniles, though one closer to maturity than the other) showed that the feathers were similar to the related Caudipteryx, with long (symmetrical) vaned feathers on the hand and tail, and the rest of the body covered in simpler, downy feathers.Paleobiology
The nature of the feathers preserved in the two immature Yixian specimens of Similicaudipteryx appeared to Xu and colleagues, who described the two feathered specimens, to change with age. The youngest specimen had relatively short primary feathers (those anchored to the hand) compared to its tail feathers. In the older specimen, the primary feathers were the same length as the tail feathers, and secondary feathers (those anchored to the lower arm) were also present. The primary feathers may have grown more slowly than the tail feathers, not reaching equal size until the animal was close to maturity, and the secondary feathers would not appear at all until this more mature stage. This suggests that the wing feathers had little use at a young age, only becoming fully developed with maturity.Additionally, the youngest specimen's vaned feathers appeared to lack barbs except at the tip, instead consisting of a solid sheet. Xu and colleagues interpreted the stark differences in the feathers of the two specimens as primarily age-related. They speculated that hatchling Similicaudipteryx would have been covered in natal down like modern birds. As the animal aged, the down would be replaced by vaned pennaceous feathers on the hands and tail, but ribbon-like and primitive in form, similar to the tail feathers of Confuciusornis
Confuciusornis
Confuciusornis is a genus of primitive crow-sized birds from the Early Cretaceous Yixian and Jiufotang Formations of China, dating from 125 to 120 million years ago...
, Epidexipteryx
Epidexipteryx
Epidexipteryx is a genus of small avialan dinosaur, known from one fossil specimen in the collection of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. Epidexipteryx represents the earliest known example of ornamental feathers in the fossil record. The type specimen is...
, and some enantiornithines
Enantiornithes
Enantiornithes is an extinct group of primitive birds. They were the most abundant and diverse avialans of the Mesozoic. Almost all retained teeth and clawed fingers on each wing, but otherwise looked much like modern birds externally. Over 50 species of Enantiornithines have been named, but some...
. These feathers would be lost through moulting as the animal aged, and replaced with more modern-style barbed feathers. The primary feathers grew more slowly than the tail feathers, not reaching equal size until the animal was close to maturity, and the secondary feathers would not appear at all until this more mature stage. This suggests that the wing feathers had little use at a young age, only becoming fully developed with maturity.
However, feather development specialist Richard Prum disputed the above interpretation of the feathers in a November 2010 letter to the journal Nature. Prum noted that the apparently ribbon-like structure of the juvenile's feathers were consistent with pennaceous feathers in the midst of moulting. In modern birds, new vaned feathers emerge from the feather follicle enclosed in a "pin feather", a solid tube covered in keratin. Usually, the tip of this tube will fall away first, leaving a structure identical to that seen in the Similicaudipteryx fossil. Later, the rest of the sheath falls away when the entire feather has fully developed. Prum also noted, as did Xu and his team, that the structure of the Similicaudipteryx feathers is fundamentally different from other prehistoric birds with ribbon-like tail feathers. In those other species, the ribbon portion is formed from a flattened and expanded rachis, or central quill, of the feather, with the feather barbs expanding out at the tip. In Similicaudipteryx, however, the "ribbon" like portion is the same width as the vaned tip. This is consistent with what is seen in feathers in the process of moulting. Prum concluded that rather than representing an instance of feathers changing in form as the animal aged, this specimen represents the first known fossil evidence of feather moulting.
Prum also noted that in modern birds, tail feathers moult sequentially, not simultaneously as in Similicauipteryx. However, the sequential moulting of modern birds is because the birds need to retain their ability to fly during the moult. For lineages more primitive than the advent of flight, like Similicaudipteryx, this would not have been an issue, and all the wing and tail feathers of primitive feathered theriopods may have moulted simultaneously.