Temporal range: Early Cretaceous, 135 Ma
|Reconstruction of Sinornis|
Sereno & Rao, 1992
|Species:||† S. santensis|
Sereno & Rao, 1992
Sinornis is a genus of enantiornithine birds. The fossils recovered from the Lower Cretaceous lake deposits from the Jiufotang Formation at Liaoning, People's Republic of China. This 135 million-year-old sparrow-sized skeleton represent a new avian which shares primitive features with Archaeopteryx as well as modern birds. Its primitive features include, but are not limited to a flexible manus with unguals, a footed pubis, and stomach ribs. As well as shedding a new light on the early evolution of avian flight and perching that lead to modern birds. Sinornis is known only from the type species, Sinornis santensis, from the words sino, 'China'; ornis, 'bird'; and the word santensis, meaning "from three temples" (for the traditional Chinese name for Chaoyoung county in Liaoning Province where the bird skeletons were found).
The holotype skeleton of Sinornis is preserved on part and counterpart slabs of fine-grained freshwater lake sediment and is associated with abundant fish, insect, and plant remains. The skeleton remains found in Jiufotang Formation at Liaoning, People's Republic of China. The skeleton exhibits remarkable primitive features with Archaeopteryx, a genus of early bird that is transitional between feathered dinosaurs and modern birds. Until the discovery of Sinornis scientists did not know much about the evolution of flight that lead to modern birds because Archaeopteryx, which lived in the Late Jurassic period around 150 million years ago, lacks many of the modern flight and perching of modern birds. Some of the primitive features found in Sinornis include moderately recurved manual unguals, as opposed to the high-recurved one in Archaeopteryx. Sinornis shares similar pelvis with the latter genus, but its pelvic girdle is free unlike the ossified ones found in modern birds. The iliac blades are erect and the ischium that is blade-shaped rather than strap-shaped. Analogous to Archaeopteryx and theropod dinosaurs, its pubis appears to be directed more ventrally and terminates distally in a hook shaped.
As in Archaeopteryx, the skull of Sinornis has a proportionately short, toothed snout, with separate carpus and manus in the forelimb. Its skull and postcranial had short snout, teeth, and pelvic with footed pubis and gastralia. The manus is composed of freely articulating metacarpals, with well-formed phalanges and unguals on the first and second digits. Had a broad nasal that expands caudally to external nares, with a triangular caudal margin. The dorsal and central margins of caudal half of the maxilla are parallel with Jugal ramus does not taper caudally.
Flight and perching
Advanced avian characters in the skeleton of Sinornis are almost all directly related to flight or perching. The fossil provides early glimpse of flight evolution, showing the intermediate step between the evolution primitive wings of Archaeopteryx to specialized wings of modern birds. As seen in modern birds, Sinornis had modified wrist bone, with groove that lets the wrist bend sharply back, tightly tucked wings during flight or rest. Sinornis was capable of flight similar to modern birds based on breastbone and shoulder structures that provided both room and supports for bulky aerobic flight muscle. It also had reduced claws, small hands with steady middle finger that anchored important flight feather. Unlike the fused finger bone of modern birds, Sinornis had separate finger bones that were well adapted for flight, with the addition of short body tail making it easier to fly. Sinornis was also capable of perching and climbing.
Discovered by a Chinese farmer in 1992, the fossil of Sinornis sheds light on the evolutionary steps that transitioned birds into strong, specialized flyers. Paul Sereno of the University of Chicago and his colleague Rao Chenggang of the Beijing Natural History Museum were the first to describe the skeleton on Sinornis.
Before this find most of what scientists knew about the origin of birds came from Archaeopteryx, a fossil with an odd mix of reptilian and bird-like traits. Sinornis shared many traits with modern birds while retaining certain primitive characteristics of the Archaeopteryx, making it the missing link in avian evolution. The discovery of Sinornis' sparrow-sized skeleton, showed advanced features found in modern birds as well as primitive features it shares with Archaeopteryx. There were other fossils discovered before Sinornis that were younger than Archaeopteryx, but Sinornis skeleton was the most complete. After the discovery of Sinornis, younger fossils were reported but they shared minimal primitive features with Archaeopteryx. The fossil of Sinornis showed that about 15 million years after Archaeopteryx the transition to the modern wing was well under way.
Paul Sereno et al. (2001) considered a similar prehistoric bird species from the same formation, Cathayornis, to be a junior synonym of Sinornis. They interpreted the anatomies of the two as very similar and sharing key autapomorphies of the pygostyle.
However, also in 2001, Zhou and Hou continued to distinguish Cathayornis from Sinornis by the former's larger size, a shorter, straighter, finger number I, with a slightly longer claw (ungual), the absence of an atitrochanter, and other features. A paper describing a second species of Cathayornis in 2008 by Li et al. also considered the genera to be distinct.
The first thorough review of Sinornis and Cathayornis was published by Jingmai O'Connor and Gareth Dyke in 2010. O'Connor and Dyke concluded that despite the 2001 paper by Sereno and colleagues, the two birds were not synonyms and in fact differ in several clear ways, including different proportions in the wing claws and digits, differences in the pelvis, and size of the pygostyle.
The holotype skeleton of both Cathayornis y. and Sinornis was discovered in China, but in different regions. Their skeletons are small, so they were preserved similarly using molds and casts to facilitate the study the specimens.
Sinornis, the most complete known enantiornithine at the time, provides a detailed view of basal avian characteristics. Based on features present on the preserved skeleton of Sinornis, it shared similarity in flight performance and perching capabilities to sparrow-sized birds living today in arboreal habitats. Its thorax is strengthened to resist forces generated by an increase in pectoral muscle mass. As well as its coracoid expands distally to form broad, lengthened struts to the sternum. It also had a robust cranial rib and ossified sternal ribs. It has V-shaped ulnare for the metacarpus allowed greater flexion at the wrist during upstroke, which is important in small bodied fliers for decreasing drag. The presence of fully opposable hallux with a particularly large ungula and the pedal claw being strongly recurved are indicators of advanced perching function and lived primarily in an arboreal habitat.
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- Sereno, P. C., & Rao, C. (1992). "Early evolution of avian flight and perching: New evidence from the lower Cretaceous of China". Science, 255(5046), 845.
- Dyke, G.J. & Nudds, R.L. 2009: "The fossil record and limb disparity of enantiornithines, the dominant flying birds of the Cretaceous". Lethaia, Vol. 42, pp. 248–254.
- O'Connor, J. and Dyke, G. (2010). "A reassessment of Sinornis santensis and Cathayornis yandica (Aves: Enantiornithes)." Records of the Australian Museum, 62: 7–20. doi:10.3853/J.0067-1975.62.2010.1540
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