Sinornis santensis

''Sinornis'' is a genus of enantiornithean birds from the Lower Cretaceous Jiufotang Formation of the People's Republic of China.

When it was described in 1992, this 120 million-year-old sparrow-sized skeleton represented a new avian sharing "primitive" features with ''Archaeopteryx'' as well as showing traits of modern birds. Its basal features include, but are not limited to, a flexible manus with unguals, a footed pubis, and stomach ribs. ''Sinornis'' is known only from the type species, ''Sinornis santensis''. The generic name comes from the Latin ''Sino~'', 'China' and the Greek ''ornis'', 'bird'. The specific name ''santensis'' refers to the provenance from Chaoyang county in Liaoning Province as ''Santa'', meaning "Three Temples", is a traditional name of the county.

Description

The holotype skeleton of ''Sinornis'', BPV 538a-b, consists of a plate and counterplate of fine-grained freshwater lake sediment as proven by numerous fish, insect, and plant remains.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. The skeleton was found in the Jiufotang Formation at Liaoning, People's Republic of China. The skeleton exhibits remarkable basal features shared with ''Archaeopteryx'', a genus of early bird that is transitional between older 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 a similar pelvis with the latter genus, but its pelvic girdle has free elements unlike the fused ones found in modern birds. The iliac blades are erect and the ischium is blade-shaped rather than strap-shaped. Analogous to ''Archaeopteryx'' and older theropod dinosaurs, its pubis appears to be directed more ventrally and terminates distally in a hook-shaped "boot".

Skeleton

As in ''Archaeopteryx'', the skull of ''Sinornis'' has a proportionately short, toothed snout. There are broad nasal bones that expands caudally to the external nares, with a triangular caudal margin. The dorsal and central margins of the caudal half of the maxilla run parallel while its jugal ramus does not taper caudally.

The postcranial skeleton features a separate carpus and manus in the forelimb. The manus, hand, is composed of freely articulating metacarpals, with well-formed phalanges and unguals on the first and second digits. The thorax is closed at the underside, by gastralia. The pelvis has a footed pubis.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

Flight and perching

Derived bird traits in the skeleton of ''Sinornis'' are typically flight or perching adaptations. At a time when very few intermediary forms were known, the fossil seemed to provide an early glimpse of flight evolution, showing the intermediate evolutionary step between the "primitive" wings of ''Archaeopteryx'' to specialized wings of modern birds. As in modern birds, ''Sinornis'' had a modified wrist bone, with a groove that lets the wrist bend sharply back, tightly tucking the 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 support for bulky aerobic flight muscles.Chiappe, L., & Witmer, L. (2002). "The Mesozoic Aviary: Anatomy and Systematics". In ''Mesozoic birds above the heads of dinosaurs'' (pp. 184–206). Berkeley: University of California Press. It also had reduced claws and small hands with a stable second finger that anchored important flight feathers. Unlike the fused finger bones of modern birds, ''Sinornis'' had separate finger bones that were well adapted for flight, while the reduction of the tail moved the centre of gravity forwards. ''Sinornis'' was also capable of perching and climbing.

Discovery

Discovered by a Chinese farmer prior to 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 name and describe the skeleton of ''Sinornis''.

Before this find much of what scientists knew about the origin of birds came from ''Archaeopteryx'', a Jurassic species with a confusing mix of reptilian and bird-like traits. ''Sinornis'' shared many traits with modern birds while retaining certain basal characteristics of ''Archaeopteryx'', so that in 1992 it could be presented as the missing link in avian evolution.Barinaga, M. (1992). Evolutionists wing it with a new fossil bird. ''Science'', 255(5046), 796. There were other fossils discovered before ''Sinornis'' that were younger than ''Archaeopteryx'', but ''Sinornis'' skeleton was the most complete. The fossil of ''Sinornis'' was originally dated to about 135 million years, which seemed to show that about 15 million years after ''Archaeopteryx'' the transition to the modern wing was well underway. However, later research showed that the layers in which the fossil was found were in fact about 120 to 110 million years old, reducing the importance of ''Sinornis''. After the discovery of ''Sinornis'', older fossils were discovered that shared even more basal features with ''Archaeopteryx''.

Classification

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.

Paleobiology

''Sinornis'', the most complete known enantiornithean 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. Its coracoid expands distally to form broad, lengthened struts attached to the sternum. It also had a robust cranial rib and ossified sternal ribs. It has a V-shaped ulnare in the wrist for articulation with the metacarpus which allowed greater flexion during upstroke, important in small-bodied fliers for decreasing drag.Zhou, Zhonghe, Hou, Lianhai (2001) "The Discovery and Study of Mesozoic Birds in China" in ''Mesozoic Birds: above the heads of dinosaurs''. University of California Press. The presence of a fully opposable hallux with a particularly large ungual and the pedal claws being strongly recurved are indicators of an advanced perching function and shows that the bird lived primarily in an arboreal habitat.Chiappe, L. M., & Dyke, G. J. (2002). "The Mesozoic Radiation of Birds". ''Annual Review Of Ecology & Systematics'', 3391.

References

External links

Sinornis (''Time'' magazine)

{{Taxonbar, from=Q141496

Euenantiornitheans
Early Cretaceous birds of Asia
Fossil taxa described in 1992
Fossils of China
Taxa named by Paul Sereno