Yixianornis
Encyclopedia
Yixianornis is a bird
genus
from the early Cretaceous
period. Its remains have been found in the Jiufotang Formation
at Chaoyang (People's Republic of China
) dated to the early Aptian
age, around 120 million years ago. Only one species
, Yixianornis grabaui, is known at present. The species name, Grabau, is named after American Paleontologist Amadeus William Grabau
, who surveyed China in the early 20th Century.
, is one of the most well-preserved bird fossils known from the Jehol group. It is nearly complete and, unlike many other fossils, the bones are mostly uncrushed and were not split in half when the stone slabs were initially separated. It is also one of the few known Mesozoic ornithuran
bird specimens that preserve clear impressions of the wing and tail feathers.
Yixianornis was moderately sized compared to other yanornithiformes, at about the size of a chicken (20 centimetres (7.9 in) long excluding feathers), with a wingspan of about 40 cm (15.7 in). Yixianornis was grossly similar to its close relatives Yanornis and Songlingornis. All three had teeth, though in Yixianornis the tips of the jaws were toothless and pockmarked with small pits and grooves, possibly indicating the presence of a beak. The teeth were small and peg-like, and lacked any serrations. The lower jaw was thin and delicate.
The breastbone bore a strong keel for the attachment of flight muscles, and contained a distinct opening or fenestra, a unique characteristic of yanornithiformes. The upper and lower arm were about the same length. Like other ornithurines, Yixianornis had a highly fused hand, with many wrist bones joined together that are free in more primitive birds.
The hips and hind limbs preserved more primitive features than the forelimbs, supporting the idea that the modern adaptations in the locomotion seen in modern birds evolved after the many specializations needed for flight. The hallux, or reversed, perching toe of modern birds, in noticeably pointed backward in the fossil. However, there is no clear evidence that the bone itself was twisted, which would indicate that the toe was permanently reversed.
Yixianornis differed from related birds only in small skeletal characteristics. For example, the teeth in the lower jaw of Yixianornis were confined to a smaller space, while they lined more of the jaw in related species. The wishbone (furcula) of Yixianornis was more narrow than in its relatives, and its shoulder blade was much shorter, only half the length of the upper arm bone (humerus).
The flight feathers of Yixianornis are well preserved in the only known specimen. There were about five primary feathers on each wing, the longest measuring about 6.7 cm (2.6 in). Unlike in many more basal species, the feathers did not become narrower or more pointed at the tip. Rather, the feathers were very broad and rounded. There were eight tail feathers, up to 9.2 cm (3.6 in) long, anchored to a pygostyle
and rectrical bulb (see below) as in modern birds.
These adaptations are rare among Mesozoic birds, many of which are known to have had long, pointed wings and very few, if any, long tail feathers. The rectrical bulb and plough-shaped pygostyle allowing for tail fanning is also a unique characteristic of ornithurine birds, of which Yixianornis is among the earliest known. In a 2006 study, Julia Clarke, Zhou Zhonghe and Zhang Fucheng found that the ability to fan the tail, along with the broad wings, show that it probably preferred environments with dense vegetation, where high maneuverability in flight would be necessary.
Comparisons between the scleral ring
s of Yixianornis and modern birds and reptiles indicate that it may have been diurnal, similar to most modern birds.
and together with this and Songlingornis
forms a clade
of early modern birds. Clarke et al. found that Yixianornis was the most primitive bird to display an essentially modern pygostyle
and fan of tail feathers. Later, an enantiornithine called Shanweiniao
was found to have a fan tail as well, though it may have evolved independently of modern birds.
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
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...
from the early Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
period. Its remains have been found in the Jiufotang Formation
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...
at Chaoyang (People's Republic of China
People's Republic of China
China , officially the People's Republic of China , is the most populous country in the world, with over 1.3 billion citizens. Located in East Asia, the country covers approximately 9.6 million square kilometres...
) dated to the early 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...
age, around 120 million years ago. Only one species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
, Yixianornis grabaui, is known at present. The species name, Grabau, is named after American Paleontologist Amadeus William Grabau
Amadeus William Grabau
Amadeus William Grabau was a German-American paleontologist and geologist who was born on January 9, 1870 in Cedarburg, Wisconsin in the United States and died on March 20, 1946 in Peking, China. He was employed as faculty at MIT and Rensselaer Polytechnic Institute...
, who surveyed China in the early 20th Century.
Description
The type specimen (and only specimen found to date) of Yixianornis, catalog number IVPP V12631 in the collections of the Institute of Vertebrate Paleontology and PaleoanthropologyInstitute of Vertebrate Paleontology and Paleoanthropology
The Institute of Vertebrate Paleontology and Paleoanthropology of China is a prominent research institution and collections repository for fossils, including many dinosaur and pterosaurand cat poo specimens...
, is one of the most well-preserved bird fossils known from the Jehol group. It is nearly complete and, unlike many other fossils, the bones are mostly uncrushed and were not split in half when the stone slabs were initially separated. It is also one of the few known Mesozoic ornithuran
Ornithurae
Ornithurae is the name of a natural group which includes all modern birds as well as their extinct relatives with plough-shaped pygostyles, a bone at the end of the tail which allows the tail feathers to fan and retract....
bird specimens that preserve clear impressions of the wing and tail feathers.
Yixianornis was moderately sized compared to other yanornithiformes, at about the size of a chicken (20 centimetres (7.9 in) long excluding feathers), with a wingspan of about 40 cm (15.7 in). Yixianornis was grossly similar to its close relatives Yanornis and Songlingornis. All three had teeth, though in Yixianornis the tips of the jaws were toothless and pockmarked with small pits and grooves, possibly indicating the presence of a beak. The teeth were small and peg-like, and lacked any serrations. The lower jaw was thin and delicate.
The breastbone bore a strong keel for the attachment of flight muscles, and contained a distinct opening or fenestra, a unique characteristic of yanornithiformes. The upper and lower arm were about the same length. Like other ornithurines, Yixianornis had a highly fused hand, with many wrist bones joined together that are free in more primitive birds.
The hips and hind limbs preserved more primitive features than the forelimbs, supporting the idea that the modern adaptations in the locomotion seen in modern birds evolved after the many specializations needed for flight. The hallux, or reversed, perching toe of modern birds, in noticeably pointed backward in the fossil. However, there is no clear evidence that the bone itself was twisted, which would indicate that the toe was permanently reversed.
Yixianornis differed from related birds only in small skeletal characteristics. For example, the teeth in the lower jaw of Yixianornis were confined to a smaller space, while they lined more of the jaw in related species. The wishbone (furcula) of Yixianornis was more narrow than in its relatives, and its shoulder blade was much shorter, only half the length of the upper arm bone (humerus).
The flight feathers of Yixianornis are well preserved in the only known specimen. There were about five primary feathers on each wing, the longest measuring about 6.7 cm (2.6 in). Unlike in many more basal species, the feathers did not become narrower or more pointed at the tip. Rather, the feathers were very broad and rounded. There were eight tail feathers, up to 9.2 cm (3.6 in) long, anchored to a pygostyle
Pygostyle
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....
and rectrical bulb (see below) as in modern birds.
Biology and ecology
The feathers of Yixianornis are unique among those preserved in other Mesozoic bird specimens, and have allowed scientists to infer its probable lifestyle. The wings were broad and rounded, with similarly large, rounded feathers. The tail feathers were arranged in a graduated series, with the outer feathers anchored closer to the base of the tail. This would have given the tail a slightly rounded silhouette. Additionally, carbonized tissue impressions show that it had a rectrical bulb, the muscles around the tail which allow the tail feathers to be fanned out during flight, and retraced when at rest.These adaptations are rare among Mesozoic birds, many of which are known to have had long, pointed wings and very few, if any, long tail feathers. The rectrical bulb and plough-shaped pygostyle allowing for tail fanning is also a unique characteristic of ornithurine birds, of which Yixianornis is among the earliest known. In a 2006 study, Julia Clarke, Zhou Zhonghe and Zhang Fucheng found that the ability to fan the tail, along with the broad wings, show that it probably preferred environments with dense vegetation, where high maneuverability in flight would be necessary.
Comparisons between the scleral ring
Sclerotic ring
Sclerotic rings are rings of bone found in the eyes of several groups of vertebrate animals, except for mammals and crocodilians. They can be made up of single bones or small bones together. They are believed to have a role in supporting the eye, especially in animals whose eyes are not spherical,...
s of Yixianornis and modern birds and reptiles indicate that it may have been diurnal, similar to most modern birds.
Classification
It was a close relative of YanornisYanornis
Yanornis is an extinct genus of omnivorous Early Cretaceous birds, thought to be closely related to the common ancestor of all modern birds. One species, Yanornis martini,...
and together with this and Songlingornis
Songlingornis
Songlingornis is a prehistoric bird genus from the Early Cretaceous. Its fossils have been found in the Jiufotang Formation of Liaoning . The age of these rocks is somewhat disputed, but probably around the early Aptian, 125-120 million years ago...
forms a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
of early modern birds. Clarke et al. found that Yixianornis was the most primitive bird to display an essentially modern pygostyle
Pygostyle
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....
and fan of tail feathers. Later, an enantiornithine called Shanweiniao
Shanweiniao
Shanweiniao is an extinct genus of long-beaked enantiornithine bird from Early Cretaceous China. One species is known, Shanweiniao cooperorum. There is one known fossil, a slab and counterslab. The fossil is in the collection of the Dalian Natural History Museum, and has accession number DNHM...
was found to have a fan tail as well, though it may have evolved independently of modern birds.