The Yixian Formation (simplified Chinese: 义县组; traditional Chinese: 義縣組; pinyin: Yìxiàn zǔ) is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans 11 million years during the early Cretaceous period. It is known for its exquisitely preserved fossils, and is mainly composed of basalts interspersed with siliciclastic sediments.
Stratigraphic range: Barremian–Aptian
|Unit of||Jehol Group|
|Overlies||Dabeigou Formation?, Tuchengzi Formation|
Yixian Formation (China)
The potential importance of the Yixian Formation was initially recognized during the time the Empire of Japan occupied China's Rehe ("Jehol") Province after the First battle of Hopei in 1933. Many Japanese scientists had noticed fossil remains of extinct fish and reptiles, possibly the champsosaurs. These initial fossil discoveries made by Japanese scientists vanished once World War II ended in 1945.
By 1949, when administration of the area passed to the Chinese Communist Party and its leader Mao Zedong, the fossils of Yixian were studied only by Chinese scientists. It was not until the 1990s when remarkable fossils of birds and dinosaurs were excavated. Since 1996, a number of dinosaur fossils that have revolutionized knowledge of these animals have been found at Yixian; among them are the first known non-avian theropods with feathers. See Jehol Biota.
For some time, the formation was believed to be from the Late Jurassic–Early Cretaceous boundary, some 145 mya (million years ago). Radiometric dating has since resolved it to be younger; it is now considered to have been deposited in the Barremian to early Aptian, some 125–121 mya.
The Yixian Formation forms the lowest part of the Jehol group, defined by Gu (1962 and 1983) as a group of geological formations including the Jehol Coal-bearing Beds, the Jehol Oil Shale Beds, and the Jehol Volcanic Rocks. The Yixian Formation is preceded by the older Daohugou Beds, of uncertain Jurassic or Early Cretaceous age, which are sometimes considered part of the Jehol group. The Yixian Formation (including the synonymous Jingangshan, Tuhulu, Jianchang, Lower Volcanic and Volcanic Rock formations) is followed stratigraphically by the slightly younger Jiufotang Formation and the Fuxin Formation. Chiappe et al. argued in 1999 that the oldest beds of the Yixian (those bearing a fauna dominated by confuciusornithid birds) were best separated as a distinct formation, called the Chaomidianzi Formation, with a type locality at the village of Sihetun, approximately 25 km south of Beipiao City. However, this classification has fallen out of favor, and the Chaomidianzi Formation is disused as a synonym for the Jianshangou Bed of the Yixian Formation.
The Dabeigou Formation in Fengning, Hebei Province may immediately precede the Yixian, or it may be equivalent to the oldest Yixian beds. The Yixian also overlies the Tuchengzi Formation in places.
The Yixian Formation is divided into the following subunits, from most recent to oldest: (ages when available from Chang et al., 2009)
New high-precision 40Ar/39Ar dating provided ages of 125.8 ± 1.0 Ma and 126.0 ± 0.8 Ma for two basaltic samples from the Lujiatun Unit.
The Yixian Formation represents the second of three major faunal phases that characterize the Jehol Biota, mainly based on changes in invertebrate diversity. In the Yixian, ostracods (seed shrimp) had diversified considerably, despite a very low diversity in the earlier Dabeigou Formation. Other major invertebrate groups in the Yixian include clam shrimp and insects. Insects, as a group, experienced their largest diversification of the entire Mesozoic era in the Yixian. On the other hand, some invertebrate groups, such as bivalves and gastropods (snails and slugs), were numerous but low in diversity, being mainly represented by one or two dominant species (Arguniella in the case of the bivalves).
Studies of vertebrates have shown support for the division of the Jehol into phases, and the diversity of fish in the Yixian was distinct from older and younger formations, with Lycoptera as the dominant species. The Yixian preserves the first Jehol dinosaurs and pterosaurs (which have not been found in the older Dabeigou Formation), and the first major radiation of birds (only one bird species is known from the Dabeigou). The Yixian also preserves the largest (and only) mammal radiation so far known from the Jehol group. Most vertebrates showed a tendency to climb trees or become arboreal, including many tree-dwelling birds, and climbing mammals and lizards.
Plant life reached its Jehol biota peak in the Yixian. Five species of flowering plant were present (three of Archaefructus, one of Archaeamphora and one of Hyrcantha (formerly Sinocarpus), as were a variety of horsetails that closely resembled modern species. It is possible that increasing animal and plant diversity were linked. The Yixian is characterized by extensive forests, dominated by trees such as ginkgoes, conifers, cycads, and seed fern trees. Ground cover plants included lycopods, horsetails, ferns, and primitive flowering plants, which were rare compared to the others.
This plant life grew around a series of freshwater lakes, provided with abundant minerals thanks to periodic volcanic eruptions. Volcanic activity, along with periodic wildfires, and noxious gasses released from the lake bottoms caused the ecosystem to be continually destroyed and regrown. This, along with the wide diversity of habitats in the surrounding region, may have contributed to the fast diversification of life forms present in the Yixian ecosystem.
With the diversity of plant life in the Yixian well known, including examples of a variety of petrified wood and growth rings, and with the help of chemical analysis, scientists have been able to determine the climate of the formation. The Yixian flora was dominated by conifers closely related to modern species that are found mainly in subtropical and temperate upland forests. The presence of ferns, cycads, and horsetails indicates a generally humid climate. However, evidence from the growth rings of petrified wood indicates that the humidity and water supply dropped regularly. This shows that the wet, humid conditions were punctuated by dry seasons, in which the environment became more arid. Evidence from the study of oxygen isotopes has shown that the average yearly temperature during this time period was 10 degrees celsius (50 degrees Fahrenheit), significantly colder than once thought. This indicates a temperate climate with unusually cold winters for the generally warm Mesozoic era, possibly due to northern China's high latitude during this time. A study by Wu et al. (2013) concluded that orbital forcing, which is the effect on climate caused by shifts in the tilt of the Earth's axis and by the shape of the Earth's orbit, contributed to the climate fluctuations of this formation.
The Yixian Formation is well known for its great diversity of well-preserved specimens and its feathered dinosaurs, such as the large tyrannosauroid Yutyrannus, the therizinosaur Beipiaosaurus and various small birds, along with a selection of non-theropod dinosaurs, such as Bolong, Dongbeititan and Psittacosaurus. Despite popular assumption, Microraptor does not hail from this formation, instead hailing from the younger Jiufotang Formation. However, other microraptorines, such as Sinornithosaurus and Graciliraptor, did indeed inhabit the Yixian. Other biota included the troodontid Mei, the dromaeosaurid Tianyuraptor, the large compsognathid Sinocalliopteryx and the tyrannosauroid Dilong.
Akidolestes cifellii is a genus of spalacotheriid mammal preserved with a complete post-cranium and a partial skull has been discovered from the Yixian Formation of Liaoning, China. The holotype of Akidolestes cifellii is reserved in Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. The holotype has a complete skeleton with partial skull and dentition. It is notable in that it displays characteristics of monotremes but appears to be more related to modern therian mammals.Akidolestes has no modern relatives. It is an early offshoot of mammal related to therians (the subclass containing marsupials and placentals). It clearly belongs within a group of theriiform mammals known as the Spalacotherioidea. Unlike other members of this superfamily, however, Akidolestes has some very prototherian features.
The genus name, Akidolestes, is derived from akido, Greek for point, and lestes, Greek for thief. Akido- refers to the pointed snout and -lestes is a common suffix for fossil mammals. The specific epithet, cifelli, is in honor of Richard L. Cifelli, a prominent researcher in prehistoric mammals.Although it revealed some similar features to monotremes in the lumbar vertebrae, pelvis and hindlimb, Akidolestes cifellii is still placed in family Spalacotheriidae and close to Zhangheotherium and Maotherium. Those convergent synapomorphies might from shared early common ancestor. According to the analysis and comparison of anatomy and locomotory features about Akidolestes cifellii to its related taxon, there is a hypothesis that spalacotheroids might evolved earlier in Eurasia and then dispersed to North America, which is consistence with a common geo-dispersal patterns based on several mammalian groups during the Early Cretaceous period.Most fossils of Mesozoic mammals are reserved with teeth or jaw fragment only. Therefore, it is worth noting that Akidolestes cifellii is the third spalacotheroid species discovered with a complete skeleton in the Yixian Formation after Zhangheotherium and Maotherium.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 113.0 ± 1.0 Ma (million years ago), approximately. The Aptian succeeds the Barremian and precedes the Albian, all part of the Lower/Early Cretaceous.The Aptian partly overlaps the upper part of the regionally used (in Western Europe) stage Urgonian.
The Selli Event, also known as OAE1a, was one of two oceanic Anoxic events in the Cretaceous period, which occurred around 120 Ma and lasted approximately 1 to 1.3 million years. The Aptian extinction was a minor extinction event hypothesized to have occurred around 116 to 117 Ma.Beipiaognathus
Beipiaognathus (meaning Beipiao jaw) is a dubious genus of coelurosaurian theropod from the Early Cretaceous Yixian Formation of Liaoning, China.The genus was initially assigned to the Compsognathidae based on the presence of two traits: fan-shaped dorsal neural spines and a robust I-1 phalanx on the hand. However, it also differs from other compsognathids in several ways: the teeth are unserrated and conical; the ulna is proportionally longer; the II-1 phalanx on the hand is longer and more robust; and the tail is much shorter.However, Andrea Cau has informally noted a number of points in the fossil that are indicative of it having been artificially assembled, thus rendering the specimen a phylogenetically uninformative chimaera.Bolong
Bolong (meaning "Bo's dragon") is a genus of iguanodontian dinosaur known from the Early Cretaceous-age Yixian Formation of western Liaoning Province, China. It lived about 125 million years ago in the earliest Aptian.Boreopteridae
Boreopteridae (meaning "northern wings") is a group of ornithocheiroid pterosaurs from the Aptian-age Lower Cretaceous Yixian Formation of Liaoning, China.Dongbeititan
Dongbeititan is a genus of sauropod dinosaur from the Early Cretaceous-age Yixian Formation of Beipiao, Liaoning, China. It is based on holotype DNHM D2867, a partial postcranial skeleton including bones from the limbs, shoulder and pelvic girdles, and vertebrae. Its describers suggested it was as a basal titanosauriform, not as derived as Gobititan or Jiutaisaurus, but more derived than Euhelopus, Fusuisaurus, and Huanghetitan. The type species is D. dongi, and it is the first named sauropod from the Yixian Formation, which is part of the well-known Jehol Group. The genus name refers to the region Dongbei and to Greek titan, "giant". The specific name honours the Chinese paleontologist Dong Zhiming. Like other sauropods, Dongbeititan would have been a large quadrupedal herbivore.Eoenantiornis
Eoenantiornis is a genus of enantiornithean birds which lived during the early Cretaceous period (124.6 Ma ago). It is known from a single fossil specimen found in the Yixian Formation in Liaoning province, China.
In 1999, the type species Eoenantiornis buhleri was named and described by Hou Lianhu, Larry Martin, Zhou Zhonghe and John Alan Feduccia. The generic name combines a Greek ἠώς, èos, "dawn" with Enantiornis, in reference to a presumed more basal position in relation to that genus. The specific name honours the late German paleornithologist Paul Bühler.The holotype, IVPP V11537, was found at Heitizigou in Liaoning in a layer of the lower Yixian Formation dating from the early Aptian. It consists of a nearly complete and articulated skeleton with skull compressed on a plate, preserving most of the feather integument. It represents a not fully grown individual.In 2005, Eoenantiornis was completely redescribed.Originally, the species was placed in a "family" Eoenantiornithidae and even an "order" Eoenantiornithiformes. In 2005, these concepts were abandoned and it was concluded that the position was in the clade Euenantiornithes. It is one of the oldest known advanced enantiornithines.Eosipterus
Eosipterus is an extinct genus of pterosaur from the Early Cretaceous of Liaoning, China.
The genus was named in 1997 by Ji Shu'an and Ji Qiang. The type species is Eosipterus yangi. The genus name is derived from Greek eos, "dawn" and Greek pteron, "wing" with a Latin ending; and a grammar error: normally the combination would have resulted in "eopterus". The "dawn" element refers to its age but also to China being "in the east". The specific name honours paleontologist Yang Daihuan.
The genus is based on holotype GMV2117, found near Jinggangshan in western Liaoning Province, in the Yixian Formation. It was the first pterosaur discovered in that region. It consists of a partial crushed skeleton of a subadult individual on a slab, lacking skull and neck. Most vertebrae have been severely damaged and even their number cannot be determined. The authors state that eighteen detached belly ribs are present in the matrix.
The wings are robust and elongated. The wing finger has the standard four phalanges, a difference from the possibly related Beipiaopterus which has lost the fourth phalanx. Total wingspan was about 1,2 metres. The pelvis is not well preserved. The femur has a length of six centimetres; the tibia of 96 millimetres. The fibula is strongly reduced. The foot claws are slightly curved; the fifth toe has been reduced to a single claw.
The authors placed Eosipterus in a general Pterodactyloidea incertae sedis; in 1999 a placement within Pterodactylidae was suggested and even a synonymy with Pterodactylus within a hypothesis that the lower Yixian Formation dated from the late Jurassic. A cladistic study in 2006 found that it was a member of the Ctenochasmatidae — David Unwin thought it more precisely belonged to the Ctenochasmatinae — but a later study showed that it was basal to the Germanodactylidae.Gegepterus
Gegepterus was a genus of ctenochasmatid pterosaur from the Early Cretaceous-age Yixian Formation of Liaoning, China.
The genus was named in 2007 by Wang Xiaolin, Alexander Kellner, Zhou Zhonge and Diogenes de Almeida Campos. The type species is Gegepterus changi. The genus name is derived from Manchu ge ge, the title of a princess, in reference to the dainty gracility of the specimen, and a Latinised Greek pteron, "wing". The specific name honours female paleontologist Chang Meemann, who over the years established a cordial relationship between the Chinese and Brazilian authors. In 2008 Wang emended the epithet to changae, but such changes are no longer allowed by the ICZN.
It is known from two specimens. The first is holotype IVPP V 11981, which was in 2001 found in grey shales from the lower part of the formation (estimated at 125 million years old), near the city of Beipiao. It consists of a crushed and damaged partial skeleton of a subadult including skull, lower jaws, cervical and sacral vertebrae, ribs, gastralia ("belly ribs"), shoulder girdle and hindlimb remains, along with dark soft tissue remains near the skull and gastralia and in the orbit; unfortunately, the soft tissue remains show no structure except for some small, unbranched fibers at the back of the head. The jaws are very elongated; the snout is flat and concave on top, with a low and thin crest. The forehead slightly projects to the front. The cervicals are elongated.The authors assigned it to the Ctenochasmatidae on the basis of its long rostrum and numerous needle-like teeth, about 150 in total. This is the first uncontroversial report of the Ctenochasmatidae from the Yixian Formation, as the fossils of other assumed ctenochasmatids have not preserved the dentition. It was at first suspected to be the juvenile of some known species.In 2011 a second, smaller specimen was described, IVPP V 11972, which increased the known skeletal elements and showed a more extensive covering of hair-like structures.Hongshanornithidae
Hongshanornithidae is an extinct group of early ornithuromorph birds from the early Cretaceous period of China. It includes the genera Hongshanornis (the type genus) and Tianyuornis from the Yixian Formation of Inner Mongolia, Longicrusavis from the Yixian Formation of Liaoning Province, Parahongshanornis from the Jiufotang Formation of Liaoning Province, and Archaeornithura, the oldest known member, from the Huajiying Formation of Hebei Province.Jixiangornis
Jixiangornis is a genus of primitive euavialans from the Early Cretaceous. Like later avialans, it had no teeth, but it also had a long tail, unlike modern birds. Since teeth were still present in some more advanced short-tailed avialans, Jixiangornis seems to have evolved its toothlessness independently of modern birds. The long forelimb (131% of hindlimb length) indicates at least some aerial ability. Jixiangornis is currently known only from a single specimen, a complete but juvenile skeleton. The fossil was found in the Yixian Formation near Beipiao City, western Liaoning, China.Liushusaurus
Liushusaurus is an extinct genus of lizard described by Susan E. Evans and Yuan Wang in 2010. The genus has a single species, Liushusaurus acanthocaudata, and is known from eight fossils, several of which preserve soft tissue detail. The specimens were found in the Lower Cretaceous aged Yixian Formation of Northeast China. Liushusaurus is one of eight lizards that are known and have been named from the Yixian Formation, part of the diverse Jehol Biota ecosystem.Manchurochelys
Manchurochelys is an extinct genus of turtle in the order Paracryptodira. It existed during the early Cretaceous of what is now northeast China. It has been found in the Jianshangou Bed of West Liaoning's Yixian Formation. However, it is a rarely found fossil.
Manchurochelys was first named by Endo and Shikama in 1942, and contains the single species, M. manchoukuoensis (sometimes misspelled M. manchouensis). A second species, M. liaoxensis, was named in 1995 but was later shown to be a species of Ordosemys. Manchurochelys was a relative of the modern-day snapping turtle. It has been occasionally placed in the family Sinemydidae, although it is said to more likely belong in the family Macrobaenidae.Ningyuansaurus
Ningyuansaurus is a basal oviraptorosaurian dinosaur genus. It contains the single species Ningyuansaurus wangi, known from a fossil specimen from the Early Cretaceous Yixian Formation (Aptian stage, 124.6 Ma ago) of Jianchang, western Liaoning Province, People's Republic of China. It is thought to be the basalmost species of oviraptorosaur, based on its long skull and a greater number of teeth in comparison to any other known oviraptorosaur. The generic name Ningyuansaurus is derived from Ningyuan, an ancient name for Xingcheng City. The specific name honors Wang Qiuwu, the private owner of the specimen who donated it for scientific study. The specimen is currently housed in the Confuciusornis Museum in Xingcheng.Paraprotopteryx
Paraprotopteryx is a genus of enantiornithean birds from the Mesozoic of China.In 2007, the type species Paraprotopeteryx gracilis was named and described by Zheng Xiaoting, Zhang Zihui and Hou Lianhai. The generic name means "near Protopteryx", in reference to a presumed similarity with that genus. The specific name is intended to mean "pretty".The holotype is specimen STM V001. It consists of a skeleton with skull on a plate and counterplate. The investigation preceding the description of the species proved that fossil traders had added the skull of a different individual to the torso. The description is based on the rump parts. Feathers have been preserved. The rump represents a subadult individual.Though initially reported to be from the Early Cretaceous Yixian Formation, later investigation showed that the fossil actually came from the Qiaotou Member of the Huajiying Formation of Fengning, Hebei Province, China, and is therefore of uncertain age. While much of the Huajiying Formation underlies the Yixian Formation, Ji and colleagues suggested in 2008 that the Qiaotou Member correlates with the Dawangzhangzi beds of the Yixian Formation, dated to approximately 122 million years ago by Zhou Zhonghe in 2006.Sinornithosaurus
Sinornithosaurus (derived from a combination of Latin and Greek, meaning 'Chinese bird-lizard') is a genus of feathered dromaeosaurid dinosaur from the early Cretaceous Period (early Aptian) of the Yixian Formation in what is now China. It was the fifth non–avian feathered dinosaur genus discovered by 1999. The original specimen was collected from the Sihetun locality of western Liaoning. It was found in the Jianshangou beds of the Yixian Formation, dated to 124.5 million years ago. Additional specimens have been found in the younger Dawangzhangzi bed, dating to around 122 million years ago.Xu Xing described Sinornithosaurus and performed a phylogenetic analysis which demonstrated that it is basal, or primitive, among the dromaeosaurs. He has also demonstrated that features of the skull and shoulder are very similar to Archaeopteryx and other Avialae. Together these two facts demonstrate that the earliest dromaeosaurs were more like birds than the later dromaeosaurs were.
Sinornithosaurus was among the smallest dromaeosaurids, with a length of about 90 centimetres (3.0 ft). In 2010, Gregory S. Paul gave higher estimations of 1.2 metres and three kilogrammes.Sinusonasus
Sinusonasus is a genus of dinosaurs from the Early Cretaceous Period, recovered from the Yixian Formation. It lived in what is now the Liaoning Province of China. Sinusonasus was a theropod, specifically a troodontid dinosaur.
The type species, Sinusonasus magnodens, was named and described by Xu Xing and Wang Xiaolin in 2004. The generic name, derived from Latin sinus, "wave", and nasus, "nose", refers to the sinusoid form, in lateral view, of the nasals. The specific name means "big-toothed" from Latin magnus, "large" and dens, "tooth". In a later publication the species is referred to as "Sinucerasaurus" but this is a junior objective synonym.
The holotype, IVPP V 11527, was found in the Lujiatun Member of the Yixian Formation, dating from the Hauterivian. It consists of a partial skeleton including skull and lower jaw fragments and partial tail, pelvis and hindlimbs. The fossil is compressed and partially articulated.Sinusonasus is a small troodontid. In 2010, Gregory S. Paul estimated its length at one metre, its weight at 2.5 kilogrammes. The femur is 141 millimetres long.In 2004, several distinguishing traits were established. An interantorbital channel, connecting the antorbital fenestrae at each skull side, is lacking. The nasal bone has an undulating profile. The middle maxillary teeth are rather large. The chevrons on the rear caudal vertebrae are so long, measured from the front to the back, that they connect, forming a continuous plate at the underside of the tail. The neck of the thighbone is elongated.More generally, the head is relatively short, equalling 77% of the length of the thighbone. There are at least nineteen maxillary teeth per side. The front teeth are not serrated: those more to the rear only have denticles at the trailing edge. Five sacral vertebrae are present; the tail probably consisted of about thirty vertebrae. The pubic bone probably pointed obliquely to the front. The ischium is elongated. Sinusonasus has a long lower leg, indicating a good running capacity. The foot is "arctometatarsal", with a 'pinched' upper third metatarsal. The second metatarsal is distinctly shorter than the fourth. The second toe bears a retractable 'sickle claw'.Sinusonasus was in 2004 placed in the Troodontidae. It was presumed to have had a rather derived position, despite living in the Early Cretaceous. This was by the describing authors not interpreted as an indication for a long ghost lineage, troodontids developing earlier during the Jurassic than had been thought, but explained by rapid evolutionary change after a Cretaceous origin of the group.Yanoconodon
Yanoconodon is a monotypic genus of extinct early mammal whose representative species Yanoconodon allini lived during the Mesozoic in what is now China. The holotype fossil of Yanoconodon was excavated in the Yan Mountains about 300 kilometres from Beijing in the Qiaotou member of the Huajiying Formation (which the original authors considered part of the Yixian Formation) of Hebei Province, China, and is therefore of uncertain age. The Qiaotou Member may correlate with the more well-known Early Cretaceous Yixian Formation, and so probably dates to around 122 Ma ago.Yanoconodon was a eutriconodont, a group composing most taxa once classified as "triconodonts" which lived during the time of the dinosaurs. These were a highly ecologically diverse group, including large sized taxa such as Repenomamus that were able to eat small dinosaurs, the arboreal Jeholodens, the aerial volaticotherines and the spined Spinolestes. Yanoconodon is inferred to be a generalized terrestrial mammal, capable of multiple forms of locomotion.Yanoconodon's name is composed of two elements: 'Yan' is taken from the Yan Mountains in the north of the Hebei Province near where the holotype of Yanoconodon was found; 'Conodon' is an often used as a mammalian taxonomic suffix meaning 'cuspate tooth'. Its species name, "allini," is derived from mammalian researcher Edgar Allin, who was notable for his research on the mammalian middle ear.Zhenyuanopterus
Zhenyuanopterus is a genus of pterosaur which is known from Lower Cretaceous (early Aptian) Yixian Formation of Liaoning, China. It contains one species, Zhenyuanopterus longirostris, which was first described and named by Lü Junchang.