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''Avimaia'' is a genus of
fossil birds Birds evolved from certain feathered theropod dinosaurs, and there is no real dividing line between birds and non-avian dinosaurs except that some of the former survived the Cretaceous–Paleogene extinction event while the latter did not. For ...
of the
Enantiornithes The Enantiornithes, also known as enantiornithines or enantiornitheans in literature, are a group of extinct avialans ("birds" in the broad sense), the most abundant and diverse group known from the Mesozoic era. Almost all retained teeth and cla ...
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...
that lived about 115 million years ago in Northwest China. The only known species is ''A. schweitzerae''. The holotype fossil of the bird was found in the
Xiagou Formation The Xiagou Formation is the middle strata of the Xinminbao Group. It is named for its type site in Xiagou, in the Changma Basin of Gansu Province, northwestern China and is considered Early Cretaceous in age. It is known outside the specialized ...
, and is noted as the first discovered fossil bird with an unlaid egg. Abnormalities, including
egg binding Egg binding occurs in animals, such as reptiles or birds, when an egg takes longer than usual to pass out of the reproductive tract. In birds In birds, egg binding may be caused by obesity, nutritional imbalances such as calcium deficiency, envi ...
in which the egg becomes stuck within the body of the bird causing death, were found in the egg suggesting that the preserved egg may have caused this bird's demise. Egg binding is a serious and lethal condition that is fairly common in small birds undergoing stress. According to paleontologist Jasmina Wiemann of Yale University, “This is a spectacular fossil with a lot of potential for future paleobiological investigations." Further, "This new specimen is arguably one of the most interesting Cretaceous fossil birds yet discovered, providing more reproductive information than any other Mesozoic fossil bird," according to a paleontology researcher at the Chinese Academy of Sciences.


Discovery and naming

In 2006, at
Changma The East Asian Monsoon is a monsoonal flow that carries moist air from the Indian Ocean and Pacific Ocean to East Asia. It affects approximately one-third of the global population, influencing the climate of Japan, the Korean Peninsula, Taiwan, Ch ...
in
Gansu Gansu (, ; alternately romanized as Kansu) is a province in Northwest China. Its capital and largest city is Lanzhou, in the southeast part of the province. The seventh-largest administrative district by area at , Gansu lies between the Tibet ...
the skeleton was found of an enantiornithean bird. In several studies it was referred to with its field number DCAGS-IG-06-CM-012 or FDRC-06-CM-012. In 2019, the type species ''Avimaia schweitzerae'' was named and described by Alida M. Bailleul, Jingmai Kathleen O’Connor, Zhang Shukang, Li Zhiheng, Wang Qiang, Matthew C. Lamanna, Zhu Xufeng and Zhou Zhonghe. The generic name is a combination of the Latin ''avis'', "bird", and Maia, a mother goddess, in reference to the find of an egg in the abdomen of the fossil. The
specific name Specific name may refer to: * in Database management systems, a system-assigned name that is unique within a particular database In taxonomy, either of these two meanings, each with its own set of rules: * Specific name (botany), the two-part (bino ...
honours
Mary Higby Schweitzer Mary Higby Schweitzer is an American paleontologist at North Carolina State University, who led the groups that discovered the remains of blood cells in dinosaur fossils and later discovered soft tissue remains in the ''Tyrannosaurus rex'' specime ...
, one of the founders of the application of molecular biology in paleontology. The holotype, IVPP V25371, was found in a layer of the lower-middle
Xiagou Formation The Xiagou Formation is the middle strata of the Xinminbao Group. It is named for its type site in Xiagou, in the Changma Basin of Gansu Province, northwestern China and is considered Early Cretaceous in age. It is known outside the specialized ...
. It consists of a partial skeleton lacking the skull, compressed on a single plate. It conserves the rear half of the body. It is articulated and visible from the underside. Parts of the plumage and an egg are preserved. A second specimen, CAGS-IG-04-CM-007, was referred to the species. It is a partial skeleton lacking the skull. It is not articulated and preserves parts of the pelvis and the hindlimbs.


Description

''Avimaia'' is a relatively small member of the Enantiornithes. The describing authors found two distinguishing traits. They are autapomorphies, unique derived characters. The pubic bone has a slender build and is curved upwards, causing a hollow profile over the entire length of its rear edge. The rear end of the
ischium The ischium () form ...
is curved to above. In several additional traits ''Avimaia'' differs from some relatives. The
sacrum The sacrum (plural: ''sacra'' or ''sacrums''), in human anatomy, is a large, triangular bone at the base of the spine that forms by the fusing of the sacral vertebrae (S1S5) between ages 18 and 30. The sacrum situates at the upper, back part ...
consists of eight sacral vertebrae. The first metatarsal is J-shaped with its diverging lower end having two thirds of the length of the shaft, which is a relatively low ratio.


Egg

Between the pubic bones of the fossil, a flattened egg is present, running from the last back vertebra to the front of the pygostyle, the fused outer tail vertebrae. It covers a surface of 5.2 cm2. It is the first time a fossil bird egg has been discovered within the abdomen of the female. The egg is not merely an outline of organic remains: the egg shells themselves have been preserved, which is again unique. The combined shells have a thickness of 0.2 to 0.4 millimetres. A section showed the presence of four to six separate shells stacked on top of each other, each about 0.05 millimetres thick. Each shell consists of three layers, as with modern birds and generally in derived dinosaur eggs. There is an inner mammillary layer, an intermediate prismatic layer and an outer crystalline layer. The three layers combined are rather thin: earlier reported shells of Enantiornithes are thicker. Another anomaly consists in the fact that the prismatic layer and outer layer combined are thinner than the mammillary layer, whereas normally the opposite is true. On the surface of the egg a brown organic layer is visible, which was identified as the
cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...
. In the cuticle nanospheres of calcium phosphate were discovered, which are also present in the cuticle of some extant birds. Additionally, remains were identified of the egg membrane, the ''membrana testacea''. The fibres of the membrane are individually visible and have a length of 1.5 to 4 micrometres. They possibly contain part of the original proteins. It is likely that during fossilisation, the egg shells shifted in relation to each other and were otherwise distorted. The presence of four to six shells on top of each other is difficult to explain. The authors considered the possibility that the shells represented two eggs, but this hypothesis was rejected. In case of two eggs, the two top shells should have mirrored each other, as they then should have been the opposite shells of a top egg. In reality, they mirror the lower shells. Furthermore, it is generally assumed that the Eumaniraptora only have one functional oviduct. It was concluded that the stacking was caused by a pathology,
egg binding Egg binding occurs in animals, such as reptiles or birds, when an egg takes longer than usual to pass out of the reproductive tract. In birds In birds, egg binding may be caused by obesity, nutritional imbalances such as calcium deficiency, envi ...
, which is not uncommon among some modern bird species, such as the chicken, and can be caused by stress. Retention of the egg in the oviduct would have led to the deposition of multiple shells on a single egg. Such a condition is potentially fatal and it was suggested that it led to the demise of the holotype individual.


Medullary bone

In the thighbone, section showed that an inner layer of
medullary bone The medullary cavity (''medulla'', innermost part) is the central cavity of bone shafts where red bone marrow and/or yellow bone marrow (adipose tissue) is stored; hence, the medullary cavity is also known as the marrow cavity. Located in the ma ...
was present around the medullary cavity. Such a medullary bone is used by modern bird females to extract chalk from during the laying of eggs. Its presence thus confirms that the holotype is a female and the structure in its abdomen is indeed an egg.


Plumage

What little of the plumage has been preserved, seems to indicate that long tail feathers were absent. It has been suggested that in the Enantiornithes such long display feathers were limited to the males only, as an instance of sexual dimorphism. Their absence thus is a further corroboration of the hypothesis that the type specimen is female.


Phylogeny

''Avimaia'' was placed in the Enantiornithes in 2019. A cladistic analysis indicated a derived position in that
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...
, in a polytomy with ''
Neuquenornis ''Neuquenornis volans''Chiappe, Luis M.,Calvo, Jorge O. (1994) "Neuquenornis volans, a New Late Cretaceous Bird (Enantiornithes: Avisauridae) from Patagonia, Argentina""Journal of Vertebrate Paleontology" Vol. 14, No. 2 (Jun. 22, 1994), pp. 230- ...
'', ''
Enantiophoenix ''Enantiophoenix'' is a genus of enantiornithine birds. Fossil remains were recovered from Lebanon Lebanon ( , ar, لُبْنَان, translit=lubnān, ), officially the Republic of Lebanon () or the Lebanese Republic, is a country in Wes ...
'', ''
Concornis ''Concornis'' is a genus of enantiornithean birds which lived during the early Cretaceous period, in the late Barremian age about 125 million years ago. Its remains are known from the Calizas de La Huérgina Formation at Las Hoyas, Cuenca prov ...
'' and ''
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 ...
''.


Gallery


References

{{Portal bar, Birds, Cretaceous, Paleontology Enantiornitheans Aptian life Early Cretaceous birds of Asia Fossils of China Fossil taxa described in 2019