Enantiornithines
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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 The Mesozoic Era is the era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Periods. It is characterized by the dominance of archosaurian reptiles such as the dinosaurs, and of gymnosperms s ...
. Almost all retained teeth and clawed fingers on each wing, but otherwise looked much like modern birds externally. Over seventy species of Enantiornithes have been named, but some names represent only single bones, so it is likely that not all are valid. The Enantiornithes became extinct at the
Cretaceous–Paleogene boundary The Cretaceous–Paleogene (K–Pg) boundary, formerly known as the Cretaceous–Tertiary (K–T) boundary, is a geological signature, usually a thin band of rock containing much more iridium than other bands. The K–Pg boundary marks the end o ...
, along with
Hesperornithes Hesperornithes is an extinct and highly specialized group of aquatic Avialae, avialans closely related to the ancestors of modern birds. They inhabited both marine and freshwater habitats in the Northern Hemisphere, and include genus, genera such ...
and all other non-avian
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...
s.


Discovery and naming

The first Enantiornithes to be discovered were incorrectly referred to modern bird groups. For example, the first known species of Enantiornithes, ''
Gobipteryx minuta ''Gobipteryx'' (from Gobi eferring to the Gobi Desert where it was first discovered and Greek pteryx "wing") is a genus of prehistoric bird from the Campanian Age of the Late Cretaceous Period.Elżanowski, A. (1974)"Preliminary note on the Pal ...
'', was originally considered a
paleognath Palaeognathae (; ) is an infraclass of birds, called paleognaths or palaeognaths, within the class Aves of the clade Archosauria. It is one of the two extant infraclasses of birds, the other being Neognathae, both of which form Neornithes. Pala ...
related to
ostrich Ostriches are large flightless birds. Two living species are recognised, the common ostrich, native to large parts of sub-Saharan Africa, and the Somali ostrich, native to the Horn of Africa. They are the heaviest and largest living birds, w ...
es and
tinamou Tinamous () are members of the order Tinamiformes (), and family Tinamidae (), divided into two distinct subfamily, subfamilies, containing 46 species found in Mexico, Central America, and South America. The word "tinamou" comes from the Carib la ...
. The Enantiornithes were first recognized as a distinct lineage, or "subclass" of birds, by Cyril A. Walker in 1981. Walker made this discovery based on some partial remains from the late
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
period Period may refer to: Common uses * Period (punctuation) * Era, a length or span of time *Menstruation, commonly referred to as a "period" Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (o ...
of what is now
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, which he assigned to a new genus, ''
Enantiornis ''Enantiornis'' is a genus of Enantiornithes. The type and only currently accepted species ''E. leali'' is from the Late Cretaceous Lecho Formation at El Brete, Argentina. It was described from specimen PVL-4035, a coracoid, proximal scapula an ...
'', giving the entire group its name. Since the 1990s, many more complete specimens of Enantiornithes have been discovered, and it was determined that a few previously described "birds" (e.g. ''
Iberomesornis ''Iberomesornis'' ("Iberian intermediate bird") is a monotypic genus of enantiornithine bird of the Cretaceous of Spain. Discovery In 1985 the fossil of ''Iberomesornis'' was discovered by Armando Díaz Romeral in the Early Cretaceous Caliza ...
'', ''
Cathayornis ''Cathayornis'' is a genus of enantiornithean birds from the Jiufotang Formation of Liaoning, People's Republic of China. It is known definitively from only one species, ''Cathayornis yandica'', one of the first Enantiornithes found in China. Se ...
'', and ''
Sinornis ''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 "pr ...
'') were also Enantiornithes. The name "Enantiornithes" means "opposite birds", from
Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...
''enantios'' () "opposite" + ''ornithes'' () "birds" . The name was coined by
Cyril Alexander Walker Cyril Alexander Walker (8 February 1939 – 6 May 2009) was a British palaeontologist, curator of fossil birds in the Natural History Museum. He was also interested in fossil turtles. book, '' Smithsonian Handbook of Fossils''. He has also con ...
in his landmark paper which established the group. In his paper, Walker explained what he meant by "opposite": This refers to an anatomical feature – the articulation of the
shoulder The human shoulder is made up of three bones: the clavicle (collarbone), the scapula (shoulder blade), and the humerus (upper arm bone) as well as associated muscles, ligaments and tendons. The articulations between the bones of the shoulder m ...
bones – which has a concave-convex socket joint between the ''scapula'' (shoulder blade) and ''coracoid'' (the primary bone of the shoulder girdle in vertebrates other than mammals) that is the reverse of that of modern birds. Specifically, in the Enantiornithes, the scapula is
concave Concave or concavity may refer to: Science and technology * Concave lens * Concave mirror Mathematics * Concave function, the negative of a convex function * Concave polygon A simple polygon that is not convex is called concave, non-convex or ...
and dish-shaped at this joint, and the coracoid is convex. In modern birds, the coracoscapular joint has a concave coracoid and convex scapula. Walker was not clear on his reasons for giving this name in the etymology section of his paper, and this ambiguity led to some confusion among later researchers. For example,
Alan Feduccia John Alan Feduccia (born April 25, 1943) is a paleornithologist specializing in the origins and phylogeny of birds. He is S. K. Heninger Distinguished Professor Emeritus at the University of North Carolina. Feduccia's authored works include t ...
stated in 1996: Feduccia's point about the ''
tarsometatarsus The tarsometatarsus is a bone that is only found in the lower leg of birds and some non-avian dinosaurs. It is formed from the fusion of several bird bones found in other types of animals, and homologous to the mammalian tarsus (ankle bones) a ...
'' (the combined upper foot and ankle bone) is correct, but Walker did not use this reasoning in his original paper. Walker never described the fusion of the tarsometatarsus as opposite, but rather as "Only partial". Also, it is not certain that Enantiornithes had triosseal canals, since no fossil preserves this feature. As a group, the Enantiornithes are often referred to as "enantiornithines" in literature. However, several scientists have noted that this is incorrect, because following the standard rules for forming the names of animal groups, it implies reference only to the subfamily Enantiornithinae. Following the naming conventions used for modern birds as well as extinct groups, it has been pointed out that the correct term is "enantiornithean".


Origin and range

'' Praeornis'', from the Oxfordian-
Kimmeridgian In the geologic timescale, the Kimmeridgian is an age in the Late Jurassic Epoch and a stage in the Upper Jurassic Series. It spans the time between 154.8 ±0.8 Ma and 149.2 ±0.7 Ma (million years ago). The Kimmeridgian follows the Oxfordian ...
of
Kazakhstan Kazakhstan, officially the Republic of Kazakhstan, is a landlocked country primarily in Central Asia, with a European Kazakhstan, small portion in Eastern Europe. It borders Russia to the Kazakhstan–Russia border, north and west, China to th ...
, may have been the earliest known member of Enantiornithes according to Agnolin et al. (2017).Agnolin, F.L., Rozadilla, S., and Ismar de Souza Carvalho (2017). ''Praeornis sharovi'' Rautian, 1978 a fossil feather from the early Late Jurassic of Kazakhstan. ''Historical Biology''. Birds with confidently identified characteristics of Enantiornithes found in
Albian The Albian is both an age (geology), age of the geologic timescale and a stage (stratigraphy), stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early Cretaceous, Early/Lower Cretaceous epoch (geology), Epoch/s ...
of
Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
,
Maastrichtian The Maastrichtian ( ) is, in the International Commission on Stratigraphy (ICS) geologic timescale, the latest age (geology), age (uppermost stage (stratigraphy), stage) of the Late Cretaceous epoch (geology), Epoch or Upper Cretaceous series (s ...
of South America, and
Campanian The Campanian is the fifth of six ages of the Late Cretaceous epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campa ...
of
Mexico Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...
(''
Alexornis ''Alexornis'' is a genus of enantiornithine birds from the La Bocana Roja Formation of Baja California, Mexico.Tykoski, R. S., & Fiorillo, A. R. (2010). An enantiornithine bird from the lower middle Cenomanian of Texas. ''Journal of Vertebrate ...
''),
Mongolia Mongolia is a landlocked country in East Asia, bordered by Russia to the north and China to the south and southeast. It covers an area of , with a population of 3.5 million, making it the world's List of countries and dependencies by po ...
and western edge of prehistoric Asia suggest a worldwide distribution of this group or in the relatively warm regions, at least. Enantiornithes have been found on every continent except
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean (also known as the Antarctic Ocean), it contains the geographic South Pole. ...
. Fossils attributable to this group are exclusively
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
in age, and it is believed that the Enantiornithes became extinct at the same time as their non-avialan
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...
relatives. The earliest known Enantiornithes are from the
Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...
of
Spain Spain, or the Kingdom of Spain, is a country in Southern Europe, Southern and Western Europe with territories in North Africa. Featuring the Punta de Tarifa, southernmost point of continental Europe, it is the largest country in Southern Eur ...
(e.g. '' Noguerornis'') and
China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...
(e.g. ''
Protopteryx ''Protopteryx'' is an extinct bird and possibly the basalmost enantiornithean, from the Cretaceous period.Zhou, Zhonghe. (2002). "A new and primitive enantiornithine bird from the Early Cretaceous of China." ''Journal of Vertebrate Paleontology' ...
'') and the latest from the
Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cre ...
of North and South America (e.g. ''
Avisaurus ''Avisaurus'' (meaning "bird lizard") is a genus of enantiornithine avialan from the Late Cretaceous of North America. Discovery ''Avisaurus archibaldi'' was discovered in the Late Cretaceous Hell Creek Formation of North America (Maastrichtian, ...
'' and ''
Enantiornis ''Enantiornis'' is a genus of Enantiornithes. The type and only currently accepted species ''E. leali'' is from the Late Cretaceous Lecho Formation at El Brete, Argentina. It was described from specimen PVL-4035, a coracoid, proximal scapula an ...
''). The widespread occurrence of this group suggests that at least some Enantiornithes were able to cross oceans under their own power; they are the first known avialan lineage with a global distribution.


Description

Many fossils of Enantiornithes are very fragmentary, and some species are only known from a piece of a single bone. Almost all specimens that are complete, in full articulation, and with soft tissue preservation are known from Las Hoyas in
Cuenca Cuenca may refer to: People * Cuenca (surname) Places Ecuador * Cuenca Canton, in the Azuay Province ** Cuenca, Ecuador, capital of Cuenca Canton and Azuay Province ** Roman Catholic Archdiocese of Cuenca Peru * Cuenca District, Huarochirí ...
, Spain and the
Jehol group The Jehol Biota includes all the living organisms – the ecosystem – of northeastern China between 133 and 120 million years ago. This is the Lower Cretaceous ecosystem which left fossils in the Yixian Formation and Jiufotang Formation. These d ...
in
Liaoning ) , image_skyline = , image_alt = , image_caption = Clockwise: Mukden Palace in Shenyang, Xinghai Square in Dalian, Dalian coast, Yalu River at Dandong , image_map = Liaoning in China (+all claims hatched).svg , ...
(
China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...
). Extraordinary remains of Enantiornithes have also been preserved in
Burmese amber Burmese amber, also known as Burmite or Kachin amber, is amber from the Hukawng Valley in northern Myanmar. The amber is dated to around 100 million years ago, during the latest Albian to earliest Cenomanian ages of the mid-Cretaceous period. Th ...
deposits dated to 99 million years ago and include hatchlings described in 2017 and 2018, as well as isolated body parts such as wings and feet. These amber remains are among the most well-preserved of any mesozoic dinosaur. Fossils of this clade have been found in both inland and marine sediments, suggesting that they were an ecologically diverse group. Enantiornithes appear to have included waders, swimmers, granivores, insectivores, fishers, and raptors. The vast majority of Enantiornithes were small, between the size of a sparrow and a
starling Starlings are small to medium-sized passerine (perching) birds known for the often dark, glossy iridescent sheen of their plumage; their complex vocalizations including mimicking; and their distinctive, often elaborate swarming behavior, know ...
, however display considerable variation in size with some species. The largest species in this clade include '' Pengornis houi'', '' Xiangornis shenmi'', ''
Zhouornis hani ''Zhouornis'' is an extinct genus of enantiornithean dinosaurs known from the Early Cretaceous Jehol Group (Aptian stage) of western Liaoning Province, northeastern China. ''Zhouornis'' was first named by Zhang Zihui, Luis María Chiappe, Gang ...
'', and ''Mirarce eatoni'',' (with the latter species being described as similar in size to modern turkeys,) although at least a few larger species may have also existed, including a potentially crane-sized species known only from footprints in the
Eumeralla Formation The Eumeralla Formation is a geological formation in Victoria, Australia whose strata date back to the Early Cretaceous. It is Aptian to Albian in age. Dinosaur remains are among the fossils that have been recovered from the formation, particul ...
(and possibly also represented in the
Wonthaggi Formation The Wonthaggi Formation is an informal geological Formation (geology), formation in Victoria, Australia whose strata date back to the Early Cretaceous. It is part of the Strzelecki Group within the Gippsland Basin. Dinosaur remains are among the ...
by a single
furcula The (Latin for "little fork"; : furculae) or wishbone is a forked bone found in most birds and some species of non-avian dinosaurs, and is either an interclavicle or formed by the fusion of the two clavicles. In birds, its primary function is ...
). Among the smallest described specimens are unnamed hatchlings, although the holotype specimens of ''Parvavis chuxiongensis'' and ''Cratoavis cearensis'' are comparable in size to small tits or hummingbirds.


Skull

Given their wide range of habitats and diets, the cranial morphology of Enantiornithes varied considerably between species. Skulls of Enantiornithes combined a unique suite of primitive and advanced features. As in more primitive avialans like ''
Archaeopteryx ''Archaeopteryx'' (; ), sometimes referred to by its German name, "" ( ''Primeval Bird'') is a genus of bird-like dinosaurs. The name derives from the ancient Greek (''archaîos''), meaning "ancient", and (''ptéryx''), meaning "feather" ...
'', they retained several separate cranial bones, small
premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammals h ...
e (bones of the snout tip) and most species had toothy jaws rather than toothless beaks. Only a few species, such as ''
Gobipteryx minuta ''Gobipteryx'' (from Gobi eferring to the Gobi Desert where it was first discovered and Greek pteryx "wing") is a genus of prehistoric bird from the Campanian Age of the Late Cretaceous Period.Elżanowski, A. (1974)"Preliminary note on the Pal ...
'', were fully toothless and had beaks. They also had simple
quadrate bone The quadrate bone is a skull bone in most tetrapods, including amphibians, sauropsids ( reptiles, birds), and early synapsids. In most tetrapods, the quadrate bone connects to the quadratojugal and squamosal bones in the skull, and forms up ...
s, a complete bar separating each
orbit In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...
(eye hole) from each
antorbital fenestra An antorbital fenestra (plural: fenestrae) is an opening in the skull that is in front of the eye sockets. This skull character is largely associated with Archosauriformes, archosauriforms, first appearing during the Triassic Period. Among Extant ...
, and dentaries (the main toothed bones of the lower jaw) without forked rear tips. A
squamosal bone The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestral ...
is preserved in an indeterminate juvenile specimen, while a
postorbital The ''postorbital'' is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some ve ...
is preserved in '' Shenqiornis'' and ''
Pengornis ''Pengornis'' is the largest known enantiornithean bird from the Early Cretaceous of northeast China. The name derives from "Peng", which refers to a mythological bird from Chinese folklore, and "-ornis", which means bird in Greek. ''Pengorni ...
''. In modern birds these bones are assimilated into the cranium. Some Enantiornithes may have had their
temporal fenestrae Temporal fenestrae are openings in the temporal region of the skull of some amniotes, behind the orbit (eye socket). These openings have historically been used to track the evolution and affinities of reptiles. Temporal fenestrae are commonly ( ...
(holes in the side of the head) merged into the orbits as in modern birds due to the postorbitals either not being present or not being long enough to divide the openings. A
quadratojugal bone The quadratojugal is a skull bone present in many vertebrates, including some living reptiles and amphibians. Anatomy and function In animals with a quadratojugal bone, it is typically found connected to the jugal (cheek) bone from the front and ...
, which in modern birds is fused to the jugal, is preserved in '' Pterygornis''. The presence of these primitive features of the skull would have rendered the Enantiornithes capable of only limited
cranial kinesis Cranial kinesis is the term for significant movement of skull bones relative to each other in addition to movement at the joint between the upper and lower jaws. It is usually taken to mean relative movement between the upper jaw and the braincase. ...
(the ability to move the jaw independent of the cranium).


Wing

As a very large group of birds, the Enantiornithes displayed a high diversity of different body plans based on differences in ecology and feeding, reflected in an equal diversity of wing forms, many paralleling adaptations to different lifestyles seen in modern birds. In general, the wings of Enantiornithes were advanced compared to more primitive avialans like ''Archaeopteryx'', and displayed some features related to flight similar to those found in the lineage leading to modern birds, the
Ornithuromorpha Euornithes (from Greek ' meaning "true birds") is a natural group which includes the most recent common ancestor of all avialans closer to modern birds than to the enantiornithines. This group was defined in the ''PhyloCode'' by Juan Benito and ...
. While most Enantiornithes had claws on at least some of their fingers, many species had shortened hands, a highly mobile shoulder joint, and proportional changes in the wing bones similar to modern birds. Like modern birds, Enantiornithes had
alula The alula , or bastard wing, (plural ''alulae'') is a small projection on the anterior edge of the wing of modern birds and a few non-avian dinosaurs. The word is Latin and means "winglet"; it is the diminutive of ''ala'', meaning "wing". The a ...
s, or "bastard wings", small forward-pointing arrangements of feathers on the first digit that granted higher maneuverability in the air and aided in precise landings. Several wings with preserved feathers have been found in
Burmese amber Burmese amber, also known as Burmite or Kachin amber, is amber from the Hukawng Valley in northern Myanmar. The amber is dated to around 100 million years ago, during the latest Albian to earliest Cenomanian ages of the mid-Cretaceous period. Th ...
. These are the first complete
Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...
remains preserved this way (a few isolated feathers are otherwise known, unassigned to any species), and one of the most exquisitely preserved dinosaurian fossils known. The preserved wings show variations in feather pigment and prove that Enantiornithes had fully modern feathers, including barbs, barbules, and hooklets, and a modern arrangement of wing feather including long flight feathers, short coverts, a large alula and an undercoat of down. One fossil of Enantiornithes shows wing-like feather tufts on its legs, similar to ''Archaeopteryx''. The leg feathers are also reminiscent of the four-winged dinosaur ''
Microraptor ''Microraptor'' (Greek language, Greek, μικρός, ''mīkros'': "small"; Latin language, Latin, ''raptor'': "one who seizes") is a genus of small, four-winged dromaeosaurid dinosaurs. Numerous well-preserved fossil specimens have been recovere ...
'', however differ by the feathers being shorter, more disorganized (they do not clearly form a wing) and only extend down to the ankle rather than along the foot.


Tail

Fossil skeleton of '' longipterygid) with a preserved pygostyle">Longipterygidae.html" ;"title="Rapaxavis pani'' (a Longipterygidae">longipterygid) with a preserved pygostyle ">Longipterygidae">longipterygid) with a preserved pygostyle">Longipterygidae.html" ;"title="Rapaxavis pani'' (a Longipterygidae">longipterygid) with a preserved pygostyle Clarke ''et al.'' (2006) surveyed all fossils of Enantiornithes then known and concluded that none had preserved tail feathers that formed a lift-generating fan, as in modern birds. They found that all avialans outside of
Euornithes Euornithes (from Greek ' meaning "true birds") is a natural group which includes the most recent common ancestor of all avialans closer to modern birds than to the enantiornithines. This group was defined in the ''PhyloCode'' by Juan Benito an ...
(the clade they referred to as Ornithurae) with preserved tail feathers had only short coverts or elongated paired tail plumes. They suggested that the development of the
pygostyle Pygostyle is a skeletal condition in which the final few caudal vertebrae are fused into a single ossification, supporting the tail feathers and musculature. In modern birds, the rectrices attach to these. The pygostyle is the main component o ...
in Enantiornithes must have been a function of tail shortening, not the development of a modern tail feather anatomy. These scientists suggested that a fan of tail feathers and the associated musculature needed to control them, known as the ''rectrical bulb'', evolved alongside a short, triangular pygostyle, like the ones in modern birds, rather than the long, rod- or dagger-shaped pygostyles in more primitive avialans like the Enantiornithes. Instead of a feather fan, most Enantiornithes had a pair of long specialized pinfeathers similar to those of the extinct ''
Confuciusornis ''Confuciusornis'' is a genus of basal crow-sized avialan from the Early Cretaceous Period of the Yixian and Jiufotang Formations of China, dating from 125 to 120 million years ago. Like modern birds, ''Confuciusornis'' had a toothless beak, bu ...
'' and certain extant
birds-of-paradise The birds-of-paradise are members of the family Paradisaeidae of the order Passeriformes. The majority of species are found in eastern Indonesia, Papua New Guinea, and eastern Australia. The family has 45 species in 17 genera. The members of this ...
. However, further discoveries showed that at least among basal Enantiornithes, tail anatomy was more complex than previously thought. One genus, ''
Shanweiniao ''Shanweiniao'' is a genus of long-snouted enantiornithean birds 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 Nat ...
'', was initially interpreted as having at least four long tail feathers that overlapped each other and might have formed a lift-generating surface similar to the tail fans of Euronithes, though a later study indicates that ''Shanweiniao'' was more likely to have
rachis In biology, a rachis (from the [], "backbone, spine") is a main axis or "shaft". In zoology and microbiology In vertebrates, ''rachis'' can refer to the series of articulated vertebrae, which encase the spinal cord. In this case the ''rachi ...
-dominated tail feathers similar to feathers present in ''
Paraprotopteryx ''Paraprotopteryx'' is a genus of enantiornithean birds from the Mesozoic of China.Zheng, X. Zhang, Z. & Hou, L. (2007) A new enantiornithine bird with four long rectrices from the Early Cretaceous of northern Hebei, China. ''Acta Geologica Sini ...
''. ''
Chiappeavis ''Chiappeavis'' is a genus of enantiornithean bird from Early Cretaceous of northeastern China. The only species is ''Chiappeavis magnapremaxillo''. ''Chiappeavis'' is classified within the family Pengornithidae. It is known from a single, almo ...
'', a primitive pengornithid, had a fan of tail feathers similar to that of more primitive avialans like ''
Sapeornis ''Sapeornis'' is a monotypic genus of avialan dinosaurs which lived during the early Cretaceous period ( late Barremian to early Aptian, roughly 125-120 mya). ''Sapeornis'' contains only one species, ''Sapeornis chaoyangensis''. Description ...
'', suggesting that this might have been the ancestral condition, with pinfeathers being a feature evolved several times in early avialans for display purposes. Another species of Enantiornithes, '' Feitianius'', also had an elaborate fan of tail feathers. More importantly, soft tissue preserved around the tail was interpreted as the remains of a rectrical bulb, suggesting that this feature was not in fact restricted to species with modern-looking pygostyles, but might have evolved much earlier than previously thought and been present in many Enantiornithes. At least one genus of Enantiornithes, '' Cruralispennia'', had a modern-looking pygostyle but lacked a tail fan.


Biology


Diet

Given the wide diversity of skull shape among Enantiornithes, many different dietary specializations must have been present among the group. Some, like '' Shenqiornis'', had large, robust jaws suitable for eating hard-shelled invertebrates. The short, blunt teeth of ''
Pengornis ''Pengornis'' is the largest known enantiornithean bird from the Early Cretaceous of northeast China. The name derives from "Peng", which refers to a mythological bird from Chinese folklore, and "-ornis", which means bird in Greek. ''Pengorni ...
'' were likely used to feed on soft-bodied arthropods. The strongly hooked talons of
Bohaiornithidae Bohaiornithidae is a group of early enantiornithean stem-birds from the early Cretaceous Period of China. All undisputed Bohaiornithidae specimens come from the northeast of present-day China – specifically, the eastern and southern foothil ...
suggest that they were predators of small to medium-sized vertebrates, but their robust teeth instead suggest a diet of hard-shelled animals. A few specimens preserve actual stomach contents. Unfortunately, none of these preserve the skull, so direct correlation between their known diet and snout/tooth shape cannot be made. ''
Eoalulavis ''Eoalulavis'' (from the Ancient Greek: ''Éōs'', "dawn"; ''alula'', "bastard wing"; ''avis'', "bird") is a monotypic genus of enantiornithean bird that lived during the Barremian, in the Lower Cretaceous around 125 million years ago. The only ...
'' was found to have the remains of
exoskeleton An exoskeleton () . is a skeleton that is on the exterior of an animal in the form of hardened integument, which both supports the body's shape and protects the internal organs, in contrast to an internal endoskeleton (e.g. human skeleton, that ...
s from aquatic
crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...
s preserved in its digestive tract, and '' Enantiophoenix'' preserved corpuscles of amber among the fossilized bones, suggesting that this animal fed on tree sap, much like modern
sapsucker The sapsuckers are species of North American woodpeckers in the genus ''Sphyrapicus''. Taxonomy and systematics The genus ''Sphyrapicus'' was introduced in 1858 by the American naturalist Spencer Baird with the yellow-bellied sapsucker (''Sph ...
s and other birds. The sap would have fossilized and become amber. However, more recently it has been suggested that the sap moved post-mortem, hence not representing true stomachal contents. Combined with the putative fish pellets of '' Piscivorenantiornis'' turning out to be fish excrement, the strange stomachal contents of some species turning out to be
ovaries The ovary () is a gonad in the female reproductive system that produces ova; when released, an ovum travels through the fallopian tube/oviduct into the uterus. There is an ovary on the left and the right side of the body. The ovaries are endocr ...
and the supposed gastroliths of ''
Bohaiornis ''Bohaiornis'' is a genus of enantiornithean dinosaurs. Fossils have been found from the Lower Cretaceous Jiufotang Formation of western Liaoning, China. The only known species, ''Bohaiornis guoi'', was named by Dongyu Hu, Li Li, Lianhaim Hou an ...
'' being random mineral precipitates, only the ''
Eoalulavis ''Eoalulavis'' (from the Ancient Greek: ''Éōs'', "dawn"; ''alula'', "bastard wing"; ''avis'', "bird") is a monotypic genus of enantiornithean bird that lived during the Barremian, in the Lower Cretaceous around 125 million years ago. The only ...
'' displays actual stomach contents. A study on paravian digestive systems indicates that known Enantiornithes lacked a crop and a gizzard, didn't use gastroliths and didn't eject pellets. This is considered at odds with the high diversity of diets that their different teeth and skull shapes imply, though some modern birds have lost the gizzard and rely solely on strong stomachal acids. An example was discovered with what was suspected to be gastroliths in the what would have been the fossil's stomach, re-opening the discussion of the use of gastroliths by Enantiornithes. X-ray and scanning microscope inspection of the rocks determined that they were actually chalcedony crystals, and not gastroliths.
Longipterygidae Longipterygidae is a family of early enantiornithean avialans from the Early Cretaceous epoch of China. All known specimens come from the Jiufotang Formation and Yixian Formation, dating to the early Aptian age, 125-120 million years ago. Descr ...
is the most extensively studied family in terms of diet due to their rather unusual rostral anatomy, with long jaws and few teeth arranged at the jaw ends. They have variously been interpreted as piscivores, probers akin to shorebirds and as arboreal bark-probers. A 2022 study however does find them most likely to be generalistic insectivores (sans possibly '' Shengjingornis'' due to its larger size, poorly preserved skull and unusual pedal anatomy), being too small for specialised carnivory and herbivory; the atypical rostrum is tentatively speculated to be unrelated to feeding ecology. However a posterior study has found them to be herbivorous, including the presence of
gymnosperm The gymnosperms ( ; ) are a group of woody, perennial Seed plant, seed-producing plants, typically lacking the protective outer covering which surrounds the seeds in flowering plants, that include Pinophyta, conifers, cycads, Ginkgo, and gnetoph ...
seeds in their digestive system.
Avisaurids Avisauridae is a Family (biology), family of Extinction, extinct Enantiornithes, enantiornithine dinosaurs from the Cretaceous period, distinguished by several features of their ankle bones. Depending on the definition used, Avisauridae is either ...
occupied a niche analogous to modern
birds of prey Birds of prey or predatory birds, also known as (although not the same as) raptors, are hypercarnivorous bird species that actively predation, hunt and feed on other vertebrates (mainly mammals, reptiles and smaller birds). In addition to speed ...
, having the ability to lift small prey with their feet in a manner similar to hawks or owls.


Predation

A fossil from Spain reported by Sanz ''et al.'' in 2001 included the remains of four hatchling skeletons of three different species of Enantiornithes. They are substantially complete, very tightly associated, and show surface pitting of the bones that indicates partial digestion. The authors concluded that this association was a regurgitated pellet and, from the details of the digestion and the size, that the hatchlings were swallowed whole by a
pterosaur Pterosaurs are an extinct clade of flying reptiles in the order Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 million to 66 million years ago). Pterosaurs are the earli ...
or small
theropod Theropoda (; from ancient Greek , (''therion'') "wild beast"; , (''pous, podos'') "foot"">wiktionary:ποδός"> (''pous, podos'') "foot" is one of the three major groups (clades) of dinosaurs, alongside Ornithischia and Sauropodom ...
dinosaur. This was the first evidence that
Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...
avialans were prey animals, and that some Mesozoic pan-avians regurgitated pellets like owls do today.


Life history

Known fossils of Enantiornithes include
eggs An egg is an organic vessel in which an embryo begins to develop. Egg, EGG or eggs may also refer to: Biology * Egg cell, the female reproductive cell (gamete) in oogamous organisms Food * Eggs as food Places * Egg, Austria * Egg, Switzerland ...
,
embryo An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...
s, and
hatchling In oviparous biology, a hatchling is a newly hatched fish, amphibian, reptile, or bird. A group of mammals called monotremes lay eggs, and their young are hatchlings as well. Fish Fish hatchlings generally do not receive parental care, similar t ...
s. An embryo, still curled in its egg, has been reported from the
Yixian Formation The Yixian Formation (; formerly Romanization of Chinese, transcribed as Yihsien Formation or Yixiang Formation) is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans the Barremian stage of the Early Cretaceous. I ...
. Juvenile specimens can be identified by a combination of factors: rough texture of their bone tips indicating portions which were still made of cartilage at the time of death, relatively small breastbones, large skulls and eyes, and bones which had not yet fused to one another. Some hatchling specimens have been given formal names, including "'' Liaoxiornis delicatus''"; however,
Luis Chiappe Luis María Chiappe (born 18 June 1962) is an Argentine paleontologist born in Buenos Aires who is best known for his discovery of the first sauropod nesting sites in the badlands of Patagonia in 1997 and for his work on the origin and early evol ...
and colleagues considered the practice of naming new species based on juveniles detrimental to the study of Enantiornithes, because it is nearly impossible to determine which adult species a given juvenile specimen belongs to, making any species with a hatchling holotype a ''
nomen dubium In binomial nomenclature, a ''nomen dubium'' (Latin for "doubtful name", plural ''nomina dubia'') is a scientific name that is of unknown or doubtful application. Zoology In case of a ''nomen dubium,'' it may be impossible to determine whether a ...
''. Together with hatchling specimens of the Mongolian ''
Gobipteryx ''Gobipteryx'' (from Gobi eferring to the Gobi Desert where it was first discovered and Greek pteryx "wing") is a genus of prehistoric bird from the Campanian Age of the Late Cretaceous Period.Elżanowski, A. (1974)"Preliminary note on the Pala ...
'' and '' Gobipipus'', these finds demonstrate that hatchling Enantiornithes had the skeletal ossification, well-developed wing feathers, and large brain which correlate with
precocial Precocial species in birds and mammals are those in which the young are relatively mature and mobile from the moment of birth or hatching. They are normally nidifugous, meaning that they leave the nest shortly after birth or hatching. Altricial ...
or
superprecocial Precocial species in birds and mammals are those in which the young are relatively mature and mobile from the moment of birth or hatching. They are normally nidifugous, meaning that they leave the nest shortly after birth or hatching. Altricial ...
patterns of development in birds of today. In other words, Enantiornithes probably hatched from the egg already well developed and ready to run, forage, and possibly even fly at just a few days old. Findings suggests Enantiornithes, especially the toothed species, had a longer incubation time than modern birds. Analyses of Enantiornithes bone histology have been conducted to determine the growth rates of these animals. A 2006 study of ''
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 ...
'' bones showed a growth pattern different from modern birds; although growth was rapid for a few weeks after hatching, probably until
fledging Fledging is the stage in a flying animal's life between hatching or birth and becoming capable of flight. This term is most frequently applied to birds, but is also used for bats. For altricial birds, those that spend more time in vulnerable c ...
, this small species did not reach adult size for a long time, probably several years. Other studies have all supported the view that growth to adult size was slow, as it is in living precocial birds (as opposed to
altricial Precocial species in birds and mammals are those in which the young are relatively mature and mobile from the moment of birth or hatching. They are normally nidifugous, meaning that they leave the nest shortly after birth or hatching. Altricial ...
birds, which are known to reach adult size quickly). Studies of the rate of bone growth in a variety of Enantiornithes has shown that smaller species tended to grow faster than larger ones, the opposite of the pattern seen in more primitive species like ''
Jeholornis ''Jeholornis'' (meaning "Jehol bird") is a genus of avialan dinosaurs that lived between approximately 122 and 120 million years ago during the early Cretaceous Period in China. Fossil ''Jeholornis'' were first discovered in the Jiufotang Formati ...
'' and in non-avialan dinosaurs. Some analyses have interpreted the bone histology to indicate that Enantiornithes may not have had fully avian
endotherm An endotherm (from Greek ἔνδον ''endon'' "within" and θέρμη ''thermē'' "heat") is an organism that maintains its body at a metabolically favorable temperature, largely by the use of heat released by its internal bodily functions inst ...
y, instead having an intermediate
metabolic rate Metabolism (, from ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cellular processes; the co ...
. However a 2021 study rejects the idea that they had less endothermic metabolisms than modern birds. Evidence of colonial nesting has been found in Enantiornithes, in sediments from the
Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cre ...
(
Maastrichtian The Maastrichtian ( ) is, in the International Commission on Stratigraphy (ICS) geologic timescale, the latest age (geology), age (uppermost stage (stratigraphy), stage) of the Late Cretaceous epoch (geology), Epoch or Upper Cretaceous series (s ...
) of
Romania Romania is a country located at the crossroads of Central Europe, Central, Eastern Europe, Eastern and Southeast Europe. It borders Ukraine to the north and east, Hungary to the west, Serbia to the southwest, Bulgaria to the south, Moldova to ...
. Evidence from nesting sites shows that Enantiornithes buried their eggs like modern megapodes, which is consistent with their inferred superprecocial adaptations. A 2020 study on a juvenile's feathers further stresses the ontological similarities to modern megapodes, but cautions several differences such as the arboreal nature of most Enantiornithes as opposed to the terrestrial lifestyle of megapodes. It has been speculated that superprecociality in Enantiornithes might have prevented them from developing specialised toe arrangements seen in modern birds like zygodactyly. Although the vast majority of histology studies and known remains of Enantiornithes point to superprecociality being the norm, one specimen, MPCM-LH-26189, seems to represent an altricial juvenile, implying that like modern birds Enantiornithes explored multiple reproductive strategies.


Flight

Because many Enantiornithes lacked complex tails and possessed radically different wing anatomy compared to modern birds, they have been the subject of several studies testing their flight capabilities. Traditionally, they have been considered inferior flyers, due to the shoulder girdle anatomy being assumed to be more primitive and unable to support a ground-based launching mechanism, as well as due to the absence of
rectrice Flight feathers (''Pennae volatus'') are the long, stiff, asymmetrically shaped, but symmetrically paired pennaceous feathers on the Bird wing, wings or tail of a bird; those on the wings are called remiges (), singular remex (), while those ...
s in many species. * However, several studies have shown that they were efficient flyers, like modern birds, possessing a similarly complex nervous system and wing feather ligaments. Additionally, the lack of a complex tail appears to not have been very relevant for avian flight as a whole - some extinct birds like lithornids also lacked complex tail feathers but were good flyers, and they appear to have been capable of a ground based launching. Enantiornithes resemble
Ornithuromorph Euornithes (from Greek ' meaning "true birds") is a natural group which includes the most recent common ancestor of all avialans closer to modern birds than to the enantiornithines. This group was defined in the ''PhyloCode'' by Juan Benito and ...
s in many anatomical features of the flight apparatus, but a sternal keel is absent in the basal-most members, only a single basal taxon appears to have had a triosseal canal, and their robust
pygostyle Pygostyle is a skeletal condition in which the final few caudal vertebrae are fused into a single ossification, supporting the tail feathers and musculature. In modern birds, the rectrices attach to these. The pygostyle is the main component o ...
seems unable to support the muscles that control the modern tail feathers involved in flight. Though some basal Enantiornithes exhibit ancestral flight apparatuses, by the end of the Mesozoic many Enantiornithes had several features convergent with the Neornithes including a deeply keeled sternum, a narrow
furcula The (Latin for "little fork"; : furculae) or wishbone is a forked bone found in most birds and some species of non-avian dinosaurs, and is either an interclavicle or formed by the fusion of the two clavicles. In birds, its primary function is ...
with a short hypocleidium, and ulnar quill knobs that indicate increased aerial abilities. At least ''
Elsornis ''Elsornis'' is a genus of enantiornithean bird. Only one species has been named, ''Elsornis keni''. It lived during the Late Cretaceous. It is known from a partially articulated fossil skeleton found in the Gobi Desert in Mongolia. The holotyp ...
'' appears to have become secondarily
flightless Flightless birds are birds that cannot fly, as they have, through evolution, lost the ability to. There are over 60 extant species, including the well-known ratites ( ostriches, emus, cassowaries, rheas, and kiwis) and penguins. The smal ...
.


Classification

Some researchers classify Enantiornithes, along with the true birds, in the class
Aves Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight ...
. Others use the more restrictive
crown group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor ...
definition of Aves (which only includes
neornithes Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight ...
, anatomically modern birds), and place Enantiornithes in the more inclusive group
Avialae Avialae ("bird wings") is a clade containing the only living dinosaurs, the birds, and their closest relatives. It is usually defined as all theropod dinosaurs more closely related to birds (Aves) than to Deinonychosauria, deinonychosaurs, though ...
. Enantiornithes were more advanced than ''
Archaeopteryx ''Archaeopteryx'' (; ), sometimes referred to by its German name, "" ( ''Primeval Bird'') is a genus of bird-like dinosaurs. The name derives from the ancient Greek (''archaîos''), meaning "ancient", and (''ptéryx''), meaning "feather" ...
'', ''
Confuciusornis ''Confuciusornis'' is a genus of basal crow-sized avialan from the Early Cretaceous Period of the Yixian and Jiufotang Formations of China, dating from 125 to 120 million years ago. Like modern birds, ''Confuciusornis'' had a toothless beak, bu ...
'', and ''
Sapeornis ''Sapeornis'' is a monotypic genus of avialan dinosaurs which lived during the early Cretaceous period ( late Barremian to early Aptian, roughly 125-120 mya). ''Sapeornis'' contains only one species, ''Sapeornis chaoyangensis''. Description ...
'', but in several respects they were more primitive than modern birds, perhaps following an intermediate evolutionary path. A consensus of scientific analyses indicates that Enantiornithes is one of two major groups within the larger group
Ornithothoraces Ornithothoraces is a group of avialan dinosaurs that includes all enantiornithes ("opposite birds") and the euornithes ("true birds"), which includes modern birds and their closest ancestors. The name Ornithothoraces means "bird thoraxes". This r ...
. The other ornithothoracine group is
Euornithes Euornithes (from Greek ' meaning "true birds") is a natural group which includes the most recent common ancestor of all avialans closer to modern birds than to the enantiornithines. This group was defined in the ''PhyloCode'' by Juan Benito an ...
or
Ornithuromorpha Euornithes (from Greek ' meaning "true birds") is a natural group which includes the most recent common ancestor of all avialans closer to modern birds than to the enantiornithines. This group was defined in the ''PhyloCode'' by Juan Benito and ...
, which includes all living birds as a subset. This means that Enantiornithes were a successful branch of avialan evolution, but one that diversified entirely separately from the lineage leading to modern birds. One study has however found that the shared sternal anatomy was acquired independently and such a relationship needs to be reexamined. Enantiornithes classification and taxonomy has historically been complicated by a number of factors. In 2010, paleontologists
Jingmai O'Connor Jingmai Kathleen O'Connor ( ''Zōu Jīngméi''; born August 26, 1983) is a paleontologist who works as a curator at the Field Museum in Chicago, Illinois. Biography O'Connor is from Pasadena, California. Her mother is a geologist. O'Connor says ...
and Gareth Dyke outlined a number of criticisms against the prevailing practices of scientists failing to describe many specimens in enough detail for others to evaluate thoroughly. Some species have been described based on specimens which are held in private collections, making further study or review of previous findings impossible. Because it is often unfeasible for other scientists to study each specimen in person given the worldwide distribution of the Enantiornithes, and due to the many uninformative descriptions which have been published on possibly important specimens, many of these specimens become "functional ''
nomina dubia In binomial nomenclature, a ''nomen dubium'' (Latin for "doubtful name", plural ''nomina dubia'') is a scientific name that is of unknown or doubtful application. Zoology In case of a ''nomen dubium,'' it may be impossible to determine whether a ...
''". Furthermore, many species have been named based on extremely fragmentary specimens, which would not be very informative scientifically even if they were described sufficiently. Over one-third of all named species are based on only a fragment of a single bone. O'Connor and Dyke argued that while these specimens can help expand knowledge of the time span or geographic range of the Enantiornithes and it is important to describe them, naming such specimens is "unjustifiable".


Relationships

Enantiornithes is the sister group to
Euornithes Euornithes (from Greek ' meaning "true birds") is a natural group which includes the most recent common ancestor of all avialans closer to modern birds than to the enantiornithines. This group was defined in the ''PhyloCode'' by Juan Benito an ...
, and together they form a
clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
called
Ornithothoraces Ornithothoraces is a group of avialan dinosaurs that includes all enantiornithes ("opposite birds") and the euornithes ("true birds"), which includes modern birds and their closest ancestors. The name Ornithothoraces means "bird thoraxes". This r ...
(though see above). Most
phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...
studies have recovered Enantiornithes as a monophyletic group distinct from the modern birds and their closest relatives. The 2002 phylogenetic analysis by Clarke and Norell, though, reduced the number of Enantiornithes
autapomorph In phylogenetics, an autapomorphy is a distinctive feature, known as a derived trait, that is unique to a given taxon. That is, it is found only in one taxon, but not found in any others or outgroup taxa, not even those most closely related to ...
ies to just four. Enantiornithes systematics are highly provisional and notoriously difficult to study, due to their small size and the fact that Enantiornithes tend to be extremely homoplastic, or very similar to each other in most of their skeletal features due to convergent evolution rather than common ancestry. What appears fairly certain by now is that there were subdivisions within Enantiornithes possibly including some minor basal lineages in addition to the more advanced Euenantiornithes. The details of the interrelationship of all these lineages, indeed the validity of most, is disputed, although the Avisauridae, for one example, seem likely to constitute a valid group. Phylogenetic taxonomists have hitherto been very reluctant to suggest delimitations of clades of Enantiornithes.Sereno, P.C. (2005)
TaxonSearch: Stem Archosauria
''. Version 1.0, 2005-NOV- 7. Retrieved 2006-OCT-02.
One such delineation named the Euenantiornithes, was defined by Chiappe (2002) as comprising all species closer to ''
Sinornis ''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 "pr ...
'' than to ''
Iberomesornis ''Iberomesornis'' ("Iberian intermediate bird") is a monotypic genus of enantiornithine bird of the Cretaceous of Spain. Discovery In 1985 the fossil of ''Iberomesornis'' was discovered by Armando Díaz Romeral in the Early Cretaceous Caliza ...
''. Because ''Iberomesornis'' is often found to be the most primitive or basal member of the Enantiornithes, Euenantiornithes may be an extremely inclusive group, made up of all Enantiornithes except for ''Iberomesornis'' itself. Despite being in accordance with
phylogenetic nomenclature Phylogenetic nomenclature is a method of nomenclature for taxa in biology that uses phylogenetic definitions for taxon names as explained below. This contrasts with the traditional method, by which taxon names are defined by a '' type'', which c ...
, this definition of Euenantiornithes was severely criticized by some researchers, such as
Paul Sereno Paul Callistus Sereno (born October 11, 1957) is a professor of paleontology at the University of Chicago who has discovered several new dinosaur species on several continents, including at sites in Inner Mongolia, Argentina, Morocco and Niger. ...
, who called it "a ill-defined
clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
..a good example of a poor choice in a phylogenetic definition". The
cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...
below was found by an analysis by Wang ''et al.'' in 2015, updated from a previous data set created by Jingmai O'Connor. The cladogram below is from Wang ''et al.'', 2022, and includes most named taxa and recovers several previously-named clades. Letters on branches indicate the positions of "wildcard" taxa, those which have been recovered in multiple disparate positions. Key to letters: ''b'' = '' Boluochia''
''c'' = ''
Cathayornis ''Cathayornis'' is a genus of enantiornithean birds from the Jiufotang Formation of Liaoning, People's Republic of China. It is known definitively from only one species, ''Cathayornis yandica'', one of the first Enantiornithes found in China. Se ...
''
''e'' = '' Enantiophoenix''
''f'' = '' Houornis''
''h'' = ''
Longipteryx ''Longipteryx'' is a genus of prehistoric bird which lived during the Early Cretaceous (Aptian stage, 120.3 million years ago). It contains a single species, ''Longipteryx chaoyangensis''. Its remains have been recovered from the Jiufotang Forma ...
''
''i'' = '' Parabohaiornis''
''j'' = '' Pterygornis''
''l'' = ''
Vorona ''Vorona berivotrensis'' ( ; Malagasy for "bird", ''berivotrensis'', "from Berivotra") is a monotypic genus of prehistoric birds. It was described from fossils found in a Maevarano Formation quarry near the village of Berivotra, Mahajanga Provi ...
''
''m'' = ''
Yuanjiawaornis ''Yuanjiawaornis'' is an extinct genus of large Enantiornithes, enantiornithean bird known from the early Cretaceous of present-day China. It is monotypic, with only type species ''Y. virisosus'' named.Dongyu Hu, Ying Liu, Jinhua Li, Xing Xu & Li ...
''
''n'' = '' Yungavolucris''


List of genera


Incertae sedis or is a term used for a taxonomy (biology), taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertainty ...

Enantiornithes taxonomy is difficult to evaluate, and as a result few clades within the group are consistently found by phylogenetic analyses. Most Enantiornithes are not included in any specific family, and as such are listed here. Many of these have been considered Euenantiornithes, although the controversy behind this name means that it is not used consistently in studies of Enantiornithes.


Longipterygidae

The
Longipterygidae Longipterygidae is a family of early enantiornithean avialans from the Early Cretaceous epoch of China. All known specimens come from the Jiufotang Formation and Yixian Formation, dating to the early Aptian age, 125-120 million years ago. Descr ...
was a family of long-snouted early Cretaceous Enantiornithes, with teeth only at the tips of the snout. They are generally considered to be fairly basal members of the group.


Pengornithidae

The
Pengornithidae Pengornithidae is a group of early Enantiornithes, enantiornithines from the early Cretaceous Period of China, with the putative member ''Falcatakely'' possibly extending this clade's range into the Late Cretaceous of Madagascar, and several puta ...
was a family of large early Enantiornithes. They had numerous small teeth and numerous primitive features which are lost in most other Enantiornithes. Mostly known from the early Cretaceous of China, with putative Late Cretaceous taxa from Madagascar.


Bohaiornithidae

Bohaiornithids were large but geologically short-lived early Enantiornithes, with long, hooked talons and robust teeth with curved tips. They may have been equivalent to birds of prey, although this interpretation is open to much debate. The
monophyly In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent comm ...
of this group is doubtful, and it may actually be an
evolutionary grade A grade is a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly phylogenetic unit. Phylogenetics The concept of evolutionary grades ...
.


Gobipterygidae

Some members of the group are obscure or poorly described and may be synonymous with its type species, ''Gobipteryx minuta''.


Avisauridae

Avisauridae Avisauridae is a family of extinct enantiornithine dinosaurs from the Cretaceous period, distinguished by several features of their ankle bones. Depending on the definition used, Avisauridae is either a broad and widespread group of advanced ena ...
is subjected to two differing definitions of varying inclusiveness. The more inclusive definition, which follows Cau & Arduini (2008), is used here. Avisaurids were a long-lasting and widespread family of Enantiornithes, which are mainly distinguished by specific features of their tarsometatarsals (ankle bones). The largest and most advanced members of the group survived in North and South America up until the end of the Cretaceous, yet are very fragmentary compared to some earlier taxa.


Dubious genera and notable unnamed specimens

*'' Gobipipus reshetovi'': Described in 2013 from embryo specimens within eggshells from the
Barun Goyot Formation The Baruungoyot Formation (also known as Barun Goyot) is a geological formation dating to the Late Cretaceous Period. It is located within and is widely represented in the Gobi Desert Basin, in the Ömnögovi Province of Mongolia. Descriptio ...
of
Mongolia Mongolia is a landlocked country in East Asia, bordered by Russia to the north and China to the south and southeast. It covers an area of , with a population of 3.5 million, making it the world's List of countries and dependencies by po ...
. These specimens were very similar to embryonic ''Gobipteryx'' specimens, although the describers of ''Gobipipus'' (a set of controversial paleontologists including
Evgeny Kurochkin Yevgeny Nikolayevich Kurochkin (Евгений Николаевич Курочкин; 12 July 1940 - 13 December 2011) was a Russian paleornithologist at the Paleontological Institute of the Russian Academy of Sciences. He served as President of ...
and
Sankar Chatterjee Sankar Chatterjee (born May 28, 1943) is a paleontology, paleontologist, the Paul W. Horn Professor of Earth science, Geosciences at Texas Tech University and curator of Paleontology at the Museum of Texas Tech University. He earned his Doctor of ...
) consider it distinct. *'' Hebeiornis fengningensis'': A synonym of ''Vescornis'' due to having been described from the same specimen. Despite having been described in 1999, 5 years prior to the description of ''Vescornis'', the description was so poor compared to the description of ''Vescornis'' that the latter name is considered to take priority by most authors. As a result, the name ''Hebeiornis'' is considered a ''
nomen nudum In Taxonomy (biology), taxonomy, a ''nomen nudum'' ('naked name'; plural ''nomina nuda'') is a designation which looks exactly like a scientific name of an organism, and may have originally been intended to be one, but it has not been published ...
'' ("naked name"). * " Proornis" is an informally-named bird from North Korea. It may not be a member of Enantiornithes. * '' Liaoxiornis delicatus'': Described in 1999 from a specimen of Enantiornithes found in the
Yixian Formation The Yixian Formation (; formerly Romanization of Chinese, transcribed as Yihsien Formation or Yixiang Formation) is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans the Barremian stage of the Early Cretaceous. I ...
. This specimen was originally considered to be a tiny adult, but later found to be a hatchling. Other specimens have henceforth been assigned to the genus. Due to a lack of distinguishing feature, many paleontologists have considered this genus an undiagnostic ''
nomen dubium In binomial nomenclature, a ''nomen dubium'' (Latin for "doubtful name", plural ''nomina dubia'') is a scientific name that is of unknown or doubtful application. Zoology In case of a ''nomen dubium,'' it may be impossible to determine whether a ...
''. * "Wasaibpanchi": A supposed member of Enantiornithes from Pakistan; the describing paper is of dubious status. *LP-4450: A juvenile of an indeterminate specimen of Enantiornithes from the El Montsec Formation of
Spain Spain, or the Kingdom of Spain, is a country in Southern Europe, Southern and Western Europe with territories in North Africa. Featuring the Punta de Tarifa, southernmost point of continental Europe, it is the largest country in Southern Eur ...
. Its 2006 description studied the
histology Histology, also known as microscopic anatomy or microanatomy, is the branch of biology that studies the microscopic anatomy of biological tissue (biology), tissues. Histology is the microscopic counterpart to gross anatomy, which looks at large ...
of the skeleton, while later studies reported a squamosal bone present in the specimen but unknown in other Enantiornithes. *IVPP V 13939: Briefly described in 2004, this Yixian Enantiornithes specimen had advanced pennaceous feathers on its legs, similar to (albeit shorter than) those of other paravians such as ''
Microraptor ''Microraptor'' (Greek language, Greek, μικρός, ''mīkros'': "small"; Latin language, Latin, ''raptor'': "one who seizes") is a genus of small, four-winged dromaeosaurid dinosaurs. Numerous well-preserved fossil specimens have been recovere ...
'' and ''
Anchiornis ''Anchiornis'' is a genus of small, four-winged Paraves, paravian dinosaurs, with only one known species, the type species ''Anchiornis huxleyi'', named for its similarity to modern birds. The Latin name ''Anchiornis'' derives from a Greek word m ...
''. *DIP-V-15100 and DIP-V-15101: Two different wings from hatchling specimens which were described in 2015. They attracted a significant amount of media attention upon their description. They were preserved in exceptional details due to having been trapped within Burmese amber for approximately 99 million years. *HPG-15-1: A partial corpse of an Enantiornithes hatchling also preserved in Burmese amber. Although indeterminate, it attracted even more media attention than the two wings upon its description in 2017. *CUGB P1202: An indeterminate juvenile
bohaiornithid Bohaiornithidae is a group of early enantiornithean stem-birds from the early Cretaceous Period of China. All undisputed Bohaiornithidae specimens come from the northeast of present-day China – specifically, the eastern and southern foothil ...
from the
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 (see Jehol Biota). It is a member of the Jehol group. T ...
. A 2016 analysis of its feathering found elongated putative
melanosomes A melanosome is an organelle found in animal cells and is the site for synthesis, storage and transport of melanin, the most common light-absorbing pigment found in the animal kingdom. Melanosomes are responsible for color and photoprotection ...
, suggesting that a large portion of its feathering was
iridescent Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear gradually to change colour as the angle of view or the angle of illumination changes. Iridescence is caused by wave interference of light in microstruc ...
. *DIP-V-15102: Another corpse of an indeterminate hatchling preserved in Burmese amber. Described in early 2018. *MPCM-LH-26189 a/b: A partial skeleton of a hatchling from Las Hoyas in Spain, including both slab and counter-slab components. Its 2018 description revealed how various features developed in Enantiornithes as they aged. Such features include the
ossification Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...
of the
sternum The sternum (: sternums or sterna) or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major bl ...
from various smaller bones, and the fusion of tail vertebrae into a
pygostyle Pygostyle is a skeletal condition in which the final few caudal vertebrae are fused into a single ossification, supporting the tail feathers and musculature. In modern birds, the rectrices attach to these. The pygostyle is the main component o ...
.


References


External links

{{Taxonbar, from=Q132268 Bird clades Cretaceous dinosaurs