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 ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...
.
Almost all retained teeth and clawed fingers on each wing, but otherwise looked much like modern birds externally. Over eighty 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, along with
Hesperornithes and all other non-avian
dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
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'', was originally considered a
paleognath related to
ostrich
Ostriches are large flightless birds of the genus ''Struthio'' in the order Struthioniformes, part of the infra-class Palaeognathae, a diverse group of flightless birds also known as ratites that includes the emus, rheas, and kiwis. There a ...
es and
tinamou
Tinamous () form an order of birds called Tinamiformes (), comprising a single family called Tinamidae (), divided into two distinct subfamilies, containing 46 species found in Mexico, Central America, and South America. The word "tinamou" comes ...
. The Enantiornithes were first recognized as a distinct lineage, or "subclass" of birds, by
Cyril A. 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 cont ...
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 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
period of what is now
Argentina
Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, t ...
, which he assigned to a new genus, ''
Enantiornis'', 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'' ("Spanish 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 Cal ...
'', ''
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 ...
'') 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 ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic p ...
''enantios'' () "opposite" + ''ornithes'' () "birds" . The name was coined by
Cyril Alexander Walker 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 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 stated in 1996:
Feduccia's point about the ''
tarsometatarsus'' (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
Enantiornithine-like birds found in
Albian
The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous Epoch/ Series. Its approximate time range is 113.0 ± 1.0 Ma to 100.5 ± ...
of
Australia,
Maastrichtian
The Maastrichtian () is, in the ICS geologic timescale, the latest age (uppermost stage) of the Late Cretaceous Epoch or Upper Cretaceous Series, the Cretaceous Period or System, and of the Mesozoic Era or Erathem. It spanned the interv ...
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. Campani ...
of
Mexico
Mexico ( Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guate ...
(''
Alexornis''),
Mongolia
Mongolia; Mongolian script: , , ; lit. "Mongol Nation" or "State of Mongolia" () is a landlocked country in East Asia, bordered by Russia to the north and China to the south. It covers an area of , with a population of just 3.3 millio ...
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, it contains the geographic South Pole. Antarctica is the fifth-largest co ...
. Fossils attributable to this group are exclusively
Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
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 period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
relatives. The earliest known Enantiornithes are from the
Early Cretaceous
The Early Cretaceous (geochronological name) or the Lower Cretaceous ( chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma.
Geology
Pr ...
of
Spain
, image_flag = Bandera de España.svg
, image_coat = Escudo de España (mazonado).svg
, national_motto = '' Plus ultra'' ( Latin)(English: "Further Beyond")
, national_anthem = (English: "Royal March")
, ...
(e.g. ''
Noguerornis
''Noguerornis'' is a genus of enantiornithine bird possibly related to ''Iberomesornis''. It lived during the Early Cretaceous (early Barremian age) about 130 mya and is known from fossils found in the La Pedrera de Rúbies Formation at El Monts ...
'') and
China (e.g. ''
Protopteryx'') and the latest from the
Late Cretaceous
The Late Cretaceous (100.5–66 Ma) is the younger 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 ''creta'', ...
of North and South America (e.g. ''
Avisaurus'' and ''
Enantiornis''). 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,
Spain
, image_flag = Bandera de España.svg
, image_coat = Escudo de España (mazonado).svg
, national_motto = '' Plus ultra'' ( Latin)(English: "Further Beyond")
, national_anthem = (English: "Royal March")
, ...
and the
Jehol group in
Liaoning (
China). 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. ...
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
Sparrow may refer to:
Birds
* Old World sparrows, family Passeridae
* New World sparrows, family Passerellidae
* two species in the Passerine family Estrildidae:
** Java sparrow
** Timor sparrow
* Hedge sparrow, also known as the dunnock or hedg ...
and a
starling
Starlings are small to medium-sized passerine birds in the family Sturnidae. The Sturnidae are named for the genus ''Sturnus'', which in turn comes from the Latin word for starling, ''sturnus''. Many Asian species, particularly the larger ones, ...
,
however display considerable variation in size with some species. The largest species in this clade include ''
Pengornis houi
''Pengornis'' is the largest known enantiornithine 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.
''Pengorn ...
'',
''
Xiangornis shenmi
''Xiangornis'' is an enantiornithine bird from the Lower Cretaceous Jiufofang Formation of Western Liaoning, China
China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous cou ...
'', ''
Zhouornis hani
''Zhouornis'' is an extinct genus of enantiornithine bird known from the Early Cretaceous Jehol Group (Aptian stage) of western Liaoning Province, northeastern China. ''Zhouornis'' was first named by Zihui Zhang, Luis M. Chiappe, Gang Han and A ...
'',
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 (and possibly also represented in the
Wonthaggi Formation by a single
furcula
The (Latin for "little fork") or wishbone is a forked bone found in most birds and some species of non-avian dinosaurs, and is formed by the fusion of the two pink
clavicles. In birds, its primary function is in the strengthening of the Thorax, ...
). 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 skulls 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'', 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 mammal has ...
e (bones of the snout tip) and most species had toothy jaws rather than toothless beaks. Only a few species, such as ''
Gobipteryx minuta'', were fully toothless and had beaks. They also had simple
quadrate bones, a complete bar separating each
orbit
In celestial mechanics, an orbit 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 object or position in space such a ...
(eye hole) from each
antorbital fenestra, and dentaries (the main toothed bones of the lower jaw) without forked rear tips. A
squamosal bone 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 v ...
is preserved in ''
Shenqiornis'' and ''
Pengornis''. In modern birds these bones are assimilated into the cranium. Some Enantiornithes may have had their
temporal fenestrae (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, 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 (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 adaptions 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. 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
alulas, 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. ...
. These are the first complete
Mesozoic
The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Creta ...
dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
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'', 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 '' '' (a 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 (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 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'' and certain extant
birds-of-paradise.
However, further discoveries showed that at least among basal Enantiornithes, tail anatomy was more complex than previously thought. One genus, ''
Shanweiniao'', 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 grc, ῥάχις [], "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 c ...
-dominated tail feathers similar to feathers present in '' Paraprotopteryx''. '' Chiappeavis'', a primitive pengornithid
Pengornithidae is a group of early 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 putative pengornith ...
, had a fan of tail feathers similar to that of more primitive avialans like '' Sapeornis'', 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
''Feitianius'' is a bird genus, belonging to the Enantiornithes, that during the Early Cretaceous lived in the area of modern China. A single species has been named in the genus, ''Feitianius paradisi''.
In 2005, fragmentary enantiornithean fos ...
'', 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'' were likely used to feed on soft-bodied arthropods.[ The strongly hooked talons of Bohaiornithidae 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'' was found to have the remains of ]exoskeleton
An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton ( endoskeleton) in for example, a human. In usage, some of the ...
s from aquatic crustacean
Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean gro ...
s preserved in its digestive tract, and ''Enantiophoenix
''Enantiophoenix'' is a genus of enantiornithine
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 fro ...
'' preserved corpuscles of amber among the fossilized bones, suggesting that this animal fed on tree sap, much like modern sapsuckers 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 and the supposed gastroliths of '' Bohaiornis'' being random mineral precipitates, only the '' Eoalulavis'' 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 is the most extensively studied family in terms of diet due to their rather unusual resotral 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.
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 (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the Order (biology), order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cre ...
or small theropod
Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally ...
dinosaur. This was the first evidence that Mesozoic
The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Creta ...
avialans were prey animals, and that some Mesozoic pan-avians regurgitated pellets like owls do today.
Life history
Known fossils of Enantiornithes include eggs, embryo
An embryo is an initial stage of development of 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 sperm ...
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 ...
s. An embryo, still curled in its egg, has been reported from the Yixian Formation
The Yixian Formation (; formerly transcribed as Yihsien Formation) is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans the late Barremian and early Aptian stages of the Early Cretaceous. It is known for its ...
. 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
''Liaoxiornis'' is a dubious genus of enantiornithine bird. The only named species is ''Liaoxiornis delicatus'', described by Hou and Chen in 1999.Hou, L., Chen, P.-J. (1999) "Liaoxiornis delicatus gen. et sp. nov., the smallest Mesozoic bird. C ...
''"; however, Luis Chiappe 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 s ...
''.[
Together with hatchling specimens of the Mongolian '' Gobipteryx'' and '' Gobipipus'',] these finds demonstrate that hatchling Enantiornithes had the skeletal ossification, well-developed wing feathers, and large brain which correlate with precocial
In biology, altricial species are those in which the young are underdeveloped at the time of birth, but with the aid of their parents mature after birth. Precocial species are those in which the young are relatively mature and mobile from the mome ...
or superprecocial 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.[
Analyses of Enantiornithes bone histology have been conducted to determine the growth rates of these animals. A 2006 study of '' Concornis'' bones showed a growth pattern different from modern birds; although growth was rapid for a few weeks after hatching, probably until fledging, 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
In biology, altricial species are those in which the young are underdeveloped at the time of birth, but with the aid of their parents mature after birth. Precocial species are those in which the young are relatively mature and mobile from the mome ...
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'' and in non-avialan dinosaurs. Some analyses have interpreted the bone histology to indicate that Enantiornithes may not have had fully avian endothermy, instead having an intermediate metabolic rate
Metabolism (, from el, μεταβολή ''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 cel ...
. 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 younger 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 ''creta'', ...
(Maastrichtian
The Maastrichtian () is, in the ICS geologic timescale, the latest age (uppermost stage) of the Late Cretaceous Epoch or Upper Cretaceous Series, the Cretaceous Period or System, and of the Mesozoic Era or Erathem. It spanned the interv ...
) of Romania
Romania ( ; ro, România ) is a country located at the crossroads of Central, Eastern, and Southeastern Europe. It borders Bulgaria to the south, Ukraine to the north, Hungary to the west, Serbia to the southwest, Moldova to the east, a ...
. 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 supreprecociality 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 rectrices 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.
Due to the difference in sternal and shoulder girdle anatomy, many Enantiornithes used a flight style unlike that of any modern bird species, though more typical flight styles were present as well.
At least '' Elsornis'' appears to have become secondarily flightless
Flightless birds are birds that through evolution lost the ability to fly. There are over 60 extant species, including the well known ratites ( ostriches, emu, cassowaries, rheas, and kiwi) and penguins. The smallest flightless bird is ...
.
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 lightweig ...
. Others use the more restrictive crown group 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 lightweig ...
, anatomically modern birds), and place Enantiornithes in the more inclusive group Avialae. Enantiornithes were more advanced than '' Archaeopteryx'', '' Confuciusornis'', and '' Sapeornis'', 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. The other ornithothoracine group is Euornithes or Ornithuromorpha, 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 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''". 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, and together they form a clade called Ornithothoraces (though see above). Most phylogenetic
In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
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 autapomorphies to just four.
Enantiornithes systematics are highly provisional and notoriously difficult to study, due to the fact that Enantiornithes tend to be extremely homoplastic
Homoplasy, in biology and phylogenetics, is the term used to describe a Phenotypic trait, feature that has been gained or lost independently in separate lineages over the course of evolution. This is different from Homology (biology), homology, w ...
, 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 enantiornithine clades.][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 ...
'' than to ''Iberomesornis
''Iberomesornis'' ("Spanish 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 Cal ...
''. Because ''Iberomesornis'' is often found to be the most primitive or basal enantiornithine, Euenantiornithes may be an extremely inclusive group, made up of all Enantiornithes except for ''Iberomesornis'' itself. Despite being in accordance with phylogenetic nomenclature, 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 and a National Geographic "explorer-in-residence" who has discovered several new dinosaur species on several continents, including at sites ...
, who called it "a ill-defined clade ..a good example of a poor choice in a phylogenetic definition".
The cladogram
A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...
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
''Enantiophoenix'' is a genus of enantiornithine
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 fro ...
''
''f'' = '' Houornis''
''h'' = '' Longipteryx''
''i'' = '' Parabohaiornis''
''j'' = '' Pterygornis''
''l'' = '' Vorona''
''m'' = '' Yuanjiawaornis''
''n'' = '' Yungavolucris''
List of genera
Incertae sedis
' () or ''problematica'' is a term used for a 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, uncertain ...
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 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 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 cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gr ...
of this group is doubtful, and it may actually be an evolutionary grade.
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 enan ...
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 Barun Goyot Formation (also known as Baruungoyot Formation or West Goyot Formation) 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 ...
of Mongolia
Mongolia; Mongolian script: , , ; lit. "Mongol Nation" or "State of Mongolia" () is a landlocked country in East Asia, bordered by Russia to the north and China to the south. It covers an area of , with a population of just 3.3 millio ...
. These specimens were very similar to embryonic ''Gobipteryx'' specimens, although the describers of ''Gobipipus'' (a set of controversial paleontologists including Evgeny Kurochkin and Sankar Chatterjee) consider it distinct.
*''Hebeiornis
''Vescornis'' is a genus of enantiornithine bird. One species is named, ''V. hebeiensis''. The holotype fossil is in the collection of the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences. Its catalog number is CAGS 1307 ...
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, 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 with an adequate des ...
'' ("naked name").
* " Proornis" is an informally-named bird from North Korea. It may not be a member of Enantiornithes.
* ''Liaoxiornis delicatus
''Liaoxiornis'' is a dubious genus of enantiornithine bird. The only named species is ''Liaoxiornis delicatus'', described by Hou and Chen in 1999.Hou, L., Chen, P.-J. (1999) "Liaoxiornis delicatus gen. et sp. nov., the smallest Mesozoic bird. C ...
'': Described in 1999 from a specimen of Enantiornithes found in the Yixian Formation
The Yixian Formation (; formerly transcribed as Yihsien Formation) is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans the late Barremian and early Aptian stages of the Early Cretaceous. It is known for its ...
. 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 s ...
''.
* "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
EL, El or el may refer to:
Religion
* El (deity), a Semitic word for "God"
People
* EL (rapper) (born 1983), stage name of Elorm Adablah, a Ghanaian rapper and sound engineer
* El DeBarge, music artist
* El Franco Lee (1949–2016), American po ...
Formation of Spain
, image_flag = Bandera de España.svg
, image_coat = Escudo de España (mazonado).svg
, national_motto = '' Plus ultra'' ( Latin)(English: "Further Beyond")
, national_anthem = (English: "Royal March")
, ...
. Its 2006 description studied the histology
Histology,
also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures vi ...
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'' and '' Anchiornis''.
*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 from the Jiufotang Formation
The Jiufotang Formation ( Chinese: 九佛堂组, pinyin: ''jiǔfótáng zǔ'') is an Early Cretaceous geological formation in Chaoyang, Liaoning which has yielded fossils of feathered dinosaurs, primitive birds, pterosaurs, and other organisms (see ...
. A 2016 analysis of its feathering found elongated putative melanosomes, suggesting that a large portion of its feathering was iridescent
Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear to gradually change color as the angle of view or the angle of illumination changes. Examples of iridescence include soap bubbles, feathers, butterfl ...
.
*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 ...
of the sternum
The sternum 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 blood vessels from injury. ...
from various smaller bones, and the fusion of tail vertebrae into a pygostyle.
References
External links
{{Taxonbar, from=Q132268
Cretaceous birds
Early Cretaceous first appearances
Maastrichtian extinctions