| Enantiornithes | |
|---|---|
 | |
| Fossil specimen of abohaiornithid (Zhouornis hani) | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | Avialae |
| Clade: | Ornithothoraces |
| Clade: | †Enantiornithes Walker, 1981 |
| Subgroups | |
and see text | |
TheEnantiornithes, also known as enantiornithines or enantiornitheans in literature, are a group of extinctavialans ("birds" in the broad sense), the most abundant and diverse group known from theMesozoic era.[3][4][5] 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 theCretaceous–Paleogene boundary, along withHesperornithes and all other non-aviandinosaurs.
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 apaleognath related toostriches andtinamou.[6] The Enantiornithes were first recognized as a distinct lineage, or "subclass" of birds, byCyril A. Walker in 1981. Walker made this discovery based on some partial remains from the lateCretaceousperiod of what is nowArgentina, 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,Cathayornis, andSinornis) were instead Enantiornithes.
The name "Enantiornithes" means "opposite birds", fromAncient Greekenantios (ἐνάντιος) "opposite" +ornithes (ὄρνιθες) "birds" . The name was coined byCyril Alexander Walker in his landmark paper which established the group.[7] In his paper, Walker explained what he meant by "opposite":
Perhaps the most fundamental and characteristic difference between the Enantiornithes and all other birds is in the nature of the articulation between thescapula [...] and thecoracoid, where the 'normal' condition is completely reversed.[7]
This refers to an anatomical feature – the articulation of theshoulder bones – which has a concave-convex socket joint between thescapula (shoulder blade) andcoracoid (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 isconcave and dish-shaped at this joint, and the coracoid is convex.[3]: 249–50 In modern birds, the coracoscapular joint has a concave coracoid and convex scapula.[8][9]
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:
The birds are so named because, among many distinctive features, there is a unique formation of the triosseal canal and the metatarsals are fused proximally to distally, the opposite of that in modern birds[10]
Feduccia's point about thetarsometatarsus (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.[3]
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 subfamilyEnantiornithinae. Following the naming conventions used for modern birds as well as extinct groups, it has been pointed out that the correct term is "enantiornithean".[11][12]
Praeornis, from theOxfordian-Kimmeridgian ofKazakhstan, may have been the earliest known member of Enantiornithes according to Agnolin et al. (2017).[13]
Birds with confidently identified characteristics of Enantiornithes found inAlbian ofAustralia,Maastrichtian of South America, andCampanian ofMexico (Alexornis[14]),Mongolia and western edge of prehistoric Asia suggest a worldwide distribution of this group or in the relatively warm regions, at least.[15] Enantiornithes have been found on every continent exceptAntarctica. Fossils attributable to this group are exclusivelyCretaceous in age, and it is believed that the Enantiornithes became extinct at the same time as their non-avialandinosaur relatives. The earliest known Enantiornithes are from theEarly Cretaceous ofSpain (e.g.Noguerornis) andChina (e.g.Protopteryx) and the latest from theLate Cretaceous of North and South America (e.g.Avisaurus andEnantiornis). 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.
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 fromLas Hoyas inCuenca, Spain and theJehol group inLiaoning (China). Extraordinary remains of Enantiornithes have also been preserved inBurmese amber deposits dated to 99 million years ago and include hatchlings described in 2017[16] and 2018,[17] as well as isolated body parts such as wings[18][19][20] and feet.[19][21] 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 asparrow and astarling,[22] however display considerable variation in size with some species. The largest species in this clade includePengornis houi,[23]Xiangornis shenmi,[24]Zhouornis hani,[22] andMirarce eatoni,[25] (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 theEumeralla Formation (and possibly also represented in theWonthaggi Formation by a singlefurcula).[26] Among the smallest described specimens are unnamed hatchlings, although the holotype specimens ofParvavis chuxiongensis[27] andCratoavis cearensis[28] are comparable in size to smalltits orhummingbirds.
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 likeArchaeopteryx, they retained several separate cranial bones, smallpremaxillae (bones of the snout tip) and most species had toothy jaws rather than toothless beaks. Only a few species, such asGobipteryx minuta, were fully toothless and had beaks. They also had simplequadrate bones, a complete bar separating eachorbit (eye hole) from eachantorbital fenestra, and dentaries (the main toothed bones of the lower jaw) without forked rear tips. Asquamosal bone is preserved in an indeterminate juvenile specimen, while apostorbital is preserved inShenqiornis andPengornis. In modern birds these bones are assimilated into the cranium. Some Enantiornithes may have had theirtemporal 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.[29] Aquadratojugal bone, which in modern birds is fused to the jugal, is preserved inPterygornis.[30] The presence of these primitive features of the skull would have rendered the Enantiornithes capable of only limitedcranial kinesis (the ability to move the jaw independent of the cranium).[31]
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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 likeArchaeopteryx, and displayed some features related to flight similar to those found in the lineage leading to modern birds, theOrnithuromorpha. 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 hadalulas, 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.[32]
Several wings with preserved feathers have been found inBurmese amber. These are the first completeMesozoicdinosaur 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.[33] 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.[18]
One fossil of Enantiornithes shows wing-like feather tufts on its legs, similar toArchaeopteryx. The leg feathers are also reminiscent of the four-winged dinosaurMicroraptor, 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.[34]
Clarkeet 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 ofEuornithes (the clade they referred to asOrnithurae) with preserved tail feathers had only short coverts or elongated paired tail plumes. They suggested that the development of thepygostyle 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 therectrical 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 extinctConfuciusornis and certain extantbirds-of-paradise.[35]
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[36] and might have formed a lift-generating surface similar to the tail fans ofEuornithes,[37] though a later study indicates thatShanweiniao was more likely to haverachis-dominated tail feathers similar to feathers present inParaprotopteryx.[38]Chiappeavis, a primitivepengornithid, had a fan of tail feathers similar to that of more primitive avialans likeSapeornis, suggesting that this might have been the ancestral condition, with pinfeathers being a feature evolved several times in early avialans for display purposes.[38] 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.[39] At least one genus of Enantiornithes,Cruralispennia, had a modern-looking pygostyle but lacked a tail fan.[40]
Given the wide diversity of skull shape among Enantiornithes, many different dietary specializations must have been present among the group. Some, likeShenqiornis, had large, robust jaws suitable for eating hard-shelled invertebrates. The short, blunt teeth ofPengornis were likely used to feed on soft-bodied arthropods.[29] The strongly hooked talons ofBohaiornithidae suggest that they were predators of small to medium-sized vertebrates, but their robust teeth instead suggest a diet of hard-shelled animals.[2]
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 ofexoskeletons from aquaticcrustaceans preserved in its digestive tract,[41] andEnantiophoenix preserved corpuscles of amber among the fossilized bones, suggesting that this animal fed on tree sap, much like modernsapsuckers and other birds. The sap would have fossilized and become amber.[42] However, more recently it has been suggested that the sap moved post-mortem, hence not representing true stomachal contents. Combined with the putative fishpellets ofPiscivorenantiornis turning out to be fish excrement, the strange stomachal contents of some species turning out to beovaries and the supposed gastroliths ofBohaiornis being random mineral precipitates, only theEoalulavis displays actual stomach contents.[43]
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,[44] though some modern birds have lost the gizzard and rely solely on strong stomachal acids.[45] 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.[46]
Longipterygidae 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,[47] probers akin to shorebirds[48] and as arboreal bark-probers.[49] A 2022 study however does find them most likely to be generalistic insectivores (sans possiblyShengjingornis 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.[50] However a posterior study has found them to be herbivorous, including the presence ofgymnosperm seeds in their digestive system.[51]
.png)
Avisaurids occupied a niche analogous to modernbirds of prey, having the ability to lift small prey with their feet in a manner similar to hawks or owls.[52][53]
A fossil from Spain reported by Sanzet 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 apterosaur or smalltheropod dinosaur. This was the first evidence thatMesozoic avialans were prey animals, and that some Mesozoicpan-avians regurgitated pellets like owls do today.[54]
Known fossils of Enantiornithes includeeggs,[55][56]embryos,[57] andhatchlings.[58] An embryo, still curled in its egg, has been reported from theYixian Formation.[59] 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.[60] Some hatchling specimens have been given formal names, including "Liaoxiornis delicatus"; 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 anomen dubium.[60]
Together with hatchling specimens of the MongolianGobipteryx[61] andGobipipus,[62][63] these finds demonstrate that hatchling Enantiornithes had the skeletal ossification, well-developed wing feathers, and large brain which correlate withprecocial orsuperprecocial 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.[60]
Findings suggests Enantiornithes, especially the toothed species, had a longer incubation time than modern birds.[64][65]
Analyses of Enantiornithes bone histology have been conducted to determine the growth rates of these animals. A 2006 study ofConcornis bones showed a growth pattern different from modern birds; although growth was rapid for a few weeks after hatching, probably untilfledging, this small species did not reach adult size for a long time, probably several years.[66] Other studies have all supported the view that growth to adult size was slow, as it is in living precocial birds (as opposed toaltricial birds, which are known to reach adult size quickly).[41] 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 likeJeholornis and in non-avialan dinosaurs.[67] Some analyses have interpreted the bone histology to indicate that Enantiornithes may not have had fully avianendothermy, instead having an intermediatemetabolic rate.[68] However a 2021 study rejects the idea that they had less endothermic metabolisms than modern birds.[69]
Evidence of colonial nesting has been found in Enantiornithes, in sediments from theLate Cretaceous (Maastrichtian) ofRomania.[70] Evidence from nesting sites shows that Enantiornithes buried their eggs like modernmegapodes, which is consistent with their inferred superprecocial adaptations.[71]
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.[72]
It has been speculated that superprecociality in Enantiornithes might have prevented them from developing specialised toe arrangements seen in modern birds like zygodactyly.[73]
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.[74]
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,[75] as well as due to the absence ofrectrices in many species.[35][37][76]
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 foravian flight as a whole - some extinct birds likelithornids also lacked complex tail feathers but were good flyers,[77] and they appear to have been capable of a ground based launching.[78]
Enantiornithes resembleOrnithuromorphs 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 robustpygostyle seems unable to support the muscles that control the modern tail feathers involved in flight.[79] 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 narrowfurcula with a short hypocleidium, and ulnar quill knobs that indicate increased aerial abilities.[80][81]
At leastElsornis appears to have become secondarilyflightless.[82]
Some researchers classify Enantiornithes, along with the true birds, in the classAves. Others use the more restrictivecrown group definition of Aves (which only includesneornithes, anatomically modern birds), and place Enantiornithes in the more inclusive groupAvialae. Enantiornithes were more advanced thanArchaeopteryx,Confuciusornis, andSapeornis, 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 groupOrnithothoraces. The other ornithothoracine group isEuornithes orOrnithuromorpha, 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.[3] One study has however found that the shared sternal anatomy was acquired independently and such a relationship needs to be reexamined.[83]
Enantiornithes classification and taxonomy has historically been complicated by a number of factors. In 2010, paleontologistsJingmai O'Connor andGareth 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 "functionalnomina dubia".[84] 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".[84]
Enantiornithes is the sister group toEuornithes, and together they form aclade calledOrnithothoraces (though see above). Mostphylogenetic 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 Enantiornithesautapomorphies to just four.[85]
Enantiornithes systematics are highly provisional and notoriously difficult to study, due to their small size[28] and the fact that Enantiornithes tend to be extremelyhomoplastic, or very similar to each other in most of their skeletal features due to convergent evolution rather than common ancestry.[38] 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.[86]
One such delineation named theEuenantiornithes, was defined by Chiappe (2002) as comprising all species closer toSinornis than toIberomesornis. BecauseIberomesornis 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 forIberomesornis itself. Despite being in accordance withphylogenetic nomenclature, this definition of Euenantiornithes was severely criticized by some researchers, such asPaul Sereno, who called it "a ill-definedclade [...] a good example of a poor choice in a phylogenetic definition".[86]
Thecladogram below was found by an analysis by Wanget al. in 2015, updated from a previous data set created by Jingmai O'Connor.[30]
The cladogram below is from Wanget 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.[87]
| Enantiornithes | |
| l |
Key to letters:
b =Boluochia
c =Cathayornis
e =Enantiophoenix
f =Houornis
h =Longipteryx
i =Parabohaiornis
j =Pterygornis
l =Vorona
m =Yuanjiawaornis
n =Yungavolucris
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.[citation needed]
| Name | Year | Formation | Location | Notes | Images |
|---|---|---|---|---|---|
| Abavornis | 1998 | Bissekty Formation (Late Cretaceous,Turonian toConiacian) |  Uzbekistan | One of many fragmentary Bissekty Enantiornithes, known only fromcoracoids | |
| Alethoalaornis | 2007 | Jiufotang Formation (Early Cretaceous,Aptian) |  China | Poorly known | |
| Alexornis | 1976 | La Bocana Roja Formation (Late Cretaceous,Campanian) |  Mexico | One of the first Enantiornithes known. Once thought to be an ancient relative ofrollers andwoodpeckers |  |
| Avimaia | 2019 | Xiagou Formation (Early Cretaceous,Aptian) | China | One specimen from this genus died with an unlaid egg in its body |  |
| Bauxitornis | 2010 | Csehbánya Formation (Late Cretaceous,Santonian) |  Hungary | Fragmentary but unique in the structure of its tarsometatarsus |  |
| Brevirostruavis | 2021 | Jiufotang Formation (Early Cretaceous,Aptian) | China | Possessed an enlargedhyoid that suggests a feeding specialization similar to hummingbirds, honeyeaters, and woodpeckers |  |
| Castignovolucris | 2023 | Unnamed formation (Late Cretaceous,Campanian) |  France | May have been the size of aCanada goose | |
| Catenoleimus | 1998 | Bissekty Formation (Late Cretaceous,Turonian toConiacian) | Uzbekistan | One of many fragmentary Bissekty Enantiornithes, known only from acoracoid | |
| Cathayornis | 1992 | Jiufotang Formation (Early Cretaceous,Aptian) | China | One of the first Jehol biota Enantiornithes described. Known from many species, although some are now placed into their own genera. May have had a similar appearance and lifestyle to apitta |  |
| Concornis | 1992 | Las Hoyas (Early Cretaceous,Barremian) |  Spain | One of the most complete Las Hoyas Enantiornithes |  |
| Cratoavis[88] | 2015 | Santana Formation (Early Cretaceous,Aptian) |  Brazil | A very well-preserved South American member of the group, complete with ribbon-like tail feathers | |
| Cruralispennia[40] | 2017 | Huajiying Formation (Early Cretaceous,Hauterivian) | China | Had an unusual ornithuromorph-like pygostyle and brush-like thigh feathers. One of the oldest Enantiornithes |  |
| Cuspirostrisornis | 1997 | Jiufotang Formation (Early Cretaceous,Aptian) | China | Originally mistakenly believed to have possessed a pointed beak |  |
| Dalingheornis | 2006 | Yixian Formation (Early Cretaceous,Aptian) | China | Was well-adapted for climbing due to itsheterodactyl feet, like those of atrogon | |
| Dunhuangia[89] | 2015 | Xiagou Formation (Early Cretaceous,Aptian) | China | A genus of Enantiornithes from the Changma basin, an area which is unusually dominated by ornithuromorphs | |
| Elbretornis | 2009 | Lecho Formation (Late Cretaceous,Maastrichtian) |  Argentina | Only known from wing bones. May be synonymous with other Lecho formation Enantiornithes | |
| Elektorornis | 2019 | Burmese Amber (Late Cretaceous,Cenomanian) |  Myanmar | Known from a foot preserved in amber with an elongated middle toe | |
| Enantiornis | 1981 | Lecho Formation (Late Cretaceous,Maastrichtian) | Argentina | Although only known from a few bones, this genus is the namesake of Enantiornithes. It was also one of the largest and last representative of the group prior to their extinction | |
| Eoalulavis | 1996 | Las Hoyas (Early Cretaceous,Barremian) | Spain | Preserves feathers including analula, a specialized type of feather which controls air flow over the wing | |
| Eocathayornis | 2002 | Jiufotang Formation (Early Cretaceous,Aptian) | China | Once considered to be a basal close relative ofCathayornis, although now considered to be more distantly related | |
| Eoenantiornis | 1999 | Yixian Formation (Early Cretaceous,Aptian) | China | Well-preserved but inconsistent in phylogenetic placement |  |
| Evgenavis | 2014 | Ilek Formation (Early Cretaceous,Barremian) |  Russia | Known only from a tarsometatarsus which shares some features with those of Enantiornithes | |
| Explorornis | 1998 | Bissekty Formation (Late Cretaceous,Turonian toConiacian) | Uzbekistan | One of many fragmentary Bissekty Enantiornithes, known only fromcoracoids | |
| Falcatakely | 2020 | Maevarano Formation (Late Cretaceous,Maastrichtian) |  Madagascar | Developed a massive snout with only a single tooth, despite retaining a "primitive" skull arrangement in contrast to modern birds |  |
| Feitianius[39] | 2015 | Xiagou Formation (Early Cretaceous,Aptian) | China | Possessed an elaborate set of tail feathers, unlike the paired ribbon-like feathers of most Enantiornithes |  |
| Flexomornis | 2010 | Woodbine Formation (Late Cretaceous,Cenomanian) |  United States ( Texas) | One of the oldest North American avialans found, albeit known only from fragmentary remains | |
| Fortipesavis | 2021 | Burmese amber (Late Cretaceous,Cenomanian) | Myanmar | Had an enlarged outer toe that may have been an adaptation for perching | |
| Fortunguavis[90] | 2014 | Jiufotang Formation (Early Cretaceous,Aptian) | China | Had robust bones, including feet and claws which may have been adapted for climbing trees | |
| Grabauornis[91] | 2015 | Yixian Formation (Early Cretaceous,Barremian) | China | The proportions of the wings of this genus of Enantiornithes as well as the presence of an alula suggest that it was a good flier | |
| Gracilornis | 2011 | Jiufotang Formation (Early Cretaceous,Aptian) | China | A possible relative ofCathayornis with characteristically slender bones | |
| Gurilynia | 1999 | Nemegt Formation (Late Cretaceous,Maastrichtian) |  Mongolia | A poorly known genus of Enantiornithes, but evidently a large and late-surviving member of the group | |
| Hollanda[92] | 2010 | Barun Goyot Formation (Late Cretaceous,Campanian) | Mongolia | Originally identified as an ornithuromorph but since reinterpreted as a genus of Enantiornithes closely related toLectavis.[93] |  |
| Holbotia[94] | 2015 | Andaikhudag Formation (Early Cretaceous,Aptian) | Mongolia | Considered a smallpterosaur since its discovery in 1977 until it received a formal description in 2015. Possessed unique neck vertebrae and a primitive palate | |
| Houornis | 1997 | Jiufotang Formation (Early Cretaceous,Aptian) | China | Once considered to be dubious or a species ofCathayornis, although a 2015 study considered it to be a valid genus[95] | |
| Huoshanornis | 2010 | Jiufotang Formation (Early Cretaceous,Aptian) | China | May have been a very maneuverable flier due to the structure of its hand and sternum | |
| Iberomesornis | 1992 | Las Hoyas (Early Cretaceous,Barremian) | Spain | One of the first genera of Enantiornithes known from decent remains. Also one of the oldest and most primitive members of the group |  |
| Imparavis[96] | 2024 | Jiufotang Formation (Early Cretaceous,Aptian) | China | The earliest known enantiornithine with a toothless beak | |
| Incolornis | 1998 | Bissekty Formation (Late Cretaceous,Turonian toConiacian) | Uzbekistan | One of many fragmentary Bissekty Enantiornithes, known only fromcoracoids. One species was once considered to belong toEnantiornis | |
| Junornis[97] | 2017 | Yixian Formation (Early Cretaceous,Aptian) | China | So well preserved that its flight pattern could be reconstructed using the proportions of its feathers and wings |  |
| Kizylkumavis | 1984 | Bissekty Formation (Late Cretaceous,Turonian toConiacian) | Uzbekistan | One of the many fragmentary Bissekty Enantiornithes, known only from ahumerus fragment |  |
| Largirostrornis | 1997 | Jiufotang Formation (Early Cretaceous,Aptian) | China | Possibly related toCuspirostrisornis or a synonym ofCathayornis | |
| Lectavis | 1993 | Lecho Formation (Late Cretaceous,Maastrichtian) | Argentina | A large and long-legged member of the group, proportionally similar to modern shorebirds |  |
| Lenesornis | 1996 | Bissekty Formation (Late Cretaceous,Turonian toConiacian) | Uzbekistan | One of many fragmentary Bissekty Enantiornithes, known only from asynsacrum fragment. Originally considered to belong toIchthyornis | |
| Liaoningornis | 1996 | Yixian Formation (Early Cretaceous,Aptian) | China | Originally believed to be an ornithuran, but now considered a relative ofEoalulavis | |
| Longchengornis | 1997 | Jiufotang Formation (Early Cretaceous,Aptian) | China | May have been a synonym ofCathayornis | |
| Magnusavis[98] | 2024 | Hell Creek Formation (Late Cretaceous,Maastrichtian) | United States ( Montana) | A large enantiornithine closely related to avisaurids | |
| Martinavis | 2007 | Grès à Reptiles Formation,Lecho Formation (Late Cretaceous,Maastrichtian) | FranceArgentina | Although known only from humeri, this genus was large and lived in a broad range | |
| Microenantiornis | 2017 | Jiufotang Formation (Early Cretaceous,Aptian) | China | A small member of the group which possessed several primitive and derived features compared to other Enantiornithes | |
| Mirusavis | 2020 | Yixian Formation (Early Cretaceous,Barremian toAptian) | China | Holotype was a small osteologically immature female preserved with medullary bone tissue | |
| Monoenantiornis[99] | 2016 | Yixian Formation (Early Cretaceous,Aptian) | China | Known from a juvenile specimen which depicts how various features developed in Enantiornithes as they age | |
| Musivavis | 2022 | Jiufotang Formation (Early Cretaceous,Aptian) | China | Most similar to bohaiornithids but also has features of other groups of Enantiornithes | |
| Nanantius | 1986 | Toolebuc Formation (Early Cretaceous,Albian) |  Australia | Fragmentary, but may have been a seabird because remains from this genus have been found asichthyosaur gut content | |
| Navaornis | 2024 | Adamantina Formation (Late Cretaceous,Santonian toCampanian) | Brazil | The first toothless enantiornithean from South America, known from a three-dimensionally preserved skull |  |
| Noguerornis | 1989 | El Montsec (Early Cretaceous,Barremian) | Spain | Preserves impressions of apropatagium, a skin flap on the shoulder which forms part of a wing | |
| Novavis[100] | 2025 | Xiagou Formation (Early Cretaceous,Aptian) | China | AnIcterus-sized bird with an unusually small pubis bone | |
| Orienantius | 2018 | Huajiying Formation (Early Cretaceous,Hauterivian) | China | Many soft tissue details of specimens from this genus were revealed by UV light | |
| Otogornis | 1993 | Yijinholuo Formation (Early Cretaceous) | China | Poorly known | |
| Paraprotopteryx | 2007 | Qiaotou member of theHuajiying Formation (Early Cretaceous,Aptian?) | China | Seemingly had four ribbon-like tail feathers instead of only two as in most Enantiornithes | |
| Parvavis[101] | 2014 | Jiangdihe Formation (Late Cretaceous,Turonian toSantonian) | China | Small but fully mature at the time of its death. One of only a few Chinese Enantiornithes dated to the Late Cretaceous |  |
| Piscivorenantiornis[102] | 2017 | Jiufotang Formation (Early Cretaceous,Aptian) | China | Known from a disarticulated skeleton preserved overlying a piece of stomach content composed of fish bones, which may have been its last meal | |
| Protopteryx | 2000 | Huajiying Formation (Early Cretaceous,Hauterivian) | China | One of the oldest and most primitive members of the group | _NMNS.jpg) |
| Pterygornis[30] | 2016 | Jiufotang Formation (Early Cretaceous,Aptian) | China | One disarticulated skeleton from this genus possesses well-preserved bones of the skull, including a quadratojugal | |
| Qiliania | 2011 | Xiagou Formation (Early Cretaceous,Aptian) | China | Some of this genus's remains include well-preserved hindlimbs. the species names,Q. graffini, is named afterGreg Graffin from the bandBad Religion | |
| Sazavis | 1989 | Bissekty Formation (Late Cretaceous,Turonian toConiacian) | Uzbekistan | One of many fragmentary Bissekty Enantiornithes, known only from atibiotarsus (shin bone) |  |
| Shangyang | 2019 | Jiufotang Formation (Early Cretaceous,Aptian) | China | Unusually, the premaxillae of this genus were fused | |
| Sinornis | 1992 | Jiufotang Formation (Early Cretaceous,Aptian) | China | One of the first Jehol biota Enantiornithes described. Similar toCathayornis but usually considered to be distinct |  |
| Xiangornis | 2012 | Jiufotang Formation (Early Cretaceous,Aptian) | China | The hand of this genus was similar to that of ornithuromorphs, likely throughconvergent evolution. A large member of the group | |
| Yatenavis[103] | 2022 | Chorrillo Formation (Late Cretaceous,Maastrichtian) | Argentina | The southernmost known member of Enantiornithes and one of the youngest members of the group | |
| Yuanjiawaornis[104] | 2015 | Jiufotang Formation (Early Cretaceous,Aptian) | China | One of the largest Enantiornithes known from decent remains | |
| Yungavolucris | 1993 | Lecho Formation (Late Cretaceous,Maastrichtian) | Argentina | Had a large and unusually widetarsometatarsal (ankle bone) |  |
| Yuornis | 2021 | Qiupa Formation (Late Cretaceous,Maastrichtian) | China | A large, toothless genus of Enantiornithes, one of the most-well preserved members from the Late Cretaceous. |  |
TheLongipterygidae 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.[36]
| Name | Year | Formation | Location | Notes | Images |
|---|---|---|---|---|---|
| Boluochia | 1995 | Jiufotang Formation (Early Cretaceous,Aptian) | China | Originally mistakenly believed to have possessed a hooked beak | |
| Camptodontornis | 2010 | Jiufotang Formation (Early Cretaceous,Aptian) | China | Originally calledCamptodontus, although that genus name is occupied by abeetle | |
| Chromeornis | 2025 | Jiufotang Formation (Early Cretaceous,Aptian) | China | Preserves unusual gastroliths and several traces of soft tissue | |
| Dapingfangornis | 2006 | Jiufotang Formation (Early Cretaceous,Aptian) | China | May have had a thornlike structure on its forehead | |
| Longipteryx | 2001 | Jiufotang Formation (Early Cretaceous,Aptian) | China | The most common and well-known member of the family |  |
| Longirostravis | 2004 | Yixian Formation (Early Cretaceous,Aptian) | China | Like other longipterygids, it possessed a thin snout which may have been used for probing for invertebrates in mud or bark |  |
| Rapaxavis | 2009 | Jiufotang Formation (Early Cretaceous,Aptian) | China | Specialized for perching due to the structure of its feet | |
| Shanweiniao | 2009 | Yixian Formation (Early Cretaceous,Aptian) | China | Acquired multiple tail feathers which may have been capable of generating lift as in modern birds |  |
| Shengjingornis | 2012 | Jiufotang Formation (Early Cretaceous,Aptian) | China | A large member of the family |
ThePengornithidae was a family of large early Enantiornithes. They had numerous small teeth and numerous primitive features which are lost in most other Enantiornithes.[1] Mostly known from the early Cretaceous of China, with putative Late Cretaceous taxa from Madagascar.[105]
| Name | Year | Formation | Location | Notes | Images |
|---|---|---|---|---|---|
| Chiappeavis[38] | 2015 | Jiufotang Formation (Early Cretaceous,Aptian) | China | Possessed a fan-shaped tail composed of many feathers |  |
| Eopengornis | 2014 | Huajiying Formation (Early Cretaceous,Hauterivian) | China | The oldest known member of the family, and one of the oldest putative genera of Enantiornithes known. Possessed extremely well-preserved tail ribbons | |
| Parapengornis[106] | 2015 | Jiufotang Formation (Early Cretaceous,Aptian) | China | Proposed to have a woodpecker-like lifestyle due to features of the foot and tail |  |
| Pengornis | 2008 | Jiufotang Formation (Early Cretaceous,Aptian) | China | The first pengornithid discovered, and also one of the largest members of the Enantiornithes known from decent remains | |
| Yuanchuavis | 2021 | Jiufotang Formation (Early Cretaceous,Aptian) | China | Possesses an elaborate "pintail" tail fan longer than its body, which may have had a display function |  |
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.[2] Themonophyly of this group is doubtful, and it may actually be anevolutionary grade.[107]
| Name | Year | Formation | Location | Notes | Images |
|---|---|---|---|---|---|
| Beiguornis | 2022 | Longjiang Formation (Early Cretaceous,Aptian) | China | Had a short but robust manual ungual | |
| Bohaiornis | 2011 | Jiufotang Formation (Early Cretaceous,Aptian) | China | Originally considered to have been preserved withgastroliths, although later these were found to be mineral concretions | |
| Gretcheniao | 2019 | Yixian Formation (Early Cretaceous,Barremian) | China | Adapted for flapping, rather than soaring, flight. Its describers suggest paraphyly or polyphyly of Bohaiornithidae |  |
| Linyiornis[108] | 2016 | Jiufotang Formation (Early Cretaceous,Aptian) | China | A possible member of the family, known from a well-preserved skeleton complete with structures believed to be developing eggs | |
| Longusunguis | 2014 | Jiufotang Formation (Early Cretaceous,Aptian) | China | A fairly typical member of the family |  |
| Neobohaiornis[109] | 2024 | Jiufotang Formation (Early Cretaceous,Aptian) | China | Half the size ofBohaiornis | |
| Parabohaiornis | 2014 | Jiufotang Formation (Early Cretaceous,Aptian) | China | A close relative ofBohaiornis | |
| Shenqiornis | 2010 | Qiaotou member of theHuajiying Formation (Early Cretaceous,Aptian?) | China | The first known member of the family, although not considered a close relative ofBohaiornis until a few years later. Preserves a large postorbital bone | |
| Sulcavis | 2013 | Yixian Formation (Early Cretaceous,Aptian) | China | A close relative ofShenqiornis with groovedenamel on its teeth, unique among fossil birds |  |
| Zhouornis | 2013 | Jiufotang Formation (Early Cretaceous,Aptian) | China | A large member of the family with a well-preserved braincase | |
Some members of the group are obscure or poorly described and may be synonymous with its type species,Gobipteryx minuta.
| Name | Year | Formation | Location | Notes | Images |
|---|---|---|---|---|---|
| Gobipteryx | 1974 | Barun Goyot Formation (Late Cretaceous,Campanian) | Mongolia | A toothless advanced genus of Enantiornithes, possessing a robust beak which convergently evolved with those of modern birds | |
| Jibeinia | 1997 | Qiaotou member of theHuajiying Formation (Early Cretaceous,Aptian?) | China | Poorly known and described from a skeleton which has now been lost. May have been synonymous withVescornis | |
| Vescornis | 2004 | Qiaotou member of theHuajiying Formation (Early Cretaceous,Aptian?) | China | A small and short-snouted genus of Enantiornithes which may be synonymous withJibeinia |
Avisauridae 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.
| Name | Year | Formation | Location | Notes | Images |
|---|---|---|---|---|---|
| Avisaurus | 1985 | Hell Creek Formation (Late Cretaceous,Maastrichtian) | United States (Montana) | The eponymous avisaurid, as well as one of the largest members of the family. Originally considered a non-avialan dinosaur |  |
| Elsornis | 2007 | Djadochta Formation (Late Cretaceous,Campanian) | Mongolia | Although incomplete, its skeleton possesses three-dimensional preservation. Possibly flightless due to its wing proportions | |
| Enantiophoenix | 2008 | Ouadi al Gabour Formation (Late Cretaceous,Cenomanian) |  Lebanon | Was once believed to have fed on tree sap as it was preserved in association withamber beads, however this was later determined as an artefact of preservation and not an indicator of diet | |
| Gettyia | 2018 | Two Medicine Formation (Late Cretaceous,Campanian) | United States (Montana) | A new genus forAvisaurus gloriae | |
| Halimornis | 2002 | Mooreville Chalk Formation (Late Cretaceous,Campanian) | United States ( Alabama) | Would have lived in a coastal environment | |
| Intiornis | 2010 | Las Curtiembres Formation (Late Cretaceous,Campanian) | Argentina | Although closely related to some of the largest avisaurids, members of this genus were very small birds |  |
| Mirarce | 2018 | Kaiparowits Formation (Late Cretaceous,Campanian) | United States ( Utah) | The most complete known North American avisaurid |  |
| Mystiornis | 2011 | Ilek Formation (Early Cretaceous,Barremian toAptian) | Russia | Possesses a myriad of features from various groups inParaves, although most closely resembles avisaurids among sampled groups | |
| Neuquenornis | 1994 | Bajo de la Carpa Formation (Late Cretaceous,Santonian) | Argentina | Possessed long wings and a reverse hallux, indicating good flight and perching abilities | |
| Soroavisaurus | 1993 | Lecho Formation (Late Cretaceous,Maastrichtian) | Argentina | A very close relative ofAvisaurus |
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Media related toEnantiornithes at Wikimedia Commons
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