Early Cretaceous–Late Cretaceous, 130.7–66.0 Ma
|Undescribed specimen informally referred to the dubious species "Liaoxiornis delicatus" by the Museo Geominero of Madrid|
and see text
Enantiornithes were the most abundant and diverse group of avialans ("birds" in the broad sense) during the Mesozoic Era. Almost all retained teeth and clawed fingers on each wing, but otherwise looked much like modern birds externally. Over 50 species of Enantiornithines have been named, but some names represent only single bones, so it is likely that not all are valid. Enantiornithes became extinct at the Cretaceous–Paleogene boundary, along with hesperornithids and all non-avian dinosaurs, and many other groups. Enantiornithes are thought to have left no living descendants.
Discovery and naming
The first Enantiornithes to be discovered were incorrectly referred to modern bird groups (Gobipteryx minuta). They were first recognized as a distinct lineage, or "subclass" by Cyril A. Walker, in 1981, based on some partial remains from the late Cretaceous period of what is now Argentina. Since the 1990s, more complete Enantiornithes were discovered and it was demonstrated that a few previously described birds (e.g. Iberomesornis, Cathayornis, Sinornis) had enantiornithe features.
"Enantiornithes" means "opposite birds", from Ancient Greek enantios (ἐνάντιος) "opposite" + ornithes (όρνιθες) "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":
Perhaps the most fundamental and characteristic difference between the Enantiornithes and all other birds is in the nature of the articulation between the scapula [...] and the coracoid, where the 'normal' condition is completely reversed.
This refers to an anatomical feature – the articulation of the shoulder bones – which has a concave-convex socket joint that is the reverse of that of modern birds. Specifically, in Enantiornithes, the facet where the scapula (shoulder blade) meets the coracoid (the primary bone of the shoulder girdle in vertebrates other than mammals) is a convex knob and the corresponding point on the shoulder blade is concave and dish-shaped. In modern birds, the way the joint articulates is reversed.
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
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.
Many enantiornithine fossils are found in highly fragmentary states, and some taxa are known only from a piece of a single bone. Particularly exquisite specimens that are complete, in full articulation and with soft tissue preservation are known from Las Hoyas in Cuenca (Spain) and the Yixian Formation in Liaoning (PRC). They have been found in both inland and marine sediments, suggesting that they were an ecologically diverse group. Enantiornithes appear to include waders, swimmers, fish-catchers, and raptors. The smallest are described as sparrow-sized, but some were much larger, such as Avisaurus which had an estimated wingspan of 1.2 meters (4 ft). The vast majority of enantiornithine species were rather small, between the size of a sparrow and a starling, while the largest members of this clade are Pengornis houi, Xiangornis shenmi and Zhouornis hani.
Given their wide range of habitats and diets, the skulls of Enantiornithes varied considerably between species. Enantiornithine skulls combined a unique suite of primitive and advanced characteristics. As in more primitive avialans like Archaeopteryx, they retained the postorbital bone and a small premaxilla, and most species had toothy jaws rather than toothless beaks. Only a few species, such as Gobipteryx minuta, were fully toothless.
The wings of Enantiornithes were relatively advanced compared to primitive avialans like Archaeopteryx, and displayed some features related to flight similar to those found in the lineage leading to modern birds, the Ornithurae. While most Enantiornithes retained claws on at least some fingers, many species had shortened hands, a highly mobile shoulder anatomy, and other proportional changes in the wing bones similar to modern birds. Like modern birds, Enantiornithes had alulas, or "bastard wings", a small forward-pointing arrangement of feathers on the first digit that granted higher maneuverability in the air and aided precise landing. As a very large group of birds, Enantiornithes displayed a 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.
One enantiornithine fossil shows wing-like feather tufts on its legs, similar to Archaeopteryx. Leg feathers are also reminiscent of the four-winged dinosaur Microraptor, however, in the enantiornithine differ from the feathers are shorter, more disorganized (do not clearly form a wing) and only extend down to the ankle rather than along the foot.
Clarke et al. (2006) surveyed all enantiornithine fossils 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 called Ornithurae) with preserved tail feathers had only short coverts or elongated paired tail plumes. Thus, 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. The primitive Euornithes Yixianornis, Hongshanornis, and Schizooura are the earliest known avialans with a fan of tail feathers.
At least one enantiornithine, Shanweiniao, is known to have had four long tail feathers that overlapped each other. It is possible that these formed a lift-generating surface similar to the tail fans of Euornithes, though the fan-like tail of this species may have evolved independently of the modern bird lineage.
Origin and range
Enantiornithes have been found in North America, South America, Europe, Asia, and Australia. Known fossils attributable to this group are exclusively Cretaceous and it is believed that Enantiornithes became extinct at the same time as their non-avian dinosaur relatives. One biogeographic study in the 1990s suggested that the distribution of Enantiornithes implies a Middle Jurassic origin for the clade, but this theory has not been widely accepted by paleoornithologists; a Late Jurassic/Early Cretaceous origin is more in line with the fossil record. The earliest known Enantiornithes are from the Early Cretaceous of Spain (e.g. Noguerornis, a basal genus) and China (e.g. Protopteryx) and the latest from the Late Cretaceous of North and South America (e.g. Avisaurus). The widespread occurrence suggests that the Enantiornithes were able to cross oceans on their own power; they are the first avialan lineage with a global distribution.
Given the wide diversity of skull shape among Enantiornithes, many different dietary specializations must have been present among the group. Some forms, like Shenqiornis, had large, robust jaws suitable for eating hard-shelled invertebrates. In longipterygids, the snouts were long and thin with teeth restricted to the tip of the jaws, and they were likely mud-probers (small-toothed species) and fishers (large-toothed species). The short, blunt teeth of Pengornis were likely used to feed on soft-bodied arthropods.
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 exoskeletons of aquatic crustaceans preserved in its digestive tract, and Enantiophoenix preserves 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.
A specimen from Las Hoyas reported by Sanz et al. (2001) includes the remains of four hatchling enantiornithine skeletons of three different species. 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 or small theropod dinosaur. This was the first evidence that Mesozoic avialans were prey animals, and that some Mesozoic pan-avians regurgitated pellets like owls do today.
Described enantiornithine fossils include eggs, embryos, and hatchlings. An enantiornithine embryo, still curled in its egg, has been reported from the Yixian Formation. Juvenile specimens can be identified by a combination of factors: rough texture of their bone tips indicating portions which were still made of cartilage at the time of death, relatively small breastbones, large skulls and eyes, and bones which had not yet fused to one another. Some hatchling specimens have been given formal names, including "Liaoxiornis delicatus"; however, Luis Chiappe and colleagues considered the practice of naming new species based on juveniles detrimental to the study of enantiornithines, 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.
Together with hatchling specimens of the Mongolian Gobipteryx and Gobipipus, these finds demonstrate that enantiornithine hatchlings had the skeletal ossification, well-developed wing feathers, and large brain which correlate with precocial or superprecocial patterns of development in birds of today. In other words, at least some enantiornithines probably hatched from the egg already well developed and ready to run, forage, and possibly even fly in a just a few days old.
Analyses of enantiornithe bone histology have been conducted to determine the growth rates of the 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 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.
Because enantiornithes lack tails and possess radically different forelimb, pectoral and shoulder girdle morphology, they have been subjected to several studies testing their aerial capacities.
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 retrices.
However, recent studies showcase that they were extremely efficient flyers like modern birds, possessing a similarly complex nervous system and wing feather ligaments. Additionally, the lack of a tail appears to not have been very relevant for avian flight as a whole - some extinct neornithes like lithornids also lacked tail feathers and were good flyers -, and they appear to have been capable of a ground based launching.
Due to the different in sternal and shoulder-girdle anatomy, many enantiornithes resorted to a flight style unknown in modern bird species, though more typical flight styles were present as well.
Some researchers classify Enantiornithes along with the true birds in the class Aves, but those that use the more restrictive crown group definition of Aves place them in the more inclusive Avialae. Enantiornithes were more advanced than Archaeopteryx or Confuciusornis, but in several respects more primitive than all 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, 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.
|Cladogram from Cau & Arduini (2008)|
Enantiornithine 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 enantiornithine taxa 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".
Enantiornithes is the sister group to Euornithes, and together they form a clade called Ornithothoraces. Most phylogenetic 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 enantiornithine autapomorphies to just four.
Enantiornithine systematics are highly provisional. What appears fairly certain by now is that there were subdivisions within Enantiornithes possibly including some minor basal lineages in addition to the more apomorphic 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.
One such delineation named the Euenantiornithes, was defined by Chiappe (2002) as comprising all species closer to Sinornis than to Iberomesornis. Because Iberomesornis is often found to be the most primitive or basal enantiornithine, Euenantiornithes may be an extremely inclusive group, made up of all enantiornithines 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, who called it
|Cladogram from Wang et al. 2014 (updated version of O’Connor et al. 2013)|
The taxonomic list of enantiornithine groups presented here follows a summary published by Thomas R. Holz, Jr. in 2011, except in cases where noted.
- Primitive Enantiornithes
- Cerebavis (Russia)
- Dalingheornis (China)
- Dapingfangornis (China)
- Elsornis (Mongolia)
- Eoalulavis (Spain)
- Gobipipus (Mongolia)
- Houornis (China)
- Iberomesornis (Spain)
- Jibeinia (China)
- Liaoningornis (China)
- Paraprotopteryx (China)
- Protopteryx (China)
- Qiliania (China)
- Sazavis (Uzbekistan)
- Xiangornis (China)
- Primitive Euenantiornithes
- Abavornis (Uzbekistan)
- Alethoalaornis (China)
- Alexornis (Mexico)
- Catenoleimus (Uzbekistan)
- Cathayornis (China)
- Elbretornis (Argentina)
- Enantiornis (Argentina)
- Eocathayornis (China)
- Explorornis (Uzbekistan)
- Flexomornis (Texas)
- Fortunguavis (China) 
- Grabauornis (China)
- Gracilornis (China)
- Gurilynia (Mongolia)
- Huoshanornis (China)
- Incolornis (Uzbekistan)
- Kuszholia (Uzbekistan)
- Kizylkumavis (Uzbekistan)
- Largirostrornis (China)
- Lectavis (Argentina)
- Lenesornis (Uzbekistan)
- Liaoxiornis (China)
- Longchengornis (China)
- Martinavis (Argentina/France/New Mexico)
- Nanantius (Australia)
- Noguerornis (Spain)
- Otogornis (China)
- Parvavis (China)
- Sinornis (China)
- Yungavolucris (Argentina)
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