The Enantiornithes (literally, "opposite birds", by which name they are popularly known), also known as enantiornithines or enantiornitheans in literature, are a group of extinct avialans ("birds" in the broad sense). They were the most abundant and diverse avialan group known from the Mesozoic era and the first to spread across the globe. Almost all retained teeth and clawed fingers on each wing, but otherwise looked much like modern birds externally. Internally, they notably differed in the arrangement of shoulder bones, with a reversed or "opposite" ball-and-socket joint. In Enantiornithes, the scapula functions as a socket and the coracoid as a ball, whereas in modern birds, these bones function respectively as a ball and a socket.
They were mostly small, about the size of sparrows. Their fossils are found only in the Cretaceous, and they diverged from the ancestors of modern birds early in that period, more than 130 million years ago."80-Million-Year-Old Enantiornithine Fossil Fills Gap between Archaeopteryx and Modern Birds". News Staff, Sci.News, 14 Nov 2024 Over seventy species of Enantiornithes have been named, but some names represent only single bones, so it is likely that not all are valid. The Enantiornithes became extinct at the Cretaceous–Paleogene boundary, along with Hesperornithes and all other non-avian dinosaurs.
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 ostriches and tinamou. The Enantiornithes were first recognized as a distinct lineage, or "subclass" of birds, by Cyril A. Walker in 1981. Walker made this discovery based on some partial postcranial remains from the Maastrichtian (late Cretaceous period) Lecho Formation of El Brete, southern Salta Province, Argentina, 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, and Sinornis) were instead Enantiornithes.
The name "Enantiornithes" means "opposite birds", from Ancient Greek 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 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 Praeornis, from the Oxfordian-Kimmeridgian of Kazakhstan, may have been the earliest known member of Enantiornithes according to Agnolin et al. (2017).Agnolin, F.L., Rozadilla, S., and Ismar de Souza Carvalho (2017). Praeornis sharovi Rautian, 1978 a fossil feather from the early Late Jurassic of Kazakhstan. Historical Biology.
Birds with confidently identified characteristics of Enantiornithes have been found in Albian of Australia, Maastrichtian of South America, Campanian of Mexico (Alexornis), Mongolia, and the western edge of prehistoric Asia, suggesting a worldwide distribution of this group or in the relatively warm regions, at least. Enantiornithes have been found on every continent except Antarctica. Fossils attributable to this group are exclusively Cretaceous in age, and it is believed that the Enantiornithes became extinct at the same time as their non-avialan dinosaur relatives. The earliest known Enantiornithes are from the Early Cretaceous of Spain (e.g. Noguerornis) and China (e.g. Protopteryx) and the latest from the Late Cretaceous 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 and the Jehol group in Liaoning (China). Extraordinary remains of Enantiornithes have also been preserved in Burmese amber deposits dated to 99 million years ago and include hatchlings described in 2017 and 2018, as well as isolated body parts such as wings and feet. These amber remains are among the most well-preserved of any mesozoic dinosaur. Fossils of this clade have been found in both inland and marine sediments, suggesting that they were an ecologically diverse group.
Enantiornithes appear to have included waders, swimmers, granivores, insectivores, fishers, and raptors. The vast majority of Enantiornithes were small, between the size of a sparrow and a starling, however display considerable variation in size with some species. The largest species in this clade include Pengornis houi, Xiangornis shenmi, Zhouornis hani, 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). Among the smallest described specimens are unnamed hatchlings, although the holotype specimens of Parvavis chuxiongensis and Cratoavis cearensis are comparable in size to small tits or hummingbirds.
Skull
Given their wide range of habitats and diets, the cranial morphology of Enantiornithes varied considerably between species. Skulls of Enantiornithes combined a unique suite of primitive and advanced features. As in more primitive avialans like Archaeopteryx, they retained several separate cranial bones, small premaxillae (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 (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 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 adaptations to different lifestyles seen in modern birds. In general, the wings of Enantiornithes were advanced compared to more primitive avialans like Archaeopteryx, and displayed some features related to flight similar to those found in the lineage leading to modern birds, the Ornithuromorpha. 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. These are the first complete Mesozoic dinosaur 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
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.