User:Luxquine/sandbox/Enantiornithes Feathers Draft

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Description[edit]

A life restoration of Iberomesornis, an early enantiornithine

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[1] and 2018,[2] as well as isolated body parts such as wings[3][4][5] and feet.[4][6] 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,[7] however display considerable variation in size with some species. The largest species in this clade include Pengornis houi,[8] Xiangornis shenmi,[9] Zhouornis hani,[7] and Mirarce eatoni,[10] (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).[11] Among the smallest described specimens are unnamed hatchlings, although the holotype specimens of Parvavis chuxiongensis[12] and Cratoavis cearensis[13] are comparable in size to small tits or hummingbirds.

Skull[edit]

A reconstruction of the skull of Bohaiornis, a bohaiornithid.

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 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.[14] A quadratojugal bone, which in modern birds is fused to the jugal, is preserved in Pterygornis.[15] 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).[16]

Wing[edit]

A micro-CT scan of an amber-encased enantiornithine wing showing rachises, skin, muscle and claws.

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.[17]

Several wings with preserved feathers have been found preserved 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.[18] 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.[3]

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.[19]

Tail[edit]

Fossil skeleton of Rapaxavis pani (a 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.[20]

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[21] and might have formed a lift-generating surface similar to the tail fans of Euronithes,[22] though a later study indicates that Shanweiniao was more likely to have rachis-dominated tail feathers similar to feathers present in Paraprotopteryx.[23] Chiappeavis, a primitive pengornithid, 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.[23] 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.[24] At least one genus of Enantiornithes, Cruralispennia, had a modern-looking pygostyle but lacked a tail fan.[25]

Biology[edit]

Diet[edit]

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. In Longipterygidae, 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.[14] 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.[26]

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 from aquatic crustaceans preserved in its digestive tract,[27] and Enantiophoenix 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.[28] 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.[29]

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,[30] though some modern birds have lost the gizzard and rely solely on strong stomachal acids.[31] 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.[32]

Predation[edit]

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 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.[33]

Life history[edit]

Fossilized eggs of Gobipteryx minuta, Dinosaurium (Prague)

Known fossils of Enantiornithes include eggs,[34][35] embryos,[36] and hatchlings.[37] An embryo, still curled in its egg, has been reported from the Yixian Formation.[38] 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.[39] 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 a nomen dubium.[39]

Together with hatchling specimens of the Mongolian Gobipteryx[40] and Gobipipus,[41][42] these finds demonstrate that hatchling Enantiornithes 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, Enantiornithes probably hatched from the egg already well developed and ready to run, forage, and possibly even fly at just a few days old.[39]

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.[43] 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).[27] 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.[44] Some analyses have interpreted the bone histology to indicate that Enantiornithes may not have had fully avian endothermy, instead having an intermediate metabolic rate.[45] However a 2021 study rejects the idea that they had less endothermic metabolisms than modern birds.[46]

Evidence of colonial nesting has been found in Enantiornithes, in sediments from the Late Cretaceous (Maastrichtian) of Romania.[47] Evidence from nesting sites shows that Enantiornithes buried their eggs like modern megapodes, which is consistent with their inferred superprecocial adaptations.[48]

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.[49]

It has been speculated that superprecociality in Enantiornithes might have prevented them from developing specialised toe arrangements seen in modern birds like zygodactyly.[50]

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.[51]

Flight[edit]

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,[52] as well as due to the absence of rectrices in many species.[20][22][53]

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,[54] and they appear to have been capable of a ground based launching.[55]

Due to the difference in sternal and shoulder girdle anatomy, many Enantiornithes used a flight style unlike that of any modern bird species[clarification needed], though more typical flight styles were present as well.[56]

At least Elsornis appears to have become secondarily flightless.[57]

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