Chinese Flyers: Feathered, Endothermic, Evolutionary Pioneers.

Greetings. Jeholornis is a genus of avialan dinosaurs that existed between 122 and 120 million years ago during the Cretaceous Period in what is now China. The genus was first discovered in the Jiufotang Formation in Hebei Province, with additional specimens having been found in the older Yixian Formation. The three species of Jeholornis had long tails and a few small teeth, and were approximately the size of modern-day turkeys, making them among the largest avialans known until the Late Cretaceous. Their diet included seeds of cycads and other similar plants.

Jeholornis were relatively large, basal avialans, with a maximum adult length of up to 85 centimeters and an estimated weight of roughly 9 kilograms. Their skulls were short and high, similar to basal paravians like Epidexipteryx and to early oviraptorosaurs like Incisivosaurus. The lower jaws were short, stout, and curved downward, an evolutionary adaptation for eating seeds. Jeholornis prima lacked teeth in their upper jaws, and had only three small teeth in their lower jaws, while Jeholornis palmapenis had a few teeth in the middle of the maxilla but none in the premaxilla. The upper teeth of J. palmapenis seem to have been angled slightly forward as in some other basal avialans. The teeth in all three species of Jeholornis were small, blunt and peg-like with no serrations.

The upper limbs of Jeholornis were robust and longer than the lower limbs, possessing well-developed shoulder girdles indicating strong wing musculature. Interestingly, the wishbone was U-shaped in J. prima and J. palmapenis but not in J. curvipes. Their fingers were short compared to those of Archaeopteryx and also far more robust. The lower legs were not particularly long, indicating that these animals were not specialized runners. The halluces of Jeholornis were short, but their claw was more strongly curved than those of the other toes. Unlike deinonychosaurs and some other Mesozoic avialans, the claw of the second toe was not enlarged relative to the other claws. Their tail anatomy was more like those of dromaeosaurids than the iconic Archaeopteryx, with more strongly interlocking vertebrae, and though they had a similar number of tail vertebrae, between 20 and 24, those of Jeholornis were much longer overall than those of Archaeopteryx. 

A scientific study conducted in 2009 has shown that Jeholornis (along with Archaeopteryx) had relatively slow ontogenic development, meaning that they grew very slowly, compared to most modern birds, which grow very quickly. The extant kiwi birds however, have slow development, and it has been speculated that Jeholornis could have possessed a similar metabolism.

The tails of several fossilized specimens preserve a fan of feathers (rectrices) at the tip, shorter than those on the forelimbs. The feather fan is similar to those of Microraptor and Caudipteryx, being restricted to the tip of the tail, unlike those of Archaeopteryx and Similicaudipteryx which have rectrices extending down much of the tail length. In at least one species, Jeholornis palmapenis, there were 11 distinct tail feathers. The feathers were short and pointed, and arched away from the body of the tail, so that the entire array of tail feathers resembled a palm frond. The tail feathers did not overlap, and so could not have formed a lift-generating surface, so the tail was probably used mainly for courtship display purposes. 

The shoulder girdles of Jeholornis were well developed and probably allowed for better flight capabilities than seen in Archaeopteryx. The flight apparatus of the Jeholornis was overall quite similar to that of Confuciusornis in form and function, with forelimbs longer than hindlimbs, and short, robust hands. However, like other basal (non-ornithothoracean) avialans and theropod dinosaurs, the shoulder blades of Jeholornis were oriented along the sides of the body, rather than on top of its back. This meant that the shoulder girdle was slung low, which would have allowed only for a typical dinosaurian motion of the shoulder. Primitive avialans like Archaeopteryx, Confuciusornis, and Jeholornis would not have been able to lift their arms vertically to achieve true flapping flight, though semi-powered gliding or parachuting would have been possible.

Examination of the claw curvature in Jeholornis suggests it may have been able to perch and may have been at least partly arboreal, spending much of its time in trees. One key adaptation of modern-day perching avian dinosaurs is the reversed, opposable first toe, or hallux. The three species of Jeholornis all possessed a reversed hallux, a physical characteristic that strongly suggests that modern-day birds were about to appear in Cretaceous skies, earlier than paleontologists ever dreamed. 

Unlike so many other wonderful species, this particular genus, and its descendants, did make it through the Cretaceous/Paleogene extinction event. Their linage continues to live on in today's skies. 

Jeholornis. A genus of avialan dinosaurs that were almost as advanced as modern-day species of birds. A true evolutionary pioneer.

Thank you for your time and consideration.