They are not particularly clear pieces of amber. Burmite, or Burmese amber, is prized for its clarity. Instead, there are large, dark spots, air bubbles and various bits and pieces floating around. But these two chunks of amber, found in 2015 at the site of Angbamo in the northern province of Kachin in Myanmar, contain something much more valuable (at least, in the eyes of palaeontologists): feathers attached to bones.
Burmese amber is the fossilized resin of trees, possibly from Metasequoia, a species of redwood, from the Middle Cretaceous period (about 99 million years ago). This amber has been known to sometimes include the remains of unfortunate animals and plants that became stuck in the resin; flowers, fungi, ticks, velvet worms and the occasional feather have been described from Burmese amber.
So if fossil feathers are nothing new, and if we already know about feathers being preserved in amber, then what’s all the fuss about? Many finds of fossil feathers consist of a single feather, which makes identifying their former owners not always easy (identifying feathers from living birds can already be challenging). Moreover, due to their fragile nature, fossil feathers preserved in 3D are extremely rare. Most of the fossil feathers are only known to science as 2D preserved specimens, which obscures important details regarding feather anatomy and colouring. For the first time, the new specimens from Myanmar preserve both the delicate anatomy of the feathers in their original three dimensions, and are attached to wing bones, thus allowing us to identify the animals.
The two new Myanmar specimens, nicknamed “angel wing” and “Rose”, both contain partial wings, with wing bones, soft tissue and a range of plumage. The lack of clearly defined articular surfaces of the bones, as well as their small size, indicates that the two wing specimens belonged to juveniles. Although this makes attributing them to a particular species difficult, the length of the wing bones, most specifically the lengths of the two metacarpals, indicate that they represent a species of enantiornithines.
Enantiornithes are a group of extinct flying dinosaurs who became extinct at the end of the Cretaceous. During the Cretacous, they were the dominant group of flying dinosaurs, with more than 60 species recognized so far, from sites all over the world, including Spain, China, the United States and Argentina. Many species were rather small, the size of a sparrow, but some attained larger body sizes. Enantiornithines displayed a tremendous diversity in morphology, which indicates that they inhabited a range of habitats very much like our modern day birds (Neornithes) do. For instance, the large-bodied Bohaiornis may have been a carnivorous species, Longipteryx’s pointy snout indicates a piscivorous diet, and Pengornis was likely to have fed on soft-shelled invertebrates. However, enantiornithines differed from modern birds in significant ways. The articulation of the shoulder blade with the coracoid, a bone in the shoulder girdle absent in mammals, is reversed with regard of the condition seen in modern birds. This configuration led avian palaeontologist Cyril Walker to describe this group of fossil birds as Enantiornithes, or ‘opposite birds’. In addition, many enantiornithines retain teeth in their mandible and claws on their forelimbs, features that are omnipresent in dinosaurs but were subsequently lost on the evolutionary path towards modern birds (although hand claws are rather selectively lost, since many living birds still have hand claws).
Yet despite their differences with modern birds, the new amber fossils show that enanthiornithean wings were remarkably similar to those of living birds. The amber specimens preserve a number of different feather types, including flimsy down feathers near the skin, covert feathers on both the upper and lower side of the wings, and strongly asymmetrical flight feathers. The flight feathers have a stiff central axis (the rachis), and up close one can even discern tiny hooklets, or barbules, which interlock with eachother to create a strong feather surface that can withstand high forces. These features of the flight feathers are similar to those observed in modern birds and indicate that these enantiornithines were capable of powered flight, in contrast to more primitive enantiornithines. Interestingly, there is nothing in the plumage to indicate that the birds were juveniles (i.e. there is no sign of moulting), in contrast to the attached bones. This indicates that the adult plumage had developed quickly, and that the juvenile plumage phase had been significantly shortened or possibly even skipped entirely. This would suggest that enanthiornitheans were precocious and would have been able to fend for themselves shortly after being born, and thus required very little parental care.
References:
Lida Xing, Ryan C. McKellar, Min Wang, Ming Bai, Jingmai K. O’Connor, Michael J. Benton, Jianping Zhang, Yan Wang, Kuowei Tseng, Martin G. Lockley, Gang Li, Weiwei Zhang & Xing Xu. 2016. Mummified precocial bird wings in mid-Cretaceous Burmese amber. Nature Communications 7, doi:10.1038/ncomms12089
Zhonghe Zhou, Julia Clarke & Fucheng Zhang. 2008. Insight into diversity, body size and morphological evolution from the largest Early Cretaceous enantiornithine bird. Journal of Anatomy, Vol. 212(5):565–577
Zhiheng Li, Zhonghe Zhou, Min Wang, & Julia A. Clarke.2014. A New Specimen of Large-Bodied Basal Enantiornithine Bohaiornis from the Early Cretaceous of China and the Inference of Feeding Ecology in Mesozoic Birds. Journal of Paleontology 88(1):99-108
