We’re back with News from the World of Old Feathers, and Lida Xing and colleagues strike again. They describe a new amber specimen of a Cretaceous bird with parts of the head, feet and wings beautifully preserved. Why is this important? After all, we have seen feathers in amber before, we have seen wings of juvenile birds in amber and last year we even had a piece of an actual dinosaur in amber. Haven’t we reached peak amber? I like to think we have not*.
What Xing and his team of paleontologists from China, Canada and the US describe in a new paper is a hatchling Enantiornithine bird that became trapped in sticky conifer resin about 99 million years ago, in what is now Burma. Although Enantiornithes looked superficially like modern birds – which is to say they were feathered and likely to have been good fliers – the anatomy of their shoulder girdle is different, they were toothed and the fingers in their wing had claws. The newly described specimen, nicknamed Belone, offers an unparalleled glimpse into feather development and molting in young Enantiornithes.
Belone measures roughly 6cm from head to tail, and consists of the skull, neck, wings and feet. The rest of the body eroded away before it became fully encapsulated in resin, but it left a thin trace in the fossilised resin. Careful lighting reveals a rather life-like outline of this little bird: wings outstretched, beak open, feet drawn up and claws out as though ready to pounce.
Despite its small size, the details of the skin and soft tissue that are preserved in Belone are dazzling. On the right side of its head, the elliptical external opening of the ear is visible, surrounded by tracts of feathers. With the help of micro-CT scanning, researchers could even see the semicircular canals of the inner ear. The eye appears large, and while the view is not entirely clear, the researchers think the eyelid may have been closed. The right foot shows a fully reversed first digit (hallux), a feature common in many modern birds such as songbirds but that had been difficult to assess in the flattened, two-dimensional fossils of Enantiornithes’ feet discovered thusfar. The lower leg and foot are covered in scaly skin, with distinct toe pads and curved claws (suggesting an arboreal lifestyle) with intact keratin sheaths. It’s a foot that wouldn’t look out of place as an entry for #feetsfriday.
But what we’re really here for are the feathers. Belone was a very young bird; the bones of the skull and the lower leg are unfused, indicating that Belone may have been only a few days, perhaps a week old when it died. In modern birds, chicks hatch covered in a layer of downy feathers, also known as first generation or neoptile feathers. They develop during embryogenesis and are relatively simple structures consisting of a series of flexible branches (the barbs) that sprout from the feather base. Yet despite Belone’s very young age, its neoptile plumage is rather sparse and most of it appears more akin to contour feathers (more developed feathers with a stiff central shaft) than the more fuzzy down feathers. It is unclear if a downy plumage had already been shed, or if Enantiornithes possessed a different molting pattern to modern birds.
Among the neoptile feathers occur isolated bristle-like filaments, whose origin and function are unclear. In modern birds, bristles are specialised feathers that aid sensory tasks. That is unlikely to have been the case for Belone; instead, they may represent evolutionary holdovers from the proto-feathers seen in some dinosaurs.
The outstretched wing beautifully shows the primary and secondary flight feathers. The eight visible primaries are asymmetrically shaped (important for aerodynamics) and remarkably, colour seems to have been preserved: a white-ish band can be seen against a walnut brown background in the tips of the feathers. The secondary flight feathers closer to the body are a pale brown, with their margins a darker brown. On the wing, even the tiny “thumb feather” or alula can be seen.
In the tail region, a single tail flight feather (rectrix) appears to emerge from a keratin sheath, which serves as protection for the feather as it matures. The fact that the diameter of this budding tail flight feather is already larger than that of the flight feathers in the wing indicates that it may have been an ornamental tail feather. Ornamental tail feathers have been observed in juvenile Enantiornithes before, but the presence of an erupting tail feather in this very young bird suggests that these feathers were not present upon hatching. Instead, they must have appeared in the first molt. Moreover, if the budding tail flight feather is indeed an ornamental feather, the timing of the development of such feathers in Enantiornithes differs markedly from what we know about modern birds, where ornamental feathers do not typically appear until the bird is skeletally and sexually mature.
Belone preserves a mix of feather types unlike any seen in living birds. The combination of neoptile feathers and more mature feathers, such as fully developed flight feathers and ornamental feathers, indicate that Enantiornithine hatchlings were highly precocial. In modern birds, hatchlings range from helpless (altricial) to mobile (precocial). In precocial species, such as megapodes, the young hatch with their eyes open and are ready to leave the nest within a short period of time. Altricial hatchlings are relatively immobile and need parental care to survive. We know very little about parenting behaviour in Enantiornithes, but if Belone is anything to go on, Enantiornithine hatchlings were ready for takeoff.
This has been News from the World of Old Feathers. Until next time.
* In my humble opinion, peak amber would be reached at the intersect of maximum organism completeness and optimum amber clarity. An example would be a complete Archaeopteryx in crystal-clear amber. Or a sauropod dinosaur, if you’re into that.
References
Lida Xing et al. 2017. A mid-Cretaceous enantiornithine (Aves) hatchling preserved in Burmese amber with unusual plumage. Gondwana Research, doi: 10.1016/j.gr.2017.06.001
