A fully feathered enantiornithine foot and wing fragment preserved in mid-Cretaceous Burmese amber

Abstract

Over the last three years, Burmese amber (~99 Ma, from Myanmar) has provided a series of immature enantiornithine skeletal remains preserved in varying developmental stages and degrees of completeness. These specimens have improved our knowledge based on compression fossils in Cretaceous sedimentary rocks, adding details of three-dimensional structure and soft tissues that are rarely preserved elsewhere. Here we describe a remarkably well-preserved foot, accompanied by part of the wing plumage. These body parts were likely dismembered, entering the resin due to predatory or scavenging behaviour by a larger animal. The new specimen preserves contour feathers on the pedal phalanges together with enigmatic scutellae scale filament (SSF) feathers on the foot, providing direct analogies to the plumage patterns observed in modern birds, and those cultivated through developmental manipulation studies. Ultimately, this connection may allow researchers to observe how filamentous dinosaur ‘protofeathers’ developed—testing theories using evolutionary holdovers in modern birds.

A New Enantiornithine (Aves) Preserved in Mid-Cretaceous Burmese Amber Contributes to Growing Diversity of Cretaceous Plumage Patterns

“…The specimen described here, YLSNHM00813, is the ninth reported Burmese amber specimen preserving avian skeletal remains. Although the skeletal remains are poorly preserved, the cumulative morphological features (e.g., penultimate pedal phalanges longest, pedal ungual phalanges long and recurved) suggest referral to the Enantiornithes, the dominant clade of Cretaceous land birds (O'Connor et al, 2011) and so far the only clade identified with certainty in Burmese amber (Xing et al, 2016(Xing et al, , 2017(Xing et al, , 2018a(Xing et al, , 2019a. The position of the forelimbs and hindlimbs in the amber block suggests that these remains belong to a single individual (Figure 1).…”

“…Eight remiges are also preserved in HPG-15-1, but it is unclear if the tract is complete in the latter specimen (Xing et al, 2017). Nine primaries are preserved in DIP-V-15100, which also preserves secondaries and thus is likely complete (Xing et al, 2016), and ten primaries are reported in DIP-V15105 (Xing et al, 2019a). Specimen HPG-15-2 may have ten or perhaps 11 primaries, but it is difficult to distinguish between the primaries and secondaries due to feather overlap within this specimen (Xing et al, 2019c).…”

“…In DIP-V-15100, this pale spot extends on to some of the primary coverts, whereas in DIP-V-15101 a secondary pale band is also present extending across the dorsal surface of the wing apical to the alula (Xing et al, 2016). A diffuse brown coloration is present throughout the wing plumage in DIP-V-15102 and DIP-V-15105b, but the barbs have paler cores in DIP-V-15102 (Xing et al, 2018a(Xing et al, , 2019a. The holotype of Elektorornis chenguangi (HPG-15-2) also preserves some wing feathers with two large-scale pale wing spots halfway down the length of the primaries, and the trailing edge vane of each primary has a pale margin (Xing et al, 2019c).…”

“…Although in tropical forests today we typically envision birds that are brightly colored, in most Burmese amber specimens the feathers are preserved with various shades of brown and this base coloring is supplemented with pale markings (inferred to represent areas of reduced or absent pigmentation) (Xing et al, 2017). Observed markings include transverse bands (LV-0321, HPG-15-1, DIP-V-15101) (Xing et al, 2016(Xing et al, , 2017(Xing et al, , 2020, spots (LV-0321, HPG-15-1, HPG-15-2, DIP-V-15100 and DIP-V-15101) (Xing et al, 2016(Xing et al, , 2019a(Xing et al, ,c, 2020 and possibly longitudinal stripes (LV-0321) (Xing et al, 2020). The brownish appearance of the feathers may be somewhat due to the yellow of the amber itself.…”

“…These include differences in whether the bird was alive or dead when it became trapped in the amber, whether it was partially scavenged, and how long it took for the remains to be fully encapsulated in the resin. As a result, skeletal preservation varies from excellent (Xing et al, 2019a) to absent, as in one specimen that preserves only a mold of the soft tissues (Xing et al, 2019b).…”