A mid-Cretaceous enantiornithine foot and tail feather preserved in Burmese amber

Lida Xing,Kecheng Niu,Jingmai K O’Connor,Huijuan Mai,Ryan C Mckellar

doi:10.1038/s41598-019-51929-9

Abstract

Since the first skeletal remains of avians preserved in amber were described in 2016, new avian remains trapped in Cretaceous-age Burmese amber continue to be uncovered, revealing a diversity of skeletal and feather morphologies observed nowhere else in the Mesozoic fossil record. Here we describe a foot with digital proportions unlike any previously described enantiornithine or Mesozoic bird. No bones are preserved in the new specimen but the outline of the foot is recorded in a detailed skin surface, which is surrounded by feather inclusions including a partial rachis-dominated feather. Pedal proportions and plumage support identification as an enantiornithine, but unlike previous discoveries the toes are stout with transversely elongated digital pads, and the outer toe appears strongly thickened relative to the inner two digits. The new specimen increases the known diversity and morphological disparity among the Enantiornithes, hinting at a wider range of habitats and behaviours. It also suggests that the Burmese amber avifauna was distinct from other Mesozoic assemblages, with amber entrapment including representatives from unusual small forms.

Highlights

The discovery of skeletal remains preserved in Cretaceous-age amber is one of the most intriguing developments in 21st century palaeontology, with current research barely scratching the surface of the potential of these discoveries to shed light on the biology of long extinct organisms
What is most unusual about YLSNHM01001 is that digit IV appears to be more robust than digits II and III
The partial foot preserved in YLSNHM01001 reveals yet another unusual pedal morphology that is undocumented in the diversity of stem birds known from the Early Cretaceous Jehol Biota, the most diverse currently known Mesozoic avifauna, or elsewhere

Summary

Introduction

The discovery of skeletal remains preserved in Cretaceous-age amber is one of the most intriguing developments in 21st century palaeontology, with current research barely scratching the surface of the potential of these discoveries to shed light on the biology of long extinct organisms. Enantiornithines in amber indicate that flight feathers in these stem birds had rachises that were less rigid (i.e., nearly cylindrical), and barbules that were poorly developed (i.e., hooklets connecting barbules are minimal or absent) e.g.1,2, as compared to modern birds. These specimens have permitted the detailed study of two feather types associated with the Enantiornithes, rachis-dominated feathers, and scutellate scale filaments. Foot of an avian theropod, and a range of feathers representing multiple morphotypes These inclusions are preserved in a single piece of amber from the Angbamo site, Tanai Township, Myitkyina District, Kachin Province of northern Burma (Myanmar). We describe the remains, which we argue are best referred to Enantiornithes, and explore possible functions for the unusual morphology observed

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