Ornamental feathers in Cretaceous Burmese amber: resolving the enigma of rachis-dominated feather structure

Lida Xing,Pierre Cockx,Jingmai O’Connor,Ryan C Mckellar

doi:10.1186/s42501-018-0014-2

Abstract

Over the last 20 years, compression fossils of feathers surrounding dinosaurs have greatly expanded our understanding of the origin and evolution of feathers. One of the most peculiar feather morphotypes discovered to date are rachis dominated feathers (RDFs), which have also been referred to as proximally ribbon-like pennaceous feathers (PRPFs). These elongate feathers are only found in the tail plumage, typically occurring in pairs with both streamer (not proximally ribbon-like) and racket-plume morphologies recognized. Here we describe a large sample set of isolated and paired RDFs from Upper Cretaceous Burmese amber (~ 99 Ma). Amber preserves the finest details of these fragile structures in three dimensions, demonstrating that RDFs form a distinct feather morphotype with a ventrally open rachis, and with significant variability in pigmentation, microstructure, and symmetry.

Highlights

The discovery of numerous feathered dinosaur specimens in China (e.g., Ji and Ji 1996; Ji et al 1998; Chen et al 1998; Xu et al 2014) and other regions (e.g., van der Reest et al 2016) has greatly improved knowledge of early dinosaur feathers and their evolution, displaying a wide range of feather types (Zhang et al 2006)
The feathers reveal an open rachis with no pith cavity like that uniformly present in previously described feathers entombed in amber, differing significantly from previous hypotheses regarding rachis dominated feathers (RDFs) structure based on compression fossils
3.1.1 Rachis morphology Compression fossils of RDFs have led to the interpretation that these feathers represent modified pennaceous feathers comparable to modern bird feathers but with an enlarged rachis (O’Connor et al 2012), implying a broad, hollow rachis with a crescentic cross-section

Summary

Introduction

The discovery of numerous feathered dinosaur specimens in China (e.g., Ji and Ji 1996; Ji et al 1998; Chen et al 1998; Xu et al 2014) and other regions (e.g., van der Reest et al 2016) has greatly improved knowledge of early dinosaur feathers and their evolution, displaying a wide range of feather types (Zhang et al 2006). Early interpretations of these feathers were confused by incompleteness and poor preservation (Zhang and Zhou 2000), recent discoveries of more complete RDF compression fossils. The line that runs down the middle of the rachis in RDFs has been viewed as a groove (Wang et al 2014), supported by the discovery of slightly three-dimensional impressions of RDFs alongside a juvenile enantiornithine from Brazil (Carvalho et al 2015), like the depression seen on the ventral surface of modern flight feathers (Lucas and Stettenheim 1972). Amber preserves three-dimensional specimens with microstructure and pigmentation (Thomas et al 2014) and has the potential to clarify problematic structures and reveal fine morphological details unlikely to preserve in compression fossils. The feathers reveal an open rachis with no pith cavity like that uniformly present in previously described feathers entombed in amber, differing significantly from previous hypotheses regarding RDF structure based on compression fossils

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Discussion

Conclusion