On the purported presence of fossilized collagen fibres in an ichthyosaur and a theropod dinosaur

Fiann M Smithwick,Gerald Mayr,Evan T Saitta,Michael J Benton,Jakob Vinther,Andrew Smith

doi:10.1111/pala.12292

Abstract

AbstractSince the discovery of exceptionally preserved theropod dinosaurs with soft tissues in China in the 1990s, there has been much debate about the nature of filamentous structures observed in some specimens. Sinosauropteryx was the first non‐avian theropod to be described with these structures, and remains one of the most studied examples. Despite a general consensus that the structures represent feathers or feather homologues, a few identify them as degraded collagen fibres derived from the skin. This latter view has been based on observations of low‐quality images of Sinosauropteryx, as well as the suggestion that because superficially similar structures are seen in Jurassic ichthyosaurs they cannot represent feathers. Here, we highlight issues with the evidence put forward in support of this view, showing that integumentary structures have been misinterpreted based on sedimentary features and preparation marks, and that these errors have led to incorrect conclusions being drawn about the existence of collagen in Sinosauropteryx and the ichthyosaur Stenopterygius. We find that there is no evidence to support the idea that the integumentary structures seen in the two taxa are collagen fibres, and confirm that the most parsimonious interpretation of fossilized structures that look like feather homologues in Sinosauropteryx is that they are indeed the remains of feather homologues.

Highlights

THE debate on the origins of birds is one of the oldest in palaeontology (Huxley 1870; Heilmann 1926), and there has been a substantial increase in research on this question in the past two decades thanks to the discovery of numerous exceptionally preserved fossils in China
We highlight issues with the evidence put forward in support of this view, showing that integumentary structures have been misinterpreted based on sedimentary features and preparation marks, and that these errors have led to incorrect conclusions being drawn about the existence of collagen in Sinosauropteryx and the ichthyosaur Stenopterygius
We find that there is no evidence to support the idea that the integumentary structures seen in the two taxa are collagen fibres, and confirm that the most parsimonious interpretation of fossilised structures that look like feather homologues in Sinosauropteryx is that they are the remains of feather homologues

Summary

Introduction

THE debate on the origins of birds is one of the oldest in palaeontology (Huxley 1870; Heilmann 1926), and there has been a substantial increase in research on this question in the past two decades thanks to the discovery of numerous exceptionally preserved fossils in China (reviewed, Zhou et al 2003; Xu 2006). The integumentary structures in Sinosauropteryx were interpreted by Lingham-Soliar et al (2007) as remnants of a frill along the dorsum, reinforced with structural collagen fibres, accounting for the way in which the integument extends dorsally from the vertebrae This was suggested to add stiffening support to the long tail, to protect against injury, as well as to provide a decorative display organ (Lingham-Soliar et al 2007). Scales have been purportedly identified on another specimen and used as evidence of a frill as well as to refute the presence of feathers (Lingham-Soliar 2013)

Objectives

Conclusion