Data archiving statement:
Data for this study are available in the Dryad Digital Repository:
https://doi.org/10.5061/dryad.3bk3j9kf1The most commonly preserved soft tissues associated with ornithischian dinosaurs are skin remains. The apparent resistance of hadrosaur skin to decay, and its abundance in the fossil record relative to that of other tetrapods, has been attributed to factors such as thickness and composition. Here we report additional intrinsic factors within hadrosaur skin: 3Dâpreserved eumelaninâbearing bodies, dermal cells and blood vessel fragments in an organic matrix composed of protein fossilization products. The skin is much thinner than that of living mammals of similar size. It is likely that the preservation of hadrosaur skin is related to the arrangement of the layers composing it.
The skin is the largest organ in the body of any vertebrate and carries out important functions such as homeostasis, sensory reception and visual signaling (Landmann 1986). The skin is composed of three layers: the epidermis (outer), dermis and the subcutis (innermost). The epidermis is composed of stratified layers of keratinized cells (the stratum corneum, stratum intermedium and stratum basale) whereas the dermis is characterized by a stratum spongiosum of loose collagen fibres and a lower stratum compactum of tightly packed, orthogonal arrays of collagen fibres (Landmann 1986; Cooper & Greenberg 1992). Colouration of the skin is generally determined by melanophores and chromatophores and the structural layering of the integument (McNamara et al. 2016; Shawkey & D'Alba 2017). Chromatophores and melanophores are usually stored at the interface between the epidermis and dermis in vertebrates (Prum & Torres 2003, 2004; Chang et al. 2009; McNamara et al. 2016; Shawkey & D'Alba 2017). The innermost layer of the skin, the subcutaneous layer, separates the integument from the rest of the body. Integument is probably the most abundant type of soft tissue in the vertebrate fossil record (Schweitzer 2011; Davis 2014) and it is providing increasingly important evidence of reptile evolution and ecological diversity in the past (e.g. Martill 1995; Briggs et al. 1997; Martill et al. 2000; Bell 2012; Lindgren et al. 2013, 2014, 2018; Davis 2014; McNamara et al. 2016, 2018; Yang et al. 2019).
Fossilized skin is preserved in three main ways: as impressions, compressions or permineralized, threeâdimensional skin (Schweitzer 2011). Impressions are occasionally found in association with bones or footprints (e.g. Lockley et al. 2004; Paik et al. 2017). Compressions are preserved as a carbonârich layer surrounding bones (e.g. Briggs et al. 1997; Qiang et al. 1998; Wang et al. 2019). Permineralized skin is the rarest of the three modes of preservation (Schweitzer 2011; Bell 2012; Davis 2014) and usually occurs in association with three dimensionally preserved skeletons. Permineralized skin was previously thought to be a highâfidelity mineralized replication of the original organic tissue (Schweitzer 2011). However, recent investigations of a few exceptionally preserved examples have challenged this view by recovering original organic molecules. Dinosaurian integument preserved as a result of early mineralization is known in Ornithischia, Sauropoda and Theropoda (Briggs et al. 1997; Martill et al. 2000; Bell 2012; Davis 2014; McNamara et al. 2018). The preservation of integument is most prevalent in hadrosaurs (Bell 2012; Davis 2014). However, the taphonomic processes that favour the fossilization of skin and associated molecular preservation are still poorly understood (Lyson & Longrich 2011; Bell 2012; Davis 2014). Previous investigations of dinosaur skin were performed on the basal ceratopsian Psittacosaurus (LinghamâSoliar 2008), a nodosaur (Brown et al. 2017), and coelurosaurs (NavalÃn et al. 2015; Bell et al. 2017; McNamara et al. 2018). Traces of collagenous material were found in a âmummifiedâ hadrosaurid from North Dakota, USA (Manning et al. 2009). Skin colouration patterns have been reconstructed in ornithischian dinosaurs (LinghamâSoliar & Plodowski 2010; Vinther et al. 2016; Brown et al. 2017) but not in a hadrosaurid.
YPMPU 016969 is a partial hadrosaur skeleton preserving skin in the flank region of the body (Fabbri et al. 2019, fig. S1). We investigated this specimen with the following aims: (1) to understand its taphonomy; (2) to test for soft tissue preservation and the presence of melanin; and (3) to consider previous explanations for the prevalence of hadrosaur skin preservation in the fossil record.
===========