Countershading and Stripes in the Theropod Dinosaur Sinosauropteryx Reveal Heterogeneous Habitats in the Early Cretaceous Jehol Biota

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Countershading and Stripes in the Theropod
Dinosaur Sinosauropteryx Reveal Heterogeneous
Habitats in the Early Cretaceous Jehol Biota

Authors
Fiann M. Smithwick,1 Robert Nicholls,2 Innes C. Cuthill,3 and Jakob Vinther134

^1  School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK
^2  Palaeocreations, 35 Hopps Road, Kingswood, Bristol BS15 9QQ, UK
^3  School of Biological Sciences, University of Bristol, Life Sciences Building, 24 Tyndall Avenue, Bristol BS8 1TQ, UK
^4  Lead Contact

Correspondence
jakob.vinther@bristol.ac.uk

In Brief
Smithwick et al. reconstruct the coloration of the small carnivorous dinosaur Sinosauropteryx. It had a bandit mask and striped tail and was also countershaded (see image). Using 3D models under different light, the authors show that its camouflage would have worked best in an open habitat. Paleocolor can help predict paleohabitat.

Highlights

  • We have reconstructed the color pattern of the theropod dinosaur Sinosauropteryx
  • Sinosauropteryx exhibited camouflage, including countershading and a bandit mask
  • The countershading pattern was most likely associated with an open habitat
  • Previously assumed to be forested, Jehol likely included a range of habitat types


https://doi.org/10.1016/j.cub.2017.09.032

SUMMARY

Countershading is common across a variety of lineages and ecological time [1–4]. A dark dorsum and lighter ventrum helps to mask the three-dimensional shape of the body by reducing self-shadowing and decreasing conspicuousness, thus helping to avoid detection by predators and prey [1, 2, 4, 5]. The optimal countershading pattern is dictated by the lighting environment, which is in turn dependent upon habitat [1, 3, 5, 6]. With the discovery of fossil melanin [7, 8], it is possible to infer original color patterns from fossils, including countershading [3, 9, 10]. Applying these principles, we describe the pattern of countershading in the diminutive theropod dinosaur Sinosauropteryx from the Early Cretaceous Jehol Biota of Liaoning, China. From reconstructions based on exceptional fossils, the color pattern is compared to predicted optimal countershading transitions based on 3D reconstructions of the animal’s abdomen, imaged in different lighting environments. Reconstructed patterns match well with those predicted for animals living in open habitats. Jehol is presumed to have been a predominantly closed forested environment [3, 11, 12], but our results indicate a more heterogeneous range of habitats. Sinosauropteryx is also shown to exhibit a "bandit mask," a common pattern in many living vertebrates, particularly birds, that serves multiple functions including camouflage [13–18]. Sinosauropteryx therefore shows multiple color pattern features likely related to the habitat in which it lived. Our results show how reconstructing the color of extinct animals can inform on their ecologies beyond what may be obvious from skeletal remains alone.

RESULTS

Plumage Distribution

To reconstruct the color patterns of Sinosauropteryx, we analyzed three of the best-preserved specimens available (Figures 1A and 1D and S1A). To reconstruct the color patterns accurately, first the distribution of pigmented plumage was described in detail for each specimen (Supplemental Descriptions). Each specimen shows extensive preservation of dark, presumably organically preserved fibers identified as feathers/feather homologs in distinct areas of the animal (Figures 1A and 1D and S1). Alternative interpretations of these structures as degraded skin collagen have recently been shown to be unfounded [19]. Preservation of feathers as organic films is due to the presence of the pigment melanin, and thus only originally pigmented feathers are found preserved in this manner [7, 8]. Visible absence of feathers in certain regions of the fossil is therefore likely due to unpigmented plumage that did not preserve, rather than a true absence of feathers in life [7, 8]. Alternatively, the areas lacking feathers could have been naked (there is no evidence of scales being preserved [19]) but would similarly be inferred to have been unpigmented. Because the feathering likely also served an insulatory role, an extensive distribution seems most plausible. Mapping the distribution of preserved pigmented feathers is therefore considered to reflect the extent of colored plumage on the animal, with other areas being covered by white (unpigmented) feathers.

Color Pattern Reconstruction

Illustrations of NIGP 127586 and NIGP 127587 show the pattern of plumage distribution across the fossils (Figures 1B and 1E). From this distribution, a complete reconstruction was created (Figure 2); this was done blind to any predictions from the modeling of illumination. The consistency of plumage patterns observed across multiple specimens gives confidence to the reconstructed color pattern. The pattern of pigment across the face appears to show a band of pigmented plumage running from the dorsal area of the head anterioventrally, which then angles toward the eye before running to the posterioventral margin of the lower jaw (Figures 3A–3E). The banded tail shows a transition from narrow to widely spaced bands from the proximal to distal regions, with the ventral pigmentation becoming denser toward the end of the tail. The ventral extent of the pigmented plumage, representing the likely countershading transition, appears to be relatively high on the flank, at around two-thirds of the way down the abdomen (Figures 3F–3I).
Predicted Lighting Environment

For countershading to be effective in obliterating 3D cues of an animal’s presence, the pattern of pigmentation from the dorsal to ventral body regions should match the illumination gradient created by the lighting environment in which it lives [1, 3, 5, 6]. This allows the determination of likely habitats of animals based on quantification of color patterns [1, 3]. Those that inhabit open environments with direct lighting conditions generally exhibit a sharp transition from dark to light color high up on the flanks of the body [1, 3]. Conversely, animals inhabiting a more closed habitat with diffuse lighting coming in at many angles often show a smoother gradation from dark to light lower down on the body [1, 3]. To predict the optimal pattern of countershading, we created and photographed 3D models of the abdomen of Sinosauropteryx under different lighting conditions. The reconstructed color patterns based on NIGP 127586 and NIGP 127587 (Figures 2 and 3H–3I) more closely match the pattern of countershading predicted from images of the models taken under direct light conditions than those of diffuse lighting conditions (Figure 4), indicative of animals living in open habitats [1, 3]. The addition of synthetic fur (representing feathers) made little difference to each countershading prediction (Figure 4). For direct overhead sun, the mean predicted transition point to lighter coloration was 72% (95% confidence interval [CI] 61%–83%) of the way from dorsal to ventral side. For direct sun at 30° it was 60% (95% CI 45%–75%), and for diffuse illumination it was 85% (95% CI 81%–88%). Only the direct illumination confidence intervals include the observed transition point (~67%).

DISCUSSION

Color Patterns of the Face

The presence of pigmented feathers surrounding the orbit and running in a band across the face conforms to "bandit masks" seen in many modern birds and mammals [15–18]. Multiple functions have been proposed for bandit masks in modern taxa [13, 14, 16–18]. One such function is as an anti-glare device [15, 18]. Reducing the glare from the feathers around the eye would be particularly useful to an animal living in environments with abundant direct sunlight, as is seen often in diurnal extant birds and mammals [13, 18]. Additionally, it has been suggested that glare is especially high in riparian habitats, because light reflectance is increased by proximity to water, as may have been the case in the lacustrine environment in which Sinosauropteryx fossils were deposited [15]. Pigmented bands that run directly across the orbital region may also help to mask the presence of the eyes as a form of camouflage against both predators and potential prey [20, 21]. Eye stripes are common in modern birds, which most often also have dark eyes, making them likely harder for visual predators or prey to detect, and given that eyes elicit responses from both in many situations, it is a plausible hypothesis [13]. Other possible functions of dark patches around the eyes of extant animals include aposematism and intraspecific signaling [13, 17]. Bandit masks have been suggested as Page:Countershading and Stripes in the Theropod Dinosaur Sinosauropteryx Reveal Heterogeneous Habitats in the Early Cretaceous Jehol Biota.pdf/4 Page:Countershading and Stripes in the Theropod Dinosaur Sinosauropteryx Reveal Heterogeneous Habitats in the Early Cretaceous Jehol Biota.pdf/5 Page:Countershading and Stripes in the Theropod Dinosaur Sinosauropteryx Reveal Heterogeneous Habitats in the Early Cretaceous Jehol Biota.pdf/6 Page:Countershading and Stripes in the Theropod Dinosaur Sinosauropteryx Reveal Heterogeneous Habitats in the Early Cretaceous Jehol Biota.pdf/7 Page:Countershading and Stripes in the Theropod Dinosaur Sinosauropteryx Reveal Heterogeneous Habitats in the Early Cretaceous Jehol Biota.pdf/8 Page:Countershading and Stripes in the Theropod Dinosaur Sinosauropteryx Reveal Heterogeneous Habitats in the Early Cretaceous Jehol Biota.pdf/9 Page:Countershading and Stripes in the Theropod Dinosaur Sinosauropteryx Reveal Heterogeneous Habitats in the Early Cretaceous Jehol Biota.pdf/10

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