Page:Countershading and Stripes in the Theropod Dinosaur Sinosauropteryx Reveal Heterogeneous Habitats in the Early Cretaceous Jehol Biota.pdf/2



Fiann M. Smithwick, Robert Nicholls, , Innes C. Cuthill, and Jakob Vinther

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). Rh