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
