Page:Countershading and Stripes in the Theropod Dinosaur Sinosauropteryx Reveal Heterogeneous Habitats in the Early Cretaceous Jehol Biota.pdf/4

 being primarily aposematic in mammalian taxa living in exposed open habitats and are especially prevalent in mammalian carnivores, which co-exist with larger carnivores [17, 22, 23], as is likely to have been the situation for Sinosauropteryx. A number of modern mammals combine bandit masks with defensive nauseous discharges [22], but it is not possible to ascertain whether this was the case with Sinosauropteryx, and aposematism is generally thought to be rare in modern birds [13], making aposematism unlikely in Sinosauropteryx. Alternatively, conspicuous face markings could serve as a warning of a physical deterrent, such as a weapon or armor [17, 22, 23]. Although the theropod had an enlarged claw on each hand [24], the animal's small size makes it unlikely that it posed any real threat to its likely much larger theropod predators, making this function of the bandit mask unlikely.

Function of the Banded Tail

Banded tails are poorly understood in modern animals and likely serve several functions, including social signaling, dazzle camouflage, and outline breaking/disruptive camouflage [15–18]. Banded tails have been proposed as a way of confusing predators or drawing attention away from more vital body parts [18].

The tail of Sinosauropteryx was the longest of any known theropod relative to body length [24]. Due to this length, it is unlikely that the animal could hold it in a perfectly horizontal position consistently, which would be necessary for a countershaded pattern to be effective. This may explain why the tail is banded rather than showing the countershaded pattern seen on the animal's flanks. The great length of the tail in combination with the distinct and presumably conspicuous color bands may be explained as a distraction strategy, a method of attracting attention as far from the less-conspicuous head and body as possible. Alternatively, the banding could have served as a form of disruptive camouflage, as is seen in a number of modern animals, breaking up the outline of the tail to make it less recognizable to potential predators [15–18]. A combined function of camouflage and instraspecific signaling has also been suggested in some extant bird taxa with banded patterns [25]. However, we find no osteological evidence for an ability to lift or pose the tail, which would have limited its utility in display.

Countershading in Sinosauropteryx

A clear darker dorsum and absence of pigmented plumage ventrally, with the light ventral side extending to the tail until at least the tenth caudal vertebra, conforms to what would be expected for countershaded camouflage adapted to reduce detection from visual predators and from potential prey [1, 3–5]. Visual hunting was likely important for predators of Sinosauropteryx. Several tyrannosauroids are contemporaneous with Sinosauropteryx [26]. Although these tyrannosauroids were small for the clade [26], they would likely have been more than capable of tackling the diminutive compsognathid, which appears to have not reached sizes much greater than a meter in length [24]. Modern avian predators rely heavily on their exceptional vision to hunt, and as such it is likely that their forebears, the theropods, also had excellent visual capabilities [27]. It has been shown that a number of tyrannosauroids had visual capabilities similar to modern raptorial birds [28], and as such strong selection for camouflage would have been likely in their prey. In fact, Rh