21Animal body coloration is a complex trait resulting from the interplay of multiple colour-producing mechanisms.
22Increasing knowledge of the functional role of animal coloration stresses the need to study the proximate causes of 23 colour production. Here we present a description of colour and colour producing mechanisms in two non-avian 24 archelosaurs, the freshwater turtles Trachemys scripta and Pseudemys concinna. We compare reflectance spectra; 25 cellular, ultra-, and nano-structure of colour-producing elements; and carotenoid/pteridine derivatives contents in 26 the two species. In addition to xanthophores and melanocytes, we found abundant iridophores which may play a 27 role in integumental colour production. We also found abundant dermal collagen fibres that may serve as 28 thermoprotection but possibly also play role in colour production. The colour of yellow-red skin patches results from 29 an interplay between carotenoids and pteridine derivatives. The two species differ in the distribution of pigment cell 30 types along the dorsoventral head axis, as well as in the diversity of pigments involved in colour production, which 31 may be related to visual signalling. Our results indicate that archelosaurs share some colour production mechanisms 32 with amphibians and lepidosaurs, but also employ novel mechanisms based on the nano-organization of the 33 extracellular protein matrix that they share with mammals. 34 35 *Jindřich Brejcha 2 36 65 Turtles are an early-diverging clade of Archelosauria, the evolutionary lineage of tetrapods leading to 66 crocodiles and birds [22]. Although many turtles have a uniform dull colour, conspicuous striped and spotted 67 patterns are common in all major lineages of turtles (for a comprehensive collection of photographs see [23-26]).68 These conspicuous colour patterns may be present in the hard-horny skin of shells, and/or in the soft skin of the 69 head, limbs or tail. The dark areas of the skin of turtles may have a threefold origin consisting either of dermal, 70 epidermal, or both epidermal and dermal melanocytes. Colourful bright regions are thought to be the result of the 71 presence of xanthophores in the dermis [27] and their interplay with dermal melanophores [28]. Iridophores have 72 never been shown to play role in coloration of turtles [27,29].
73Pigment-bearing xanthophores were first described in the dermis of the Chinese softshell turtle (Pelodiscus 74 sinensis) [29]. Xanthophores have also been found sporadically in the dermis of the spiny softshell turtle Apalone 75 spinifera, the Murray river turtle (Emydura macquarii) and in the painted turtle (Chrysemys picta) [27]. Such scarcity 76 3 of carotenoid/pteridine derivatives-containing cells is in contrast with chemical analyses of the yellow and red 77 regions of the integument of the red-eared slider (Trachemys scripta elegans) and C. picta [30,31]. Two major 78 classes of carotenoids have been described in the integument of these turtles: short wavelength absorbing 79 apocarotenoids and longer wave...