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, 279 (1731), 1114-21

The Original Colours of Fossil Beetles

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The Original Colours of Fossil Beetles

Maria E McNamara et al. Proc Biol Sci.

Abstract

Structural colours, the most intense, reflective and pure colours in nature, are generated when light is scattered by complex nanostructures. Metallic structural colours are widespread among modern insects and can be preserved in their fossil counterparts, but it is unclear whether the colours have been altered during fossilization, and whether the absence of colours is always real. To resolve these issues, we investigated fossil beetles from five Cenozoic biotas. Metallic colours in these specimens are generated by an epicuticular multi-layer reflector; the fidelity of its preservation correlates with that of other key cuticular ultrastructures. Where these other ultrastructures are well preserved in non-metallic fossil specimens, we can infer that the original cuticle lacked a multi-layer reflector; its absence in the fossil is not a preservational artefact. Reconstructions of the original colours of the fossils based on the structure of the multi-layer reflector show that the preserved colours are offset systematically to longer wavelengths; this probably reflects alteration of the refractive index of the epicuticle during fossilization. These findings will allow the former presence, and original hue, of metallic structural colours to be identified in diverse fossil insects, thus providing critical evidence of the evolution of structural colour in this group.

Figures

Figure 1.
Figure 1.
Beetle specimens from Cenozoic Lagerstätten analysed using scanning- and transmission electron microscopy, and reflectance microspectrophotometry in this study. (a–h) Well-preserved specimens; the cuticle is highly reflective and metallic colours, bright. (a) Buprestid from Enspel (GKE 2009 PE 5889). (b) Chrysomelid elytron from Clarkia (YPM 2010 P37b 005). (c) Unidentified beetle from Clarkia (YPM 2010 P37b 010). (d) Chrysomelid from Messel (MeI 15 552). (e) Chrysomelid from Messel (MeI 15 553). (f–h) Chrysomelids from Eckfeld ((f) NMM PE 2000 364, (g) NMM PE 2000 385 and (h) NMM PE 2000 636). (i) Poorly preserved chrysomelid elytron from Eckfeld (NMM PE 2000 470); metallic colour is dull. (j) Poorly preserved chrysomelid from Eckfeld; cuticle is matt and friable and metallic colour is not preserved (NMM PE 2000 449). Scale bars: (a,c) 5 mm; (b,dj) 1 mm.
Figure 2.
Figure 2.
(a,c,d,g–i) Scanning and (b,e,f,j) transmission electron micrographs of cuticle ultrastructures in fossil beetles with well-preserved metallic colours. (a,b) Vertical sections through the cuticle showing epicuticle (ep), exocuticle (ex) and endocuticle (en). r, resin. (c) Fractured vertical section through the cuticle showing laminated epicuticle (ep) and exocuticle (ex). s, sediment. (d) Detail of surface of lamina in epicuticle showing fibrillar network perforated by pore canals (arrow). (e) Vertical section through the cuticle showing alternating electron-dense and electron-lucent laminae in epicuticle (ep), and uniformly electron-dense laminae in exocuticle (ex). r, resin. (f) Vertical section through the epicuticle showing multi-laminate outer epicuticle (arrow). s, sediment. (g) Vertical section through the exocuticle showing laminations. (h) Oblique fractured section through the epicuticle (ep) and exocuticle (ex) showing arcuate patterns and pore canals (arrow) in the exocuticle. (i) Internal-facing surface of the base of the exocuticle showing pore canals and, inset, pore canal filaments. (j) Membranes of the basal endocuticle (en) and/or basement membrane. Note, granular texture of the endocuticle. Scale bars: (a,b,e) 1 µm; (c,d,g,h) 2 µm; (f,j) 500 nm; (i) 5 µm, inset, 1 µm.
Figure 3.
Figure 3.
(a,c,e,f) Scanning and (b,d) transmission electron micrographs of beetle specimens that do not exhibit well-preserved metallic colours. (a,b) Cuticle from a specimen with poorly preserved metallic colour. (a) Vertical fractured section through the cuticle showing textural distinction between exocuticle (ex) and endocuticle (en), and pore canals (arrow). (b) Subtle banding in the epicuticle (ep). Note lamination in the exocuticle is not preserved. s, sediment. (c,d) Cuticle from a specimen where metallic colours are not preserved, showing orthogonally arranged bundles of chitin fibrils in the endocuticle (en). No ultrastructures are preserved in the exocuticle (ex) or epicuticle. r, resin. (e,f) Cuticle from a curculionid from Eckfeld. (e) Preserved lamination in the exocuticle; there is no evidence for a laminated epicuticle. (f) Internal-facing surface of the exocuticle showing pore canals. Scale bars: (a,e) 1 µm; (b) 500 nm; (c) 5 µm; (d,f) 2 µm and inset in (d) 500 nm.
Figure 4.
Figure 4.
Two-dimensional Fourier analysis and reflectance microspectrophotometry of fossil multi-layer reflectors in beetle specimens with well-preserved metallic colours. (a,c,e,g,i,k,m) transmission electron micrographs of epicuticular multi-layer reflectors preserved in specimens in figure 1(a–g). (b,d,f,h,j,l,n) Measured and Fourier predicted reflectance spectra for nanostructures in parts (a,c,e,g,i,k,m). Scale bars: (a,c,e,g,i,k,m) 500 nm.

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