Abstract
Viewed under UV light the diverse and exceptionally well-preserved molluscs from the Late Jurassic Cordebugle Konservat Lagerstätte (Calvados, Normandy, France) reveal fluorescent fossil shell colour patterns predating the oldest previously known instance of such patterns by 100 Myr. Evidently, residual colour patterns are observable in Mesozoic molluscs by application of this non-destructive method, provided the shells are not decalcified or recrystallized. Among 46 species which are assigned to twelve gastropod families and eight bivalve families, no less than 25 species yielded positive results. Out of nine colour pattern morphologies that have been distinguished six occur in gastropods and three in bivalves. The presence of these variant morphologies clearly indicates a significant pre-Cenozoic diversification of colour patterns, especially in gastropods. In addition, the occurrence of two distinct types of fluorescence highlights a major difference in the chemical composition of the pigments involved in colour pattern formation in gastropods. This discovery enables us to discriminate members of higher clades, i.e. the Vetigastropoda emitting red fluorescence from the Caenogastropoda and Heterobranchia emitting whitish-beige to yellow fluorescence. Consequently, fluorescent colour patterns may help to allocate part of the numerous enigmatic Mesozoic gastropod taxa to their correct systematic position.
Highlights
Colour patterns of animals play an important role in numerous ecological and evolutionary processes such as sexual selection, camouflage or UV-protection
The aim of the present study is to: (1) systematically investigate the shells from the late Jurassic of Cordebugle under UV light, and detect and document any residual colour patterns; (2) provide an illustrated overview of all residual colour patterns found in the molluscs from Cordebugle; (3) demonstrate that this kind of preservation is basically not scarce in material from Mesozoic sediments, provided that the shells are well preserved; and (4) show that the type of fluorescence and variant morphologies of colour patterns may serve as additional characters for refined systematic assignment and determination of the studied species
Within the taxa showing colour patterns under UV light, positive results were achieved for 49.37% of the specimens in gastropods and 31% of the specimens in bivalves
Summary
Colour patterns of animals play an important role in numerous ecological and evolutionary processes such as sexual selection, camouflage or UV-protection. It is difficult to document and consider colour pattern evolution through time, because most of the pigments involved in their formation decay and disappear very quickly after death. This is certainly the case among molluscs (Gastropoda, Bivalvia), several thousand Recent species of which display highly variable shell colour patterns, whereas most fossils seem devoid of them, apart from exceptionally well-preserved specimens showing remnants of patterns in natural light (e.g., [1,2,3,4,5,6,7,8,9]). A number of scholars, have demonstrated that masked patterns can be enhanced by bleaching Cenozoic shells in sodium hypochlorite and exposing them to UV light [10,11,12,13,14,15,16,17].
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