Distributional patterns of eye lens variation in different morphs of the phacopine trilobite Acuticryphops acuticeps (Kayser 1889) are investigated. The specimens were collected from the latest Frasnian preceding the Upper Kellwasser (UKW) global extinction event in the Frasnian– Famennian (Late Devonian) stratotype section at Coumiac, southern France. In six successive populations a gradual reduction in the mean number of lenses occurs within the short time span of a single conodont Zone. This morphological change cannot be imputed either to the size of individual specimens or to variation in cephalic morphology. Thus morphs with different numbers of eye lenses are considered intraspecific. However, the intrapopulational percentage relation between morphs does not remain constant, as the coefficient of variation in lens number continuously increases from one population to the next. Cases of individuals with asymmetric eyes appear in the two latest assemblages prior to the UKW level. The fact that the total variation of the shape of the cephalon is not affected in these assemblages suggests that the morphological changes observed in the visual complex may account for the relaxation of selective pressures on this trait. Such a change in the regime of selection would have been accompanied by a lessening of the processes that control the development of this trait. As the phenomenon of eye reduction is not restrained by local conditions at Coumiac (Montagne Noire) but occurs contemporaneously to various extents in other crustal blocks such as Rhenish Slate Mountains (Avalonia), Thuringia (Armorica), and Morocco (Northern Gondwana), it is considered as constituting an adaptation to global eustatic deepening that occurred in the terminal Frasnian just before the global extinction event.

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