Evidence of possible protectcive pigmentation in the Messinian cyprinodontid Aphanius crassicaudus (Agassiz, 1832)

The cyprinodontid Aphanius crassicaudus is extremely abundant in the deposits recording the Messinian Salinity Crisis (MSC) event throughout the Mediterranean Basin. This species was defined by Louis Agassiz (1832) in his monumental “Recherches sur les poissons fossiles” based on material collected in the “argile plastique” outcropping in the nearby of Senigallia, in the Marche region. In the following 180 years articulated skeletal remains belonging to this fish have been collected from many localities in Italy, Spain, Greece and Algeria, mostly in laminated clayey and marly deposits originated in lagoonal and estuarine settings during both the evaporitic and post-evaporitic phases of the MSC. The presence of Aphanius crassicaudus in the Messinian deposits of the Tertiary Piedmont Basin has been reported by many authors (e.g, Gaudant, 1979). Recent excavations carried out by the students of the course “Laboratorio di Scavo Paleontologico” (LT Scienze Naturali, Università degli Studi di Torino) in the euxinic shales of the Vena del Gesso Formation cropping out along the Tanaro River, resulted in the discovery of a few hundred specimens of Aphanius crassicaudus, associated with exquisitely well-preserved plants and insect remains. The euxinic conditions that resulted in the preservation of the organic residues of soft tissues of plants and insects also favored the preservation of part of the original pigmentation of the scales, abdominal cavity, and dorsal and caudal fins in several specimens of Aphanius crassicaudus. In particular, some of these recently collected specimens exhibit three prominent, nearly vertical black stripes that run the depth of the caudal fin plus a nearly horizontal black stripe that run the dorsal-fin base. In extant fishes, black coloration results from the dispersion of melanin in the melanophores. Scattered small black dots are commonly present on body scales, possibly representing preserved individual melanophores, as well as the black stripes observed in the caudal and dorsal fins may represent the chemical residues of the original melanin pigments. Preserved pigmentation patterns have been described in fossil fishes from several Tertiary localities (e.g., Grande, 1984). A wide variety of pigmentation patterns are developed in fishes, mostly used for visual communication and camouflage (e.g., Breder, 1949; Owen, 1982). Most probably, the vertical and horizontal stripes observed in Aphanius crassicaudus can be interpreted as evidence for both cryptic and disruptive coloration. Dark pigmentation in contrast with the far less pigmented region of the body would have permitted effective countershading, and vertical caudal stripes would have helped to disrupt the outline of the posterior part of the body of the fish. Extant fishes use these types of coloration for concealment or to confuse potential predators. In summary, the specimens of Aphanius crassicaudus recently collected from the euxinic shales of the Vena del Gesso Formation exposed along the banks of the Tanaro River provided the first evidence of the presence of (possible) protective pigmentation in this common Messinian cyprinodontid fish.