Epibiont habitation patterns and their implications for life habits and orientation among trigoniid bivalves

Abstract

The bivalve superfamily Trigoniacea has persisted from the Late Paleozoic to the Recent. Late Jurassic and terminal Cretaceous mass extinctions decimated this once-dominant group in shallow marine facies; only a single genus with seven species survives today in the Austral Province. Trigoniacea retain a vestigial byssus and primitive but efficient schizodont dentition. They have been widely considered as infaunal bivalves, burrowing with a very large foot to shallow depths, with inhalant and exhalant apertures at or slightly below the sediment-water interface (SWI). Yet the Trigoniacea are poorly adapted for this life habit. The mantle in living species is unfused and non-siphonate, and some fossil Trigoniacea have permanent shell gapes over these apertures, enhancing the probability of sediment fouling of feeding and respiratory structures. Some living Neotrigonia, e.g., N. margaritacea, solve this problem by having a semi-infaunal life habit, with the inhalant and exhalant apertures elevated above the SWI and the zone of active sediment transport. Semi-infaunal species commonly have epibionts cohabiting the exposed posterior-posteroventral portion of the shell. Numerous well-preserved species of South American Mesozoic Trigoniacea have phototropically and geotropically oriented epibionts on co-attached valves, strongly suggesting a semi-infaunal life mode for at least some members of these taxa. These shell symbionts allow orientation of extinct trigoniid shells relative to the SWI during life, as well as analysis of their depth of burial. Careful analyses of the kinds, size classes, orientation, and dispersion of various epibionts on fossil Trigoniacea thus yield important new information on their life habits, and demonstrate that semi-infaunal life modes were far more common than previously supposed.