Bromalites from the Middle Triassic of Poland and the rise of the Mesozoic Marine Revolution

Durophagous predation has been an important cause of significant evolutionary changes in the history of life. One of the most dramatic predation-driven changes in marine ecosystems occurred during the middle and late Mesozoic, which has been called the Mesozoic Marine Revolution (MMR). At this time, diversification of various predators elicited many escalation-related adaptations among prey taxa including trends toward infaunalization, elaboration of armor shells, and environmental restriction. However, the rise of this phenomenon has been the source of considerable debate. Initially, it has been argued that these major evolutionary changes began in the Jurassic and continued to accelerate in the Cretaceous. Although recent reports have shown that the MMR may have actually started soon after the end-Permian extinction in some groups, Triassic records of predation on benthic organisms are still very scant. Here, we report several bromalites (most probably regurgitalites) from the Middle Triassic of the Gogolin Formation, Upper Silesia, southern Poland. They are in a form of distinct and packed accumulations of intermingled fossil remains composed primarily of various angular bivalve shell fragments with sharp, non-abraded margins as well as crinoid ossicles with common signs of breakage. These dispersions of material are up to 99 mm in maximum diameter. It is suggested that these accumulations represent orally ejected waste produced by durophagous animals, most probably by durophagous sharks, colobodontid fish, placodonts, and some pachypleurosaurs or sauropterygian reptiles. All of these taxa have been recorded in the Middle Triassic of Poland. The feasibility of these vertebrates as potential agents of the present bromalites is discussed. We suggest that many morphological and behavioral innovations in the Triassic gastropods, bivalves and crinoids are escalation-related adaptations to durophagous predators and that the Mesozoic Marine Revolution was a far more prolonged evolutionary event than its name indicates.

► Bromalites found in the Middle Triassic of Poland.
► Innovations in the Triassic benthic fauna are adaptations to durophagous predation.
► Mesozoic Marine Revolution had already started during the Middle Triassic.
► MMR was a far more gradual evolutionary event than previously thought.