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Fishes are a major component of modern aquatic ecosystems where they regulate some key biological processes such as the proliferation of algae. Their diet is an important and highly variable aspect of their ecology and role in the environment. Yet fossil fishes are often interpreted as ecologically restricted to a single function. Fossil fishes from the order Pycnodontiformes are considered as specialised shell-crushers that drove the evolution of shelled animals in the escalation event known as the Mesozoic Marine Revolution. But this hypothesis is still untested for want of a reliable tool. The manner in which vertebrates wear their teeth provides a direct link between a resilient, easy to fossilise structure and their feeding ecology. It has recently become possible to quantify and compare the roughness of this wear at the microscopic scale. The potential of dental microtexture analysis as a tool to compare and make inferences of dietary preferences in fishes is further investigated here. Here we show that the technique can track subtle differences in diet between morphologically similar animals and correctly assign them to a relevant trophic niche in a taxon, size and environment-independent way. Dental microtexture analysis is applied to teeth from pycnodontiform fishes to test their role in Mesozoic foodwebs. The hypothesis that they were specialised shell-crushers is refuted. A broader range of realised trophic ecologies is highlighted in pycnodonts. They had more generalist or herbivorous habits and potentially were among the first major consumers of macroalgae, a key ecological role filled by extant analogues which allows light to reach zooxanthellate corals on modern reefs. The results of early investigations comparing dental microtexture between different histologies in extant and early gnathostomes are also presented, and show that trophic diversity and shell-crushing are 360-million years old phenomena that need to be considered in scenario of macro-evolution.