Trigonida

Trigonida is an extinct order of extinct marine bivalve molluscs. These organisms lived during the Triassic and Jurassic periods, making them a significant component of Mesozoic marine ecosystems. They are characterized by their triangular, or wedge-shaped, shell, from which their name is derived. This shell morphology often implies a burrowing lifestyle, with the apex of the shell positioned upwards for attachment to sediment or to orient the animal for feeding.

Fossil evidence suggests that Trigonida were epifaunal or infaunal organisms, meaning they lived either on the surface of the seafloor or buried within it. Their diet likely consisted of plankton or detritus, filtered from the water column or sediment through their siphons. The internal anatomy of Trigonida is inferred from shell structures and muscle scars, providing clues about their musculature and feeding mechanisms. While they were relatively common in certain marine environments of the Triassic and Jurassic, they eventually disappeared, with their ecological niche potentially being filled by other bivalve groups. The study of Trigonida contributes to our understanding of bivalve evolution and the paleoecology of ancient marine habitats.