The new nuculid bivalve Trigonucula (Gonionucula) aciloides new subgenus and species from the upper Triassic Nayband Formation of Central Iran is described. Gonionucula differs from Trigonucula s.s. by the presence of oblique ribs, which either deviate from a concentric pattern at the anterior shell part, or, as in the newly described type species, consist of a complex pattern of chevron-like ribs on the central part of the flank, plus an additional set of opisthocline to more or less acline ribs on posterior part of the flank. The oblique ribs of Gonionucula probably aided energy-efficient burrowing, which was advantageous in competing for food resources with less efficiently burrowing detritus feeders. The number of burrowing bivalve species with oblique ribs generally rose towards the end of the Triassic, which probably reflects increasing competition at a time when most niches were re-occupied after the end-Permian mass extinction event. The fact that oblique ribs were rare in Paleozoic bivalves suggests that the level of competition was higher in post-Paleozoic biota, which is in accordance with a general increase in metabolic rates, predation pressure, and ecospace occupation in the course of the Mesozoic.

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