Why Carboniferous Gigantoproductini grew so large and thick shelled? A possible explanation for gigantism in fossils

The species of Gigantoproductus are well-known giants within the Brachiopoda. This is a dilemma as living brachiopods have a low-energy lifestyle. Why these taxa have acquired such a large size (over 30 cm in width) and thick shells (over 1 cm) has never been investigated in detail. Even if brachiopod metabolic rates were higher during the Paleozoic than today, and they experienced a general directional trend towards larger size, the massive size reached by species of Gigantoproductus is still surprising. This increase in size during the Paleozoic has been related to increases in oxygen availability, high primary productivity, or high predation pressure; nevertheless, gigantism in fossil and Recent plants and animals remains poorly understood. By examining their diet, we are seeking to understand the mechanisms that enabled these low- metabolism brachiopod species to become giants. Were they suspension feeders, similar to all other brachiopods or did endosymbiosis provide a lifestyle that allowed them to have higher metabolic rates and become giants? We suggest that the answer to this persistent conundrum may be located in the identification of the biosignatures of symbionts through combined analyses of the carbonand nitrogen-isotopic compositions of the occluded organic matrix within the columnar layer of the pristine fossil shells. This consists of remarkably long and a few hundreds of micrometers wide substructured columnar units.