PALEOCLIMATICALLY INDUCED SIZE INCREASE IN THE EARLY CARBONIFEROUS BRACHIOPOD CLADE GIGANTOPRODUCTUS

The brachiopod genus Gigantoproductus is one of the most stratigraphically useful Early Carboniferous brachiopod clades. Species within this lineage are restricted to the late Viséan (Asbian-Brigantian-Middle Chesterian) strata of North America, Europe, Asia, and Africa. Some of the youngest members of this group display hingelines that measure up to 17 cm in length. With purported ancestry in the middle Viséan (Arundian-Holkerian), this lineage exhibits a distinct increase in both valve thickness and size through time. These macroevolutionary changes are consistent with Cope’s Rule of phyletic size increase. Unfortunately, the reasons for such changes remain unclear. Latitudinal size variations, deep water environmental settings, oxygen level of the atmosphere, and paleoclimatically induced changes have been postulated as factors creating increased individual growth. “Bergmann’s Rule” suggests that larger individuals with thinner shells characterize cooler waters at high latitudes and greater water depth. However, the widespread genus Gigantoproductus is characteristic of shallow water facies and does not appear to exhibit any distinguishable latitudinal clinal variations in size. Elevated atmospheric oxygen levels have been attributed as a cause of gigantism among Carboniferous terrestrial insects. This does not appear to be a proximate cause for size increases in Gigantoproductus, insofar as published trends in Carboniferous atmospheric oxygen levels show a significant increase beginning in the Late Serpukhovian, well after the Gigantoproductus lineage went extinct. While there is little evidence for geographic variations in size among species of Gigantoproductus, there do appear to be temporal trends. Early Viséan species assigned to this genus possess thinner and smaller shells, whereas, late Viséan to early Serpukhovian species exhibit thicker and larger shells. Such changes in shell character can be correlated with a late Viséan period of global climatic warming. The inferred warming in oceanic temperatures provided decreased levels of calcium carbonate solubility as well as increased oceanic mixing and concurrent increased nutrient levels which provided an opportunity for growth of thick-shelled individuals.