TRENDS IN DIVERSITY, SIZE, AND COMPLEXITY OF TRIASSIC FORAMINIFERA

Trends in the diversity, size, and complexity of Triassic foraminifera were studied. Diversity was calculated as the number of species and as standing diversity. Maximum linear dimensions were taken as approximations of size. As per complexity, foraminifera were graded based on test architecture and wall features. Foraminiferal diversity continually increased through the Triassic, but at a slower pace after the middle Anisian. The lower size limit of foraminifera does not change. The upper size limit and the mean size greatly increased in the early Anisian but remained constant afterward. The expansion of the size range suggests diversification into new niches and/or release from environmental pressure. The average size lies closer to the lower size limit, which suggests that small size is advantageous for some foraminifera, or that smaller species produce more evolutionary offspring. The most complex foraminifera in the Early Triassic – Anisian are multilocular. An exception is Kamurana chatalovi, which inherited a coarsely perforated wall from its Permian antecessors. New taxa with endoskeleton appeared during the Ladinian. Additional internal structures increase the strength of the test and may have allowed some lineages to expand into high-energy environments. During the Carnian, several miliolid genera with thick porcelaneous walls and coarse perforations appeared. In the Norian and Rhaetian, coarse perforations are also known in hyaline Involutina and Aulosina. Perforations could ease gas exchange or could be related to the acquisition of symbiotic algae in relation to the spreading of oligotrophic conditions and/or the evolution of suitable endosymbionts.