LINKING ECOPHYSIOLOGICAL STUDIES OF LIVING BRACHIOPODS TO PHANEROZOIC OXYGEN VARIATIONS

Ecophysiological studies of living organisms contribute to a better understanding of the evolution and adaptation of life to atmospheric oxygen variations throughout the Phanerozoic. Several studies of vertebrate groups, such as placental mammals, have established a link between the development of morphological innovations and traits that have resulted from oxygen variations in the atmosphere. For groups of marine invertebrates, however, there is less knowledge, despite their importance in the fossil record. Brachiopods, a long-standing and conservative group of marine bivalved animals, evolved shells with complex perforations (punctae), possibly related to respiration. A combination of morphological and isotopic analyses in living species of brachiopods is used to demonstrate a relationship between punctae and respiration. Synchrotron-radiation X-ray tomographic microscopy (SRXTM) reveals the species-specific nature and 3-D complexity of punctae in Recent terebratulid brachiopods and their link with oxygen exchange. The involvement of punctae in respiration has led to an array of punctual morphologies that evolved to facilitate terebratulid brachiopod adaptation to a range of environmental pressures. Importantly, the presence and structural adaptation of punctae may explain the survival and increased diversity of terebratulides in low oxygen conditions throughout three major periods of mass extinctions (end-Devonian, Permian-Triassic and Triassic-Jurassic) and oceanic anoxic events in the Mezozoic.