ORNAMENTATION IN BRACHIOPODS: A JOURNEY AMONG THREATS IN THE PHANEROZOIC

Ornamentation in brachiopods served as an effective defense against predation pressure. The present study examine the evolutionary trends in brachiopod ornamentation in the Phanerozoic. Our findings suggest that brachiopods gradually became more ornamented, driven by increasing predation pressure, throughout the Paleozoic. This directional trend towards higher ornamentation drastically altered its course across the Permian – Triassic mass extinction boundary, which coincide with the shift between the Paleozoic fauna and Modern fauna. The mean ornamentation index maintained a stasis at significantly lower values throughout the Mesozoic and Cenozoic, suggesting a reduced predation pressure on brachiopods in the post-P-T extinction in marine benthic communities. It appears that while brachiopods experienced intense predation pressure in the Paleozoic, they were not the most important components of predators' diet in the Mesozoic and Cenozoic, most likely replaced by bivalves. This long-term directional-punctuation-stasis trend is evident across all latitudinal bins, indicating the importance of predation as a selection pressure in extratropics and polar regions also, in addition to the tropical region. This overall trend also stays true when the records from siliciclastics and carbonates are evaluated separately. All major classes of brachiopod do not exhibit similar trends in the Phanerozoic, indicating a lack of correlation between the phylogenetic and phyletic trends. The taxonomic diversity of the phylum in the Phanerozoic suggest that higher ornamentation in brachiopods facilitated survival and longevity before the P-T extinction, whereas ornamentation failed to provide any advantage to overall fitness after the P-T extinction. Whether ornamentation was the trigger or it complimented the biological processes as an aftermath to determine the evolutionary trajectories in the brachiopods is a matter of debate.