Quantitative palaeoecology in the Pachycyrtella Bed, Early Permian of Interior Oman

The population dynamics and community structure of the Pachycyrtella Bed at the base of the Lower Permian Saiwan Formation (Interior Oman) are described. The populations of the brachiopod Pachycyrtella omanensis and the bivalve Dickinsartella pistacina are census populations, which show a stable structure, with individuals growing successfully to maturity due to fast initial growth rate. P. omanensis and D. pistacina are opportunist species that colonized a physically stressed environment, characterized by high mobility of the substrate, high energy and high nutrient influx at the end of the Gondwanan deglaciation. Both P. omanensis and D. pistacina have a large biovolume with respect to surface area and this may reflect an adaptation to cool climate conditions, whereas their thick shell substance could be the result of increasing biocalcification in the high O2/CO2 Early Permian atmosphere. Community structure records the combined effect of rich food resources and an unstable environment, as low (individuals) to very low (biovolume) diversity values and short suspension feeding chains characterize the pioneer palaeocommunities of the Pachycyrtella Bed. The slight increase in diversity in the upper part of the bed is interpreted to have resulted from a change in the environmental stability, probably connected to decreasing hydrodynamic energy during a small scale sea-level rise. More favourable conditions at the top of the bed are also suggested by the relative abundance of the newly recognized small-sized strophalosiid species Strophalosia ericinia n. sp., which spent part of its life firmly attached to P. omanensis and bivalves.