MACROEVOLUTION IN BENTHIC FORAMINIFERA: ATTRIBUTES FOR SPECIES LONGEVITY

Macroevolution in different invertebrate fossil groups has largely been considered by comparing genus or species durations compared to factors such as abundance and geographic distribution. Environmental versatility has also been recognized as contributing to species longevity. Yet in the benthic foraminifera, neither abundance, geographic range, nor frequency of occurrence necessarily have a significant positive relationship with duration. Abundant since the Early Paleozoic, benthic foraminifera comprise over 99% of living foraminiferal species. Their habitats range from fresh water to the deep-sea abyss, and they are at the base of food chains, comprising a considerable amount of the biomass in sediments, especially in the deep sea. Once new species survive a “trial period”, mean partial species ranges are relatively long, about 20 my for both rare and abundant taxa. This paper discusses data from fossil and living foraminifera to address the reasons why benthic foraminifera have been so successful through time. Foremost for species survival is dispersal method. Living benthics disperse mainly through the use of resting propagules, which are carried by currents and can survive months to years until conditions are favorable. Their populations exhibit a log series distribution in space and time, providing a large species pool from which to draw as environments change. Dispersal is rapid, and characteristic biogeographic and evolutionary patterns can be observed in shallow and deeper water communities. Species turnover is faster at shallower depths, likely due to a less stable environment. A further key attribute of benthic foraminifera that contributes to species survival is their ability to tolerate and even flourish under extreme and/or changing environmental conditions. For instance, some living benthic species are facultative anaerobes, thriving in oxic or anoxic environments. Estuarine forms, such as Ammotium salsum, may proliferate during both very high and very low salinities caused by changing temperatures and precipitation. And many species have little difficulty with pollutants, even incorporating high levels of heavy metals in their shells.