Life habit and spatial distribution of siphonotretid brachiopods in the Lower Ordovician of the Prague Basin, Czech Republic

Abstract

Siphonotretid brachiopod distribution in the Lower Ordovician of the Prague Basin is described and discussed; the new species Alichovia cometa is established. The preservation and distribution of the group indicate a preference for shallow-water offshore, high-energy environments: siphonotretids are abundant shelly fossils in the shallower-water, mostly marginal sites of the basin but are absent in the deeper parts. Their environmental limits were complementary to those of the graptolites; siphonotretids co-occur with epiplanktonic graptolites at only a few localities. Deeper offshore deposits with biramose dichograptids and diverse, generally delicate benthic dendroids lack siphonotretids. Indeed, siphonotretids were intolerant to dysoxia and preferred a well-aerated siliciclastic, firm sea floor in the shallower parts of the basin. They were tolerant to suspended coarser detritus, using their spines which functioned as an effective sieve. The density and ramification of spines indicate three sifting strategies. Suggested life styles above the seabed on elevated surfaces (algae, sponges, dendroids) are not supported by direct evidence. They more likely lived within more supple and soft matrices (sponges, tufts of algae) stabilised by their spines, with the spinose basket maintaining free space for inhalant and exhalant currents.