STERCOMATA MAY REVEAL A NOW LARGELY HIDDEN RECORD OF BENTHIC PROTISTS IN THE CAMBRIAN EVOLUTIONARY RADIATION

The spartan occurrence of Cambrian foraminiferans may reflect a taphonomic bias against preservation that has limited knowledge of them to a few, rare, monothalamic (single chambered), agglutinated taxa, some of which (i.e., Platysolenites) may not even be foraminifera. This contrasts with modern marine habitats in which monothalamic foraminifera and gromiids are common and inhabit most marine oxygenated and hypoxic sediment facies. A surprising 2011 development was the report of large Bathysiphon specimens (ca. 6 cm long) in late early Tremadocian (ca. 485 Ma) rocks on the Avalonian microcontinent, which extended the record of monothalmic foraminiferans ca. 255 Ma from the Triassic. The specimens are unambiguously referrable to Bathysiphon as they feature a tube of clay particles organized into a felt-like sheath with the lumen packed with “stercomata”— micrometer-scale, cytoplasmic oblate aggregates of undigested waste (primarily clay particles) comparable to those in certain monothalmids and all gromiids. Stercomata are the unicellular equivalent of metazoan coprolites, with the Ordovician record showing nearly identical cytoplasmic activity to that in Modern Bathysiphon. Ordovician Bathysiphon shows that stercomata can be remarkably resistant to destruction. Evidence of this resistance is complimented by our taphonomic investigation of stercomata from live and experimentally decayed Gromia sp. specimens from McMurdo Station, Antarctica. Despite >20 yr incubation in bacterized seawater, stercomata retained all the structural features of those in live specimens. Stercomata resist disaggregation by sonication, overnight dissolution in 5% sodium hypochlorite, boiling in 5% hydrogen peroxide, and ashing for 24 hours at 500° C. Even sonication in surfactant/detergent solutions (Triton X-100, NP-40, Quaternary O) failed to disrupt stercomata. The structural integrity of stercomata seems imparted by refractory organic matter that serves as a binding agent, which may provide isotopic signatures of past oceanic conditions. A search for stercomata as potential microfossils in disaggregated fine-grained, siliciclastic sedimentary rocks, in thin sections, and in the gut contents of soft-bodied taxa may document the presence, role, and facies distribution of benthic protists in Cambrian and older marine rocks.