Palaeoecology of a conodont age-calibrated and spectacularly preserved bryozoan-brachiopod biostromal palaeocommunity from the Asselian of Carlin Canyon, Nevada, USA

The Strathearn Formation is a carbonate-dominated succession cropping out in Carlin Canyon, northern Nevada. The upper part of this unit is mid-upper Asselian (Lower Permian), as determined by conodont biostratigraphy including occurrences of Sweetognathus expansus, Streptognathodus constrictus, and Mesogondolella striata. At the study location, the Strathearn is fine-grained carbonate, dominated by a heterozoan assemblage of bryozoans, brachiopods and echinoderms that accumulated in a mid-ramp setting below a thermocline. The studied unit is highly fossiliferous with a silicified bryozoan assemblage dominated by ramose forms including trepostomids, cryptostomids, and cystoporids, and subordinate fenestrids. The matrix between bryozoan zoaria includes carbonate mud and peloids with some microbial fabric and minor quartz silt. Brachiopods are also found throughout, mostly in life-position. The assemblages include large productides as well as small ones, and small rhynchonellide and spriferinide taxa, suggesting sparse and limited food resources, as these conditions favour the growth of brachiopods with simple lophophores, which can attain large size as the productides Reticulatia huecoensis and Kochiproductus sp. They are mostly free-living semi-infaunal taxa indicating the occurrence of soft substrates, but some forms are pedicle-attached. Biotic relationships within the palaeocommunity were dominated by competition among suspension-feeders that collected food in different tiers at a depth between fair-weather and storm wave base. Some bryozoans lie horizontal to bedding, but others have a vertical life-position. Overall, the degree of relief suggests that this unit represents multiple incipient biostromes that were never fully developed, as determined by numerous storms that are recorded as graded tempestites downslope. The assemblage is compared to others along the western and northwestern margin of Pangea. Storms, increasing turbidity, and climate change may have locally inhibited and limited long-term development of this suspension-feeding community. “I'd like to be, under the sea, in a bryozoan's garden, in the shade” apologies to The Beatles