Sclerobiont assemblages in the Middle Jurassic Carmel Formation of southwest Utah, USA show paleoecological succession and differences between substrates in a geographically isolated and paleoenvironmentally stressed shallow marine setting. Previous work in the region has shown fossil assemblages to be low diversity and dominated by the oyster Liostrea. Three groups not previously described from the Carmel Formation – sabellid tube worms, craniid brachiopods, and acrothoracican barnacle borings – are found as sclerobionts on shells, increasing the estimated faunal diversity in the region, but not the ecological diversity. Beta diversity is low, but differences can still be observed between sclerobiont assemblages on different types of hard substrate. Sclerobionts in the Carmel Formation colonize three types substrates: shells, synsedimentarily lithified carbonate hardgrounds, and ostreoliths, which are unusual circumrotatory oyster accumulations. Comparisons of richness, beta diversity, and faunal composition show significant but slight differences along a gradient from shell assemblages through hardground assemblages to ostreolith assemblages. This gradient most likely represents a successional sequence controlled by the ecosystem engineering effects of Liostrea, starting with small-bodied, low-richness, highly variable communities of opportunistic colonizers on shells and proceeding to a high biomass, higher richness, consistent climax community that is best developed on ostreoliths. This succession corresponds to increasing modification of the substrate by Liostrea; isolated individuals of Liostrea occur on shells, multigenerational accumulations form on hardgrounds, and ostreoliths are almost entirely composed of Liostrea. Other possible explanations for observed differences include differences between cryptic and exposed faunas, succession forced by abiotic factors, and paleoenvironmental differences correlated with but not directly connected to substrate type. These between-substrate differences and evidence of succession in the Carmel Formation provide insight into paleoecological processes in low-diversity and generalist-dominated communities.


Wilson, Mark


Earth Sciences


Earth Sciences | Ecology and Evolutionary Biology | Paleobiology | Paleontology


sclerobiont, paleoecology, quantitative paleobiology, Carmel Formation, Jurassic, Middle Jurassic, shallow marine, hardground, ostreolith, ecological succession, substrate differences, calcite sea

Publication Date


Degree Granted

Bachelor of Arts

Document Type

Senior Independent Study Thesis



© Copyright 2019 Galen Schwartzberg