Coprolites from a brackish ecosystem in the Pennsylvanian Joggins Formation, Nova Scotia, Canada and their palaeoecological implications

The Joggins Fossil Cliffs UNESCO World Heritage Site contains a wealth of terrestrial and aquatic fossils from the late Carboniferous (Pennsylvanian) Period. Fossils from the aquatic realm have historically been under-studied at Joggins, and the brackish ecosystem they represent is poorly understood globally. We used coprolites, which are abundant in the limestones of the Joggins Formation, to broaden our knowledge of the aquatic ecosystem and help reconstruct the food web. Coprolites were studied in hand sample and with standard transmitted-light microscopy to determine morphology and contents. Cathodoluminescence was used to examine coprolite microfabrics to interpret the degree of post-depositional alteration. Specimens range in size from 1 to 10 cm in length and were divided into six broad groups based on size and shape (in order of most to least abundant): small/equant; cylindrical; heteropolar macrospiral; large; irregular; and amphipolar spiral. Where possible, specimens were identified to ichnospecies. The host rock (limestone) and large quantities indicate that at least most, perhaps all, coprolites originated from fish, with some potentially originating from amphibious tetrapods. Bone fragments and/or coated grains were found in the majority of samples, however their relative abundance differs between morphotypes: small/equant coprolites contained the most coated grains and fewest bones and heteropolar macrospiral coprolites contained the most bones and almost no coated grains. Specific species identification of bones was not possible. The mineralogy of the coprolites is phosphatic, specifically carbonate fluorapatite, suggesting that the fish producing these coprolites were carnivorous and that there was a lack of coprolites generated by herbivores. Based on our findings of coprolite morphology and contents and what is currently known about the biotic environment, we introduce a hypothetical trophic pyramid, with rhizodonts at the apex producing the largest coprolites, and smaller fish species responsible for the small/equant-shaped coprolites at the secondary consumer level. This research provides a foundation for further studies on coprolites and other bromalites for a deeper understanding of brackish ecosystems where fish diversified further into fresh water during the Palaeozoic.