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The Ingersoll shale, an estuarine clay lens within the Upper Cretaceous Eutaw Formation of eastern Alabama that hosts a well-preserved terrestrial biota (plants, feathers, and amber), contains abundant narrow, discontinuous, straight to meandering, rarely branched, virtually flat and mostly horizontal ribbons of essentially pure, mainly framboidal pyrite. These structures likely originated as mucus films secreted by epibenthic or shallow endobenthic organisms, perhaps grazing gastropods and errant worms. Whatever their progenitors, mucus films served as labile hosts for sulfate-reducing bacteria and, consequently, were primary sites for very early iron-sulfide mineralization in Ingersoll substrates. Without mucus secretion and subsequent pyritization, evidence for relatively pervasive biogenic activity and environmental conditions would not have been preserved. By contrast, pyritization of body fossils in the Ingersoll shale terrestrial biota was limited, reflecting inhibited degradation of constituent refractory organic compounds in rapidly deposited, oxygen-deficient sediments.
The earliest known rugosans attached syn vivo to vertical stems occur in the late Silurian of Saaremaa, Estonia. These rugosans display vertical to subvertical attachment scars and are more common in the Ludfordian than in the Pridoli. The unknown hosts provided a higher tier for the feeding, making the association beneficial for the rugosans. Several rugosans were themselves syn vivo encrusted by bryozoans and unknown endobiotic tubicolous organisms, possibly cornulitids. Estonian rugosans appear to have been host size selective and preferred substrates of certain size. Silurian symbiotic rugosans are more often endobionts in stromatoporoids than epibionts on the vertical stems.
Shell morphology affects multiple aspects of the biology of ectocochleate cephalopods (e.g., floatability, life habit, post mortem behavior of the shell, etc.), so it should impact the establishment and development of sclerobiont faunas as well. In this study, the sclerobiont faunas of Weavericeras vacaense (a spherocone) and Holcoptychites agrioensis (a discocone), two early Hauterivian ammonites from the Agrio Formation (Neuquén Basin, Argentina) were compared. The coeval nautilid Cymatoceras perstriatum (studied previously), was contrasted with both ammonites. Results show that the three sclerobiont faunas had similar abundance, taxonomic composition and distribution of individuals across the shells, but H. agrioensis showed a markedly reduced richness, with a fauna almost entirely composed by the oyster Amphidonte (Ceratostreon) sp. The more evolute and compressed H. agrioensis sank more quickly than W. vacaense and C. perstriatum, which may have undergone longer periods of flotation and exposure on the sea bottom. This agrees with the lower taxonomic richness of the sclerobiont fauna and a better overall preservation of H. agrioensis specimens. Differences in the sclerobiont faunas are greater across variations in shell inflation and coiling degree than across nautilids and ammonites; therefore, the latter are important parameters for the sclerobiont fauna as well since they impact how long shells will float and be exposed on the seafloor, and therefore on their time of exposure. The presence of an almost monospecific fauna in H. agrioensis, despite its rapid sinking and burial, indicates that Amphidonte (Ceratostreon) sp. was the earliest settler, and could abundantly colonize hard substrates in a short time.