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A block of stromatolitic limestone found on the Angara River shore near Kodinsk, Siberia, derived from the exposed nearby Ust-kut Formation, has yielded a sample of 146 ellesmeroceratid nautiloid specimens. A minor contribution to the fossil assemblage from bellerophontid and hypseloconid molluscs suggests a restricted abnormal salinity environment. The associated shallow-water low diversity assemblage of the conodonts Laurentoscandodus triangularis and Utahconus(?) eurypterus indicates an age close to the Furongian–Tremadocian boundary. Echinoderm sclerites, trilobite carapaces, and hexactinellid sponge spicules were found in another block from the transitional strata between the Ust-kut and overlying terrigenous Iya Formation; these fossils indicate normal marine salinity. The conodont L. triangularis is there associated with Semiacontiodus iowensis and Cordylodus angulatus. This means that the stromatolitic strata with cephalopods are older than the early Tremadocian C. angulatus Zone but not older than the Furongian C. proavus Zone. The sample of nautiloid specimens extracted from the block shows an unimodal variability in respect to all recognizable aspects of their morphology. The material is probably conspecific with the poorly known Ruthenoceras elongatum from the same strata and region.

A block of stromatolitic limestone found on the Angara River shore near Kodinsk, Siberia, derived from the exposed nearby Ust-kut Formation, has yielded a sample of 146 ellesmeroceratid nautiloid specimens. A minor contribution to the fossil assemblage from bellerophontid and hypseloconid molluscs suggests a restricted abnormal salinity environment. The associated shallow-water low diversity assemblage of the conodonts Laurentoscandodus triangularis and Utahconus(?) eurypterus indicates an age close to the Furongian-Tremadocian boundary. Echinoderm sclerites, trilobite carapaces, and hexactinellid sponge spicules were found in another block from the transitional strata between the Ust-kut and overlying terrigenous Iya Formation; these fossils indicate normal marine salinity. The conodont L. triangularis is there associated with Semiacontiodus iowensis and Cordylodus angulatus. This means that the stromatolitic strata with cephalopods are older than the early Tremadocian C. angulatus Zone but not older than the Furongian C. proavus Zone. The sample of nautiloid specimens extracted from the block shows an unimodal variability in respect to all recognizable aspects of their morphology. The material is probably conspecific with the poorly known Ruthenoceras elongatum from the same strata and region. [ABSTRACT FROM AUTHOR]

The cameral and intrasiphonal deposits of a Pennsylvanian straight nautiloid (Pseudorthoceratidae) are studied in order to understand the formation of these deposits. The specimens from the Buckhorn Asphalt deposit (Oklahoma) are exceptionally preserved including original aragonite and microstructures. The specimen investigated survived a predation attempt and shows bite marks on the phragmocone. This is the second report of an ectocochleate cephalopod and first report of an orthoconic nautiloid which survived massive damage of conch and siphuncle. For the first time, a high-magnesium calcitic mineralogy of cameral deposits is documented. These deposits were formed in alternation with aragonite in a chamber which was perforated during the unsuccessful predation attempt. The animal formed the chamber deposits throughout its entire lifetime and the siphuncle played a major role in formation of the cameral deposits. [ABSTRACT FROM AUTHOR]