Sclerobionts, shell morphology and biostratinomy on ammonites: Two early cretaceous cases from the Neuquén basin, Argentina

Abstract

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.