Encrustation in Nautilids: a case study in the cretaceous species Cymatoceras Perstriatum, Neuquén basin, Argentina

Abstract

Despite many advances in the study of the paleoecology of nautiloids and amonoids, there is still much ground to cover. Study of their encrusters provides information on their paleoecology and taphonomy. In this work the encrusting fauna of abundant material of the nautilid Cymatoceras perstriatum (Steuer) is statistically and taxonomically analyzed in order to infer paleoecological and taphonomic features of the fossil organism. The available material of C. perstriatum consists mainly of phragmocone fragments, with rare cases in which the body chamber was preserved. Nine encrusting taxa, evenly distributed across the nautilid shell (cementing bivalves, serpulids, sabellids, cyclostome bryozoans, and agglutinated foraminifers), were observed in and on C. perstriatum. Of these taxa, oysters are dominant. Encrusters are abundant, with a mean of 12 encrusters per shell. Internal encrustation is common, especially inside the body chamber. The orientation of encrusters is variable. Intensity of encrustation varies, with some shells heavily colonized while others remained clean. The encrusting fauna is interpreted as mainly postmortem. Encrusters are distributed across the shell in a uniform way, lack particular orientation, and are common on the inside of the body chamber, suggesting that encrustation occurred both during necroplanktonic drift and as the shell rested on the sea bottom. A few encrusters were trapped between shell whorls, indicating that less extensive in vivo encrustation took place. Some differences in encrustation parameters were found among localities, corresponding to minor paleoenvironmental differences. Variable encrustation intensity suggests moderate to prolonged duration of exposure of the shells. However, those collected from an exceptional concentration at the El Salado locality were probably buried shortly after death.