Oriented attachment of a stalked cirripede on an orthoconic heteromorph ammonite - implications for the swimming position of the latter

Scalpelliform cirripedes [order Scalpelliformes BUCKERIDGE & NEWMAN, 2006] of the genus Stramentum LOGAN are known from different mid-to early Late Cretaceous (Albian-Santonian) facies in Europe, North, Central and South America, the Near East, Africa and Japan. Stramentum (S.) pulchellum (G.B. SOWERBY) occurs in the Cenomanian and Turonian of England, northern Ireland, northwest Germany and the Czech Republic. Generally, only isolated plates of the capitulum or peduncle are found, for instance in white chalk facies; finds of (near-) complete, articulated skeletons are rare. However, articulated stramentids appear to be common in two exceptional cases; either when cirripede larvae settled on planispirally coiled ammonites, reached adulthood as epizoans but were then smothered by the growing ammonite shell, or when cirripedes were embedded in black shales. Below we describe a stramentid from a black shale facies of latest Cenomanian age (Neocardioceras juddii ammonite Zone) at Lengerich (northwest Germany), which is attached by the base of the peduncle to the orthoconic shell of the heteromorph (baculitid) ammonite Sciponoceras sp. For the first time, the orientation between cirripede and ammonite shell provides evidence of the near-horizontal swimming position of this heteromorph, similar to extant sepiids. The oriented attachment of stramentids to planispirally coiled ammonites, as well as to orthocones, document that these cephalopods did not swim backwards, according to the thrust principle, but rather moved forwards.