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Published online by Cambridge University Press: 20 May 2016
Gastropods belonging to the extinct family Cirridae Cossmann, 1916 are relatively rare elements of Mesozoic marine communities. The unusual change in shell coiling from dextral to sinistral during their ontogeny represents a character which makes their recognition in the fossil record easy. This shell feature is also known from members of the Paleozoic family Porcelliidae Koken in Zittel, 1895, uniting the planispirally coiled Porcelliinae with the sinistrally coiled Agnesiinae Knight, 1956. This heterostrophy is not homologous with that of the Heterobranchia (Frýda and Blodgett, 2001). Shells of Porcelliinae and Agnesiinae bear an apertural slit, in contrast to the Mesozoic Cirridae. According to Bandel (1993a), slit-bearing members of the Porcelliidae evolved into the Cirridae without slit during the Triassic. Therefore Bandel (1993a) united both families into one natural group, the Cirroidea. That name has now been changed into Porcellioidea, according to the older family name (see discussion in Bandel and Frýda, in press). The oldest representatives of the Porcelliidae were reported from Silurian strata (Frýda, 1997). The youngest Cirridae are known from Late Cretaceous strata (Kiel and Bandel, 2002). Frýda and Blodgett (1998) described an Emsian (Early Devonian) porcellioid—Alaskacirrus bandeli—without slit as in the Mesozoic Cirridea, and pointed out that phylogenetic relationships of the Paleozoic Porcelliidae and Mesozoic Cirridae are still uncertain. The placement of the Porcellioidea within the Archaeogastropoda is based on the discovery of an archaeogastropod-type protoconch in the Paleozoic Porcelliidae (Yoo, 1994) as well as in the Mesozoic Cirridae (Bandel, 1993a). In addition, Bandel (1993a) mentioned the presence of a nacreous inner layer in a Mesozoic cirrid. This observation was from an unpublished master's thesis (Hänsel, 1992) supervised by him. Hänsel (1992) demonstrated that nacre is present in shells of the Late Cretaceous cirrid Sensuitrochus ferreri Quintero and Revilla, 1966 (Fig. 1.1), using peels of polished and etched shells to determine the shell structure. Although this observation is important for the interpretation of the higher taxonomic position of the Cirridae, it was never published. The aim of the present study is to document the presence of nacreous shell in the Mesozoic cirrid Sensuitrochus ferreri, using scanning electron microscopy.