A fresh look at Jurassic Retroceramidae and their mode of life

DAMBORENEA, S.E.; JOHNSTON, P.A.

Cambridge, Gran Bretaña

Simposio; International Meeting on the Biology and Evolution of the Bivalvia; 1999

Malacological Society of London

The taxonomic relationships and life habits of  retroceramids are reappraised on evidence from Middle Jurassic Retroceramus species from central western Argentina and elsewhere. The  Family Retroceramidae is removed from the Pteriomorphia and relocated within the Superfamily Inoceramoidea in the Subclass Cryptodonta because: a) details of the retroceramid ligamental area reveal a linear growing margin, as in inoceramids, and not sinusoidal, as in Isognomon; b) posterior pedal muscle scars are comparatively large, subcentrally  placed, and well separated from the relatively small, distally placed posterior adductor, fitting the inoceramid and praecardioid patterns, not that of pteriomorphs; c) several species show geniculations of shell profile resulting from  changes in shell convexity and ornament during ontogeny; and d) main ornament consists of pronounced comarginal rugae affecting both inner and outer shell surfaces. Shell shape and ornamentation indicate that Retroceramus species were orthothetic, probably semi-infaunal or epifaunal, and lived on mud-grade substrates in poorly oxygenated settings, as supported by taphonomic evidence. Occurrence of these bivalves with ammonoids only, or with few other benthonic megafauna, in deposits originated in dysaerobic environments suggests that retroceramids may have harboured as symbionts chemosynthetic sulphophilic bacteria, as already proposed for inoceramids on various grounds (isotopic, sedimentological, etc.) by other authors. Furthermore, peculiar modifications of the ventral region of some Retroceramus species studied, suggest a thin, flexible, ventral flap-like extension of  the shell margin which might have been related to the presence of a ventral, sulphide-pumping organ.