Abstract
Abstract. The stratigraphic distribution of the Late Callovian to Early Oxfordian dinoflagellate cyst Stephanelytron Sarjeant 1961 emend provides new evidence pertaining to its evolution. Middle and Upper Callovian times favoured the development of speciations to a short-ranging Stephanelytron community with corona(s) in ventral–posterior position (Stephanelytron brontes, S. callovianum, S. ceto and S. tabulophorum) from eurytopic species with antapical coronas (S. caytonense, S. membranoidium, S. redcliffense and S. scarburghense). The former group of species (except S. tabulophorum) may represent an example of peripatric speciation from an unfavourable mutation. The reduced stratigraphic range gives the appearance of an endemic population. The genus Lagenadinium Piel, 1985 is a junior synonym of Stephanelytron Sarjeant, 1961. A new emendation of Stephanelytron, two new combinations (S. callovianum and S. membranoidium) and two new species (?S. brontes and S. ceto) are proposed.
Highlights
During an investigation of Jurassic dinoflagellate cyst assemblages in the Tethyan marine realm, the Middle and Upper Jurassic transition was studied in southeastern France (Fig. I)
The Callovian-Oxfordian boundary described by Smelror & Leereveld (1989) in a section located at the Montagne de Crussol (Rh6ne Valley) is characterized by the Compositosphaeridium polonicum-Sentusidinium pilosum (Cp-Sp) association
This association is defined by the common occurrence of Endoscrinium galeritum (Deflandre, 1938) Vozzhennikova, 1967, Escharisphaeridia pocockii (Sarjeant, 1968) Erkmen & Sarjeant, 1980, Gonyaulacysta jurassica (Deflandre, 1938) Norris & Sarjeant, 1965, Rhynchndiniopsis cladophora (Deflandre, 1938) Below 1981, Rigaudella aemula (Deflandre, 1938) Below, 1982, Barbatacysta pilosa (Ehrenberg, 1854) Courtinat 1989, Sentusidinium rioultii (Sarjeant, 1968) Sarjeant & Stover, 1978 emend Courtinat 1989, Surculosphaeridium ?vestiturn (Deflandre 1938) Davey et a/., 1966 and Wanaea spp
Summary
During an investigation of Jurassic dinoflagellate cyst assemblages in the Tethyan marine realm, the Middle and Upper Jurassic transition was studied in southeastern France (Fig. I). Specimen photographed in equatorial view; note the rather short horn, a tear under the horn mimetic of an intercalary archeopyle and the reduced corona in ventro-posterior position (presumed on 2”’ paraplate). Specimen photographed in equatorial view; note the long horn and the reduced corona in ventro-posterior position (presumed on 2”’ paraplate).
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