Cretaceous Research (1997) 18, 493 –504 Evolution and distribution of calpionellids—the most characteristic constituents of Lower Cretaceous Tethyan microplankton 1 Daniela Reha ´ kova ´ and Jozef Michalı ´k Geological Institute of the Slovak Academy of Sciences , Du ´ bravska ´ cesta 9, 842 26 Bratislava, Slovakia Revised manuscript accepted 23 July 1996 During the evolution of the Calpionellidae, the calcite composition of their loricas, hitherto considered as one of the major taxonomic criteria of this family, changed several times. During periods of calcium carbonate hypersaturation of sea-water, the calcite deposited in the walls of the lorica could replace their original chitinous composition. From this point of view, we regard chitinous Chitinoidella, Deflandronella and Praecolomiella or Parachitinoidella as direct relatives of calcareous ‘true’ calpionellids. Taking into consideration ecological aspects of the distribution of individual calpionellid forms, we have modified the model of their middle Tithonian to early Albian evolution, within the frame of which we have recognized five diversification stages. These are (1) chitinoidellid (tiny microgranular loricas); (2) crassicollarian (simple hyaline loricas); (3) remaniellid (advanced loricas with plicate collars); (4) praecolomiellid (reset of microgranular forms); and (5) colomiellid stage (hyaline forms with more or less articulated collars). ÷ 1997 Academic Press Limited KEY WORDS: calpionellids; evolution; bioevents; biozonation; Late Jurassic; Early Cretaceous. 1. Introduction Calpionellids represent one of the most important constituents of tropical calcareous microplankton in Tethyan Upper Jurassic –Lower Cretaceous pelagic carbonates. This is the reason for their key role in biostratigraphic correlation of these deposits in the Mediterranean area (Michalı ´k, 1995). After the first application of calpionellids in stratigraphy (Remane, 1964, 1969), Allemann et al. (1971) introduced the standard calpionellid zonation, accepted and further evolved by other specialists (Allemann et al ., 1975; Pop, 1976; Trejo, 1975, 1980; Bakalova, 1977, 1986; Makarieva, 1979; Borza, 1974, 1984; Borza & Michalı ´k, 1986; Nagy, 1986; Remane et al. 1986; Altiner & O ¨ zkan, 1991; Lakova, 1993; Pop, 1994a; Reha ´ kova ´ , 1995). Bonet (1956) defined the Calpionellidae as a group of Late Tithonian to Valanginian calpionellids with hyaline calcitic loricas. Similar loricas formed by microgranular calcite (Chitinoidella Doben 1963) were regarded as homeomorphs belonging to another group (Trejo, 1976). The same conclusions (Borza, 1978; Remane in Bolli et al., 1985) were applied to Late Aptian microgranular loricas (Deflandronella Trejo, 1975, Parachitinoidella Trejo, 1972; Praecolomiella Borza, 1979) despite the fact that the latter gave rise to Early Albian hyaline specimens (Colomiella Bonet, 1956) that resemble Late Tithonian–Valanginian calpionellids. The application of these criteria led to conclusions about a restricted time range for Calpionellidae from Late Tithonian to Early Valanginian, and to even more speculative postulations about their monophyletic origin (Remane 1971). Later, Remane (in Bolli et al., 1985) considered the possibility of gradual 1 Contribution to IGCP Project 362: Tethyan and Boreal Cretaceous. 0195– 6671 / 97 / 030493 + 12 $25.00 / 0 / cr970067 ÷ 1997 Academic Press Limited