The Tethyan Macrocephalitinae: Evolutionary environmental and dispersal strategies

Abstract

The macrocephalitin evolution and dispersal are examined in context of the high resolution of Lower Callovian ammonoid successions realized recently in France, Germany, Britain and India. The macrocephalitins evolved in SW Pacific in the Bathonian and entered the Tethys at its eastern portal not earlier than during the late Upper Bathonian eustatic rise (Vail's cycle). There was rapid early expansion from S to N. The early Indo-East-African species M. triangularis and M. madagascariensis, are morphologically close to the European M. jaquoti and M. verus, little understood inflated accessories are also present, and this differentiation may already have been complete before their arrival in the Tethys. After the rapid early spread independent successions developed in different areas, with a maximum of 3 in the Submediterranean province reaching into the Medea Subzone: (1) fine ribbed compressed (Gracilis lineage), (2) fine ribbed inflated (Macrocephalus lineage) and (3) coarse ribbed inflated (Sphaericus lineage). India (chrysoolithicus lineage) and Britain (Kamptus lineage) evidence independent successions of only coarse ribbed inflated forms, which terminated at the close of Diadematus Subzone (≈ Koenigi Zone). This is attributed to the eustatic fall at the close of the subcycle LZA 3.1 under their near similar paleolatitudinal and paleogeographical setting. Two new short-lived compressed lineages M. formosus-M. semilaevis-E. eucyclum and M. formosus-M. opis (Formosus Subzone to early Anceps Zone) developed in India as probable derivation from the Submediterranean province which may also be related to the eustatic rise of the following subcycle Z.A. 3.2. © 1993.