Paragarantiana and other new latest Bajocian garantianin lineage (Ammonoidea, Middle Jurassic): A case of proterogenesis and neoteny

摘要

Two genera and six species of ammonites belonging to the subfamily Garantianinae are described from the Upper Bajocian of the Maizet-Bretteville-Sully area of Calvados (North-West France). A revision of the genus Paragarantiana proposed by Gauthier (2003) is presented. Paragarantiana alticosta (WETZEL), P. longidoides (GAUTHIER et al.), P. ipfensis (DIETZE et al.) and P. sullyensis sp. nov. are heterochronic components of a lineage characterizing the Parkinsoni Zone. Another lineage is identified as a new genus Metagarantiana composed at least by the following species: M. occidentalis (GAUTHIER), M. media sp. nov. and M. magna sp. nov., recorded from the Tetragona Subzone to the Densicosta Subzone of the Garantiana and Parkinsoni zones. The taphonomic, palaeobiological and biochronological information on Paragarantiana and Metagarantiana suggest that representatives of these two Submediterranean-NW European genera inhabited the central part of the Anglo-Paris Basin (in the sector of the present day area of Maizet-Bretteville-Sully), at least with six miodemic species showing intraspecific dimorphism. The Late Bajocian species of Metagarantiana and Paragarantiana herein identified in the western Neotethys, as members of the Mediterranean-Caucasian Subrealm, formed separate gradational series of morphological change or peramorphoclines, undergoing greater ontogenic variation and development by hypermorphosis and acceleration respectively, during the Subgaranti-Bomfordi subzonal interval of the Garantiana and Parkinsoni zones. In contrast, rapid proterogenesis undergoing smaller ontogenic variation originated and diversified some garantianin lineages, giving paedomorphic members of neotenic type, such as Paragarantiana sullyensis sp. nov. These two Submediterranean-NW European genera represent diverse and lasting anagenetic lineages initially branched off from Garantiana/Pseudogarantiana, in turn, a direct derivative of Strenoceras/Orthogarantiana.