New species of Franchia and Protozigzagiceras (Ammonoidea, Middle Jurassic): The phyletic origin of Zigzagiceratinae

Abstract

Three genera and seven species belonging to the
subfamily Zigzagiceratinae (family Perisphinctidae) are
described from the Lower Bathonian of France and Saudi
Arabia. Intraspecific dimorphism is recognized. A revision of
the genus Franchia proposed by Sturani (1967), based on the
syntypes and new specimens from south-east France, is
presented. Franchia arkelli Sturani, Franchia subalpina sp.
nov., Protozigzagiceras torrensi (Sturani), Protozigzagiceras
tethycum sp. nov., Protozigzagiceras flexum sp. nov. and Protozigzagiceras
densum sp. nov. are described from the Digne–
Castellane region of south-east France. Megazigzagiceras
subarabicum, gen. et sp. nov. is described from the Dharma
region of Saudi Arabia. The successive Early Bathonian species
of Franchia and Protozigzagiceras herein identified in
West Tethyan areas, as members of the Mediterranean–Caucasian
Subrealm, formed lasting separate peramorphoclines
characterized by increasing hydrodynamic coiling of the
shell. In contrast, rapid proterogenesis originated and diversified
the earliest Bathonian zigzagiceratin lineages, giving
paedomorphic members, commonly neotenic and more scarcely
progenetic. Procerites–Siemiradzkia seems to be the oldest
zigzagiceratin member in the French Subalpine, Iberian
and Lusitanian basins, branched off by paedomorphosis from
leptosphinctins at the Bajocian–Bathonian transition. The
Mediterranean–Caucasian genera Franchia, Zigzagiceras, Zigzagites
and Wagnericeras branched from successive species of
Protozigzagiceras, in turn, a direct derivative of Procerites.
The oldest lineages of the clade Zigzagiceratinae evolved by
iterative, rapid, paedomorphic changes and additional, lasting,
peramorphic modifications during the Early Bathonian.