Antaganides n. gen., a new genus of Middle Jurassic nautiloid, ancestor of Pseudaganides Spath, 1927, with discussion on the phyletic origin of the family Pseudonautilidae

An important collection of nautiloids from the Middle Jurassic of western France has enabled a revised understanding of the origin and evolutionary progression of the genus Pseudaganides Spath, 1927, a genus characterised by a particularly sinuous suture line. This study describes and introduces a new genus, Antaganides n. gen. at the origin of the family Pseudonautilidae. Progressive deepening of the lateral lobe and appearance of a laterodorsal saddle are the major elements of the evolutionary mode within this family. Of the species described herein, six are new. Three are related to Antaganides n. gen.: A. sauvageti n. gen., n. sp., A. fournieri n. gen., n. sp. and A. grulkei n. gen., n. sp. Three further specimens are related to Pseudaganides: P. aquitanense n. sp., P. buffeventi n. sp. and P. crechensis. Results confirm the absence of a relationship with older folded-wall nautiloids such as the genus Belmonticeras Rulleau, 2008. The stratigraphical position of Pseudaganides subbiangulatus (d’Orbigny, 1850) is questioned and for this species, a case of sexual dimorphism is considered with P. dimidiatus (Marchand & Tintant 1971). From a palaeobiogeographic viewpoint, it appears that the Antaganides n. gen.-Pseudaganides lineage first evolved on the north-western margin of the Tethyan Ocean before migration into the Indo-Malagasy domain at the beginning of the Callovian.