The Podagrosiceratinae (Ammonitida) in the Upper Toarcian-Lower Bajocian (Jurassic) of Argentina

Abstract

The Andean Podagrosiceratinac include the genera Podagrosicerar MAUBEUGE & LAMBERT. Sphaerocoeloceras JAWORSKI and Westermanniceras nov. gen. The morphological trend places the Podagrosiceratinac origin in the Late Toarcian, minute ellipto-sphae-roconic Sphaerocoelocerus, with globose phragmocone, egressing and contracted body chamber and straight primary nbs, dividing into weak secondanes that are barely internupted on mid venter. Troughout the Aalenian (Manflasensis to Malarguensis Zones) the Podagrosiceratinac, represented by Westermanniceras nov. gen. became progressively larger, more evolute and compressed with more prominent and fexsous ribbing. This trend reached its maximum in Podagrosiceras, Malarguensis Zore, a genus with sexual dimorphism. where macroconchs wenre even larger and more evolute, and where ribbing became progressively obsolete on the body chamber. This mode of evolution is interpreted as hypemorphosis, Le. a lorm of peramorphosis or an expression of beterochronic process es probably related to K-selective pressures.