First record of sexual dimorphism in the genus Anagaudryceras (Ammonoidea, Lytoceratoidea) from Antarctica

OLIVERO, EDUARDO B.; RAFFI MARÍA EUGENIA

Puerto Madryn

Congreso; RCAPA 2018; 2018

APA

As a result of ongoing systematic studies in the late-early to mid Campanian Rabot Formation, two adult morphotypes of the genusAnagaudryceras have been recorded. They coexist in the last 50 m of the Member II, Ammonite Assemblage 6, Natalites spp. Group 2 andthe beginning of the Member III, Ammonite Assemblage 7 Neokosmaticeras redondensis. These morphs are associated with Baculitessubanceps Haughton, Metaplacenticeras subtilistriatum Jimbo and Hoplitoplacenticeras sp., which indicate elsewhere the base of the midCampanian. The adult body chamber of the bigger and compressed morphotype A (Wb/Wh ~ 0.80) has band-like ribs preceded by a markedconstriction that runs parallel to the rib; the adult body chamber of morphotype B is depressed (Wb/Wh ~ 1.18?1.03) and has two types ofornamentation; strong asymmetrical, slightly flexuous ribs, preceded by constrictions and almost imperceptible spiral striations. Theseadult morphotypes develop from morphologically similar depressed juvenile shells. The neanoconch is ornamented by five or less ribsaccompanied by fine lirae. The rest of the phragmocone has fine, flexuous, prorsiradiate lirae and up to five collars per whorl. As these twomorphotypes are characterized by a) differences of ornamentation only in the adult body chamber, b) similar abundance of specimens and c)same stratigraphic range, we conclude that they represent an Anagaudryceras antidimorphic pair. This is the first evidence of sexualdimorphism in Anaguadryceras; even though no peristome or lappets are preserved, we tentatively differentiated the morphotype A as themacroconch [♀] and the morphotype B as the microconch [♂].