Middle Palaezoic cephalopods from southwestern Gondwana and their palaeobiogeographic meaning

CICHOWOLSKI, M.; URIZ, NORBERTO; RUSTÁN, JUAN JOSÉ

Fez

Simposio; 10th International Symposium "Cephalopods - Present and Past"; 2018

Wëstfalische Wilhelms Universität Münster, University Sidi Mohammed Ben Abdellah

Some groups of non-ammonoid cephalopods were usually consideredexclusive of warm waters, being only known from low paleolatitudinal regions.Some examples include the orders Ascocerida and Discosorida (e.g. Mutvei, 2012;Kröger, 2013), but also the Family Lamellorthoceratidae (see references inCichowolski & Rustán, 2017). However, recent findings from southwesternGondwana have demonstrated that a reassessment of their palaeogeographic distributionis needed. New data mainly come from the Hirnantian (?)?early Llandovery from theParaná Basin in Paraguay (the Eusebio Ayala and Vargas Peña formations), whereascocerid specimens were found. Even though their preservation is poor, someunusual features that distinguish this group can be clearly recognized. Palaeogeographicreconstructions indicate that fossil localities, characterized by a scarce and poorlydiversified fauna, were located at 60°S by the latest Ordovician (Torsvik & Cocks, 2013). Onthe other hand, Early Devonian rocks of the Argentine Precordillera revealedseveral specimens of the genus Arthrophyllum(Lamellorthoceratidae). Also in these rocks, we identified a relatively diverseand abundant cephalopod fauna composed by bactritids, pseudorthocerids, oncocerids,and orthocerids (Cichowolski & Rustán, 2014). Thus, this new informationcontrasts with previous models of the Malvinokaffric Realm, an austral circumpolarmajor paleobiogeographic unit for the Devonian, alleged as virtually devoid ofcephalopod records (Boucot & Racheboeuf, 1993). Based on theseconsiderations, we think that supposedly restricted distribution of some groupsof cephalopods during the Early and Middle Palaeozoic might be a bias artifactdue to the uneven availability of information from northern and southern worldhemispheres. Future work will strengthen these preliminary conclusions.Boucot, A.J. & Racheboeuf, P.R. (1993): Biogeographicsummary of the Malvinokaffric Realm Silurian and Devonian fossils. ? In: Suarez-Soruco, R. (ed.): Fósiles yfacies de Bolivia v. II. Invertebrados y Paleobotánica. -- Revista Técnica deYPFB, 13?14: 71?75. Cichowolski, M. & Rustán, J.J. (2014): LowerDevonian cephalopods from Precordillera, Western Argentina: FourthInternational Palaeontological Congress, Mendoza, Argentina, 2014, AbstractVolume, p. 683.Cichowolski,M. & Rustán, J. J. (2017): First report of Devonian bactritids(Cephalopoda) from South America: paleobiogeographic and biostratigraphicimplications. Journal of Paleontology, 91:417-433. Kröger,B. (2013): The cephalopods of the Boda Limestone, Late Ordovician of Dalarna,Sweden. European Journal of Taxonomy, 41: 1-110. Mutvei,H.(2012): Siphuncular structure in Silurian discosorid and ascocerid nautiloids(Cephalopoda) from Gotland, Sweden: implications for interpretation of mode oflife and phylogeny. GFF, 134: 27-37.Torsvik,T.H. & Cocks, L.R.M. (2013): Gondwana from top to base in space and time.Gondwana Research, 24: 999-1030.