Journal of Paleontology, 91(3), 2017, p. 417–433 Copyright © 2017, The Paleontological Society 0022-3360/17/0088-0906 doi: 10.1017/jpa.2017.17
First report of Devonian bactritids (Cephalopoda) from South America: paleobiogeographic and biostratigraphic implications
Marcela Cichowolski,1,2 and Juan J. Rustán3,4
1Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Departamento de Ciencias Geológicas, Área de Paleontología, Ciudad Universitaria, Pab. 2, C1428EGA, Buenos Aires, Argentina ⟨
mcicho@gl.fcen.uba.ar⟩ 2CONICET-Universidad de Buenos Aires, Instituto de Estudios Andinos “Don Pablo Groeber” (IDEAN), Buenos Aires, Argentina 3Centro de Investigaciones en Ciencias de la Tierra (CICTERRA), Centro de Investigaciones Paleobiológicas (CIPAL), CONICET, Universidad
Nacional de Córdoba, Av. Vélez Sarsfield Nº 1611, X5016GCA, Córdoba, Argentina ⟨
juanjorustan@gmail.com⟩ 4Universidad Nacional de La Rioja, M. de la Fuente s/n, CP 5300, La Rioja, Argentina
Abstract.—Devonian bactritids are described for the first time from South America. They come from siliciclastic rocks of the Talacasto Formation in the Precordillera Basin, west-central Argentina. The host strata span the Lochkovian–Emsian and contain other non-ammonoid cephalopods as well, thus refuting the alleged virtual absence of cephalopods in circumpolar Devonian basins from southwestern Gondwana (the Malvinokaffric Realm). We report Bactrites gracilis and Devonobactrites? sp., whose wide distribution contrasts with the endemic paleobiogeographic signature of some other taxonomic groups in these basins. Furthermore, new Lochkovian and Pragian records of Bactrites sp. provide new insights into the earliest bactritid records worldwide.
Introduction
Bactritid cephalopods are considered a key group of the middle– upper Paleozoic. In addition to the interesting phylogenetic aspects concerning their origin, their important role in the evo- lutionary history of cephalopods has been especially debated (e.g., Erben, 1964; Kröger and Mapes, 2007; Kröger et al., 2011; Klug et al., 2015). The present consensus assumes they evolved from an orthocerid group during the latest Silurian– Early Devonian, and, in turn, gave rise to ammonoids during the Devonian and to coleoids in the Mississippian (Erben, 1966; Mapes, 1979; Doguzhaeva, 2002; Kröger and Mapes, 2007; Klug et al., 2015). In contrast to the rich records from Laurentia and northern
Gondwana, bactritids are very poorly known from southern Gondwana. Particularly in South America, the only certain reference is a report of the genus Bactrites Sandberger, 1843 from the Pennsylvanian of Perú (Thomas, 1928). Although Cecioni (1953) reported Eobactrites? from supposedly Early Devonian beds of the Lipeón Formation (Turner, 1960) in northwestern Argentina, that genus is considered a synonym of Bactroceras Holm, 1898 (e.g., Evans, 2005), and accordingly belongs to the Orthocerida (Kröger and Mutvei, 2005). Furthermore, those beds are known to be Silurian in age (Rickards et al., 2002). A moderately small collection of non-ammonoid cephalo-
pods from the Lower Devonian of west-central Argentina (Precordillera Basin) has recently been studied. Here, we describe bactritid cephalopods, which constitute the first Devonian record of the group in South America. Preliminarily reported orthocerids, pseudorthocerids, and oncocerids from
this collection (Cichowolski and Rustán, 2014), as well as recently recognized lamellorthoceratids will be discussed at length in a forthcoming contribution. With a long history of paleontological investigation, the
rich records of the Argentine Precordillera have provided the best source of information on Devonian marine faunas from Argentina (e.g., Vaccari et al., 1994; García-López and Fernández-Martínez, 1995; Herrera, 1995a, b; Sánchez et al., 1995; Herrera et al., 1998; Rustán and Vaccari, 2010, 2012; Rustán et al., 2011a, b; Carrera et al., 2013; Salas et al., 2013; Carrera and Rustán, 2016). However, in spite of this extensive paleontological record, no cephalopods other than the waste- basket genus Orthoceras have been reported from this basin to date (Thomas, 1905; Keidel, 1921; García, 1945; Leidhold and Wetten, 1947). Paleobiogeographic analyses of Devonian faunas from the
Precordillera and elsewhere in southern South America mainly deal with the cold water Malvinokaffric Realm, a major circum- polar austral marine paleobiogeographic unit recognized from the Early–Middle Devonian in southwestern Gondwana (Richter and Richter, 1942). In addition to the high endemism of taxa at the suprageneric level, and the unusual abundance of certain taxonomic groups (such as hyoliths and conulariids), this realm was also characterized by the scarcity (or absence) of several characteristic Paleozoic groups (Boucot and Racheboeuf, 1993). Particularly striking are the lack of stromatoporoids, conodonts, and graptolites, and the extreme scarcity of goniatites. In this context, a putative “near lack of cephalopods” was
cited as a characteristic Malvinokaffric trait (Boucot and Racheboeuf, 1993, p. 72), overlooking a number of compelling reports of Early Devonian non-ammonoid records previously
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