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Journal of Paleontology 91(3):417–433
from Plagiostomoceras Teichert and Glenister, 1952 through Sphaerorthoceras beatum Ristedt, to ‘B.’ bohemicus. Unquestionable bactritids appear at Filon Douze
(Morocco) in the lower Zlíchovian (early Emsian), with the abundant presence of Devonobactrites obliquiseptatus Sand- berger and Sandberger (Klug et al., 2008; Kröger, 2008). Only a single dubious bactritid fragment was found in Pragian strata, which Kröger (2008) questionably assigned to Bactrites because of the lack of a discernible siphuncle. He explained that due to the small diameter and marginal character of this structure, it is sometimes difficult to identify in small specimens. Other than this questionable record, Klug et al. (2008) suggested that Emsian specimens of Devonobactrites obliquiseptatus are the oldest known bactritids. Kröger (2008) suggested that the Moroccan record reflects
a general pattern because, despite the intensive collecting effort and repeated monographic treatment of Siluro-Devonian cephalopod faunas, no additional bactritid specimens have been reported until now from pre-Emsian strata since the work of Ristedt (1981). Becker and House (1994) drew attention to a questionable Pragian bactritid record in the Rhenish Massif, based on a faunal list in Bender et al. (1974). However, the specimen under question has never been described or re- examined. Thus, it appears that unquestioned bactritids were pre-
viously unknown in pre-Emsian strata with the possible exception of ‘Bactrites’ bohemicus (Kröger, 2008). In this scenario, some of our specimens probably constitute
the oldest records of the group known at present. As specified above, in the “Stratigraphy and geological setting” section, these earliest records would be Lochkovian. CEGH–UNC 27092 was collected from a concretionary 20m thick part of the argillaceous lower interval of the Talacasto Formation at Quebrada de la Cortadera in San Juan Province. This interval contains corals preserved in life position within large lenticular calcareous nodules (up to 50cm wide), which are equivalent to the Lochkovian records just 70m above the base of the Talacasto Formation in the nearby Quebrada de los Algarrobos, as reported by Carrera et al. (2013). Specimens PULR-I 1–3 were also collected from the lower argillaceous interval of the same unit at Sierra de las Minitas, in La Rioja Province from a shaly marker unit bearing fossiliferous nodules containing Lochkovian trilobites such as Echidnops taphomimus Rustán and Balseiro, 2016 and Talacastops zarelae Edgecombe, Vaccari, and Waisfeld, 1994 (Rustán et al., 2011a, b). In turn, the specimen CEGH-UNC 27089, assigned to Bactrites gracilis, was collected 30 meters below Keidel’s bed at the Quebrada de Talacasto locality, of probable Pragian age. The median section of this incomplete conch shows the absence of deposits, the marginal, small and orthochoanitic siphuncle, and the low apical angle, and would be the oldest record of this species. In addition, the moderate abundance of specimens exhibiting undisputed bactritid morphology, and their relative continuity throughout the Lochkovian–Emsian section, does not support a hypothesis of homeomorphy, but accounts for a relatively rich bactritid record in nearly all the Talacasto Formation. Although important protoconch information typical for the genus is missing in our specimens, the diagnostic adult
taxonomic characters of the typical genus Bactrites can be recognized in the Lochkovian. Thus, these earliest records from Argentina close a gap in the fossil record, and “push” the discussions related to the origin of bactritids down into the earliest Devonian, as well as renewing attention to previous Silurian records such as those from Morocco and Bohemia. In addition, the discovery of relatively abundant bactritids
in pre-Emsian strata from high paleolatitudes provides new insights into the probable paleogeographic region of their origin, challenging the previous implicit scenario of a warm-water region of origin where they were more diverse and more widely distributed since the Emsian.
Acknowledgments
We are very grateful to C. Klug (Zürich, Switzerland) and R.T. Becker (Münster, Germany) for their positive criticism of the original manuscript. Both reviews greatly helped to improve this work. Many thanks to D.M. Work (Associate Editor of the Journal) for his help with English. This work was financially supported by the PICT 1993–2012 (ANPCYT) grant to J.J. Rustán. We are especially grateful to R.T. Becker for discussions and suggestions concerning bactritid taxonomy and biostratigraphy. We also thank C. Klug for bibliographic support and suggestions and B. Kröger (Helsinki, Finland) for bibliographic support. We are grateful to the Argentine researcherswho collected specimens described herein: Z.Herrera, B.G. Waisfeld, and N.E. Vaccari (Córdoba, Argentina). M.S. Plastani, S. Adamonis, L. Luci, and A. Caramés (Universidad de Buenos Aires, Argentina) assisted with photo- graphy. M. Medina (CICTERRA) prepared polished sections. This is the contribution R-173 of the Instituto de Estudios Andinos “Don Pablo Groeber” (IDEAN).
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