Othonella araguaiana (Bivalvia, Megadesmidae) from the Corumbataí Formation (Midlle Permian), Eastern Margin of the Paraná Basin: Systematic, Evolutionary and Biostratigraphic Significances

In this study, the occurrence of Othonella araguaiana Mendes, a rare bivalve species is reported for the fi rst time in the Pinzonella illusa biozone, Middle Permian Corumbataí Formation, in the State of São Paulo. This species was originally

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   Revista do Instituto de Geociências - USP - 45 -   Disponível on-line no endereço www.igc.usp.br/geologiausp Geol. USP, Sér. cient., São Paulo, v. 10, n. 2, p. 45-55, julho 2010 Othonella araguaiana   (Bivalvia, Megadesmidae) from the Corumbataí Formation (Midlle Permian), Eastern Margin of the Paraná Basin: Systematic, Evolutionary and Biostratigraphic Significances Othonella araguaiana Mendes (Bivalvia, Megadesmidae), Formação Corumbataí (Permiano Superior) da Margem Leste da Bacia do Paraná: Significado Sistemático, Evolutivo e Bioestratigráfico  Marcello Guimarães Simões 1  (btsimoes@ibb.unesp.br), Luiz Eduardo Anelli 2  (anelli@usp.br),  Juliana Machado David 3  (juliana_mdavid@yahoo.com.br) 1 Instituto de Biociências - UNESPDistrito de Rubião Júnior s/n, CEP 181610-000, Botucatu, SP, BR 2 Departamento de Geologia Sedimentar e Ambiental - Instituto de Geociências - USP, São Paulo, SP, BR 3 Instituto de Geociências e Ciências Exatas - UNESP, Rio Claro, SP, BR Received 24 August 2009; accepted 16 March 2010  ABSTRACT In this study, the occurrence of Othonella araguaiana Mendes, a rare bivalve species is reported for the fi rst time in the Pinzonella illusa  biozone, Middle Permian Corumbataí Formation, in the State of São Paulo. This species was srcinally described in coeval rocks of the Estrada Nova Formation (= Corumbataí) from the Alto Araguaia and Alto Garças regions, State of Mato Grosso. The specimens of O. araguaiana were found in the base of a bioclastic sandstone bed, a proximal tempestite, in the middle of the Corumbataí Formation, in the city of Rio Claro, São Paulo State. The silici fi ed shells and internal molds are well preserved, showing impressions of muscle scars and other internal anatomic characters (e.g., hinge), never illustrated by previous authors. In his srcinal description, Mendes (1963) called attention to the similarity between O. araguaiana and Terraia aequilateralis, a common veneroid of the Corumbataí Formation. Conversely, Runnegar and  Newell (1971) suggested that O. araguaiana  belongs to Megadesmidae, being a junior synonym of   Plesiocyprinella carinata (the commonest megadesmid of the Passa Dois Group). Our study indicates that O. araguaiana is indeed a megadesmid,  but is distinct from the P. carinata . The new occurrence of O. araguaiana demonstrates that a) the paleobiogeographic distribution of this species is wider than previously thought (that it was restricted to the northern part of Paraná Basin, Mato Grosso State); b) the molluscan fauna of the Corumbataí Formation ( P. illusa  biozone) in the State of São Paulo is more diverse and dominated by megadesmids; and c) the composition of the molluscan fauna of the Corumbataí Formation in Alto Garças, State of Mato Grosso, is essentially the same as that of the P. illusa  biozone of the eastern margin of the Paraná Basin. Keywords:  Bivalvia; Megadesmidae; Permian; Paraná Basin; Corumbataí Formation; Systematics. RESUMO  Nesse estudo, é reportada, pela primeira vez, a ocorrência de Othonella araguaiana Mendes, no Estado de São Paulo, intervalo da biozona de Pinzonella illusa , Formação Corumbataí, Permiano médio. Trata-se de bivalve de ocorrência rara, srcinalmente descrito para a Formação Estrada Nova (= Formação Corumbataí), do Alto Araguaia e Alto Garças, Estado do Mato Grosso. Novos espécimes de O. araguaiana foram encontrados em arenito bioclástico (tempestito proximal), na  parte média da Formação Corumbataí, em Rio Claro, SP. As conchas silici fi cadas e os moldes internos estão bem conser-vados, preservando a impressão da musculatura e de outros caracteres internos (e.g., charneira) nunca antes ilustrados. Em sua descrição srcinal, Mendes (1963) chamou a atenção para a semelhança entre O. araguaiana e Terraia aequilateralis, um Veneroida comum na Formação Corumbataí. Contrariamente, Runnegar e Newell (1971), notaram que O. araguaiana   Marcello Guimarães Simões et al. - 46 -   pertence aos Megadesmidae, se tratando provavelmente de um sinônimo-júnior de Plesiocyprinella carinata (o megadesmídeo mais comum do Grupo Passa Dois). Conforme nossos dados mostram, O. araguaiana  pode ser atribuída aos Megadesmidae, sendo, porém, distinta de P. carinata . A nova ocorrência de O. araguaiana é importante para esclarecer as relações de parentesco entre os Megades-midae da Bacia do Paraná e por demonstrar que: a) a distribuição paleobiogeográ fi ca da espécie não está restrita à porção setentrional da Bacia do Paraná, no Estado do Mato Grosso; b) a fauna de moluscos bivalves da Formação Corumbataí (biozona de P. illusa ) no Estado de São Paulo é mais diversa e, de fato, dominada pelos megadesmídeos, e c) a composição da fauna de moluscos da Formação Corumbataí em Alto Garças é essencialmente a mesma da biozona de P. illusa , da borda leste da Bacia do Paraná. Palavras-chave:  Bivalvia; Megadesmidae; Permiano; Bacia do Paraná; Formação Corumbataí; Sistemática. INTRODUCTION The oldest known mollusc-bearing, long-lived endemic  bivalve fauna in the world was reported in the Paraná lake/sea (Passa Dois Group), Late Paleozoic (Wesselingh, 2007). This bivalve fauna, fi rstly described by Holdhaus (1918), was revised by Runnegar and Newell (1971) and has gained renewed systematic (Simões et al., 1997, Simões, Rocha-Campos, Anelli, 1998; Mello, 1999), paleoecological (Ghilardi, 1999; Kowalewski et al., 2000; Ghilardi and Simões, 2002), and taphonomic (Torello and Simões, 1994; Simões, Torello, Rocha-Campos, 1996; Simões and Kowalewski, 1998; Simões et al., 2000a; Simões and Torello, 2003) attention during the last decades (Wesselingh, 2007).During the Permian, the Paraná lake/sea was an aquatic island of diversity and endemicity within a continental interior (Eastern Gondwana) (Runnegar and Newell, 1971; Simões, Rocha-Campos, Anelli, 1998; Wesselingh, 2007). According to Runnegar and Newell (1971), the evolution of the Passa Dois Group molluscan fauna can be considered a distinct chapter in the evolutionary history of Bivalvia Class. Runnegar and Newell (1971) compared the evolution of the Passa Dois Group bivalve fauna with that of Pontian radiation of Caspian and Black seas during the Neogene. Stanley (1979, 1990) argued that the isolated Paraná Basin was a site of similar but less dramatic molluscan radiation during the Permian. According to this author, the Paraná Basin illustrates the importance of geographic factors in the control of evolutionary radiations. However, the more realistic scenario of this in situ  evolution is obscured by the poor and biased knowledge of the systematics of the Passa Dois Group molluscan fauna. For example, most of the published papers dealing with the systematics of mid- Permian bivalves of the Paraná Basin are from the interval of Pinzonella illusa  and Pinzonella neotropica   biozones, in the middle and upper portions of the Teresina and Corumbataí formations (Rohn, 1994). Actually, even in the classical monograph of Runnegar and Newell (1971), the bivalves of these two biozones were studied in  preference to others. This is because, in such intervals, the  bivalves are found in bioclastic sandstones and coquinas, where the shells are silici fi ed and, usually, well preserved. The bivalves of the Irati (Kazubek and Simões, 2003) and Serra Alta formations (Maranhão, 1986) as well as those of the Serrinha and Morro Pelado members of the Rio do Rasto Formation (Rohn, 1994; Rohn and Simões, 1997) are still poorly studied and described. Hence, at the present time, the exact number of species is unknown (due to the  poor preservation of some earlier described taxa, mostly from the Serra Alta and Rio do Rasto formations, as well as, the fact that some important systematic studies, such as Maranhão, 1986 and Rohn, 1988, were not be published). The impact of the study on the evolutionary history of this spectacular molluscan fauna is obvious: we do not know well the biodiversity and morphological disparity that resulted from the in situ  evolution of the bivalve mollusks within the Paraná lake/sea during the Permian.Despite the comments above, the continuous search for  bivalves in the classical mid-Permian strata, cropping-out in the Corumbataí River valley, in the city of Rio Claro, State of São Paulo, has resulted in the identi fi cation of a new  bivalve species. In this brief contribution, we presented, by the fi rst time, the occurrence of rare specimens of Othonella araguaiana  Mendes, 1963, in the Permian strata of the State of São Paulo. This bivalve mollusk species was srcinally observed in coeval rocks of the Estrada Nova Formation (= Corumbataí Formation) in the Alto Araguaia and Alto Garças regions, State of Mato Grosso. This new fi nd is important because the studied shells are well preserved, helping us to add new information about the morphology and systematic af  fi nities of this species, a controversial issue since early 60’s (see Mendes, 1963; Runnegar and  Newell, 1971), as well as, the taxonomic composition of the molluscan fauna of the P. illusa  biozone. BACKGROUNDThe enigmatic Othonella araguaiana  The genus Othonella  and the species O. araguaiana  were proposed by Mendes (1963) for small (length of ~3 cm) silici fi ed shells found in coquinas and bioclastic  - 47 -  Othonella araguaiana (Bivalvia, Megadesmidae) from the Corumbataí... sandstones of the Corumbataí Formation, of the State of Mato Grosso. The srcinal material described by Mendes (1963) is housed in the Scienti fi c Collection of Geosciences Institute of the University of São Paulo. This material came from three main sites in the region of the Alto Araguaia River, State of Mato Grosso, where the Permian strata of the Paraná Basin are exposed (see Almeida, 1954).During the description of O. araguaiana , Mendes was  particularly impressed by the similarities between the hinge of this species and that of Terraia aequilateralis  Mendes, 1952, a veneroid crassatellacean bivalve. Nevertheless, Mendes (1963) did not assign O. araguaiana  to any known  bivalve order or family. Hence, until 1970, the af  fi nities of O. araguaiana  remained unclear. However, Runnegar and Newell (1971) and Simões et al. (1997) demonstrated that most of the thick-shelled, infaunal suspension-feeding bivalves of the Passa Dois Group, bearing a massive blunt tooth in the right valve, could be assigned to Megadesmidae, a compact group of Late Paleozoic marine  bivalve mollusks.Runnegar and Newell (1971, p. 29) considered O. araguaiana  as a synonym of Plesiocyprinella carinata  Holdhaus, 1918, due to the fact that the type specimens of Mendes (1963, Figures 3 and 4) have shell shape and hinge similar to those of P. carinata  (Runnegar and Newell, 1971, p. 32). In addition, these authors called attention to the poorly preserved nature of Mendes’ specimens. However, later in the same paper, Runnegar and Newell (1971, p. 33) considered Othonella  as a distinct genus of Megadesmidae. Hence, we could ask if Othonella  is indeed a valid genus or a synonym of Plesiocyprinella . The identi fi cation of new specimens, as well as, their extensive laboratory preparation made it possible for us to analyze the hinge features and muscle scars and address the issue above. MATERIAL AND METHODS In this study, we have analyzed new specimens of O. araguaiana  from the Corumbataí Formation (Figures 1 and 2) as well as, the type material of Mendes (1963) from the same lithostratigraphic unit of the State of Mato Grosso. Altogether, six specimens were examined, including fi ve silici fi ed shells and one internal mold. The internal anatomical characters of these specimens, including the muscle scars and hinge, were, described in detail for the fi rst time. The type material (four specimens) is housed in the scienti fi c collection of the Institute of Geosciences, University of São Paulo, under the code DGP-7. The two new studied specimens are reposited in the Scienti fi c Collection of the Department of Zoology, Institute of Biosciences, São Paulo State University, Botucatu campus, under the code DZP.The new specimens were found in a sandstone bed in the middle portion of the Corumbataí Formation (middle Permian), P. illusa  biozone (sense Rohn, 1994) (Figures 1 and 2). The specimens were collected and prepared using standard paleontological techniques (Feldmann, Chapman, Hannibal, 1989). In the study area, Rio Claro region, the best and most extensively studied exposures of this unit are in the Corumbataí River valley (Figures 1 and 2) (Mendes, 1952; Landim, 1970). For this study, we focused our sampling efforts on the so-called “Ferraz shell bed” (Simões and Kowalewski, 1998), a proximal tempestite bed that is intercalated within a monotonous sequence dominated by violet siltstones with wave and lenticular bedding (Figure 2). This bed is well exposed in a road cut located about 1.5 km from the abandoned Figure 1.  Schematic map of the study area, City of Rio Claro, São Paulo, Brazil, showing the location of the Ferraz shell bed (Simões and Kowalewski, 1998).   Marcello Guimarães Simões et al. - 48 -  Figure 2.  Composite stratigraphic section of the lower and middle part of the Corumbataí Formation, Rio Claro County (see Figure 1), highlighting the Ferraz shell bed (Simões and Kowalewski, 1998). Othonella shells were found in Unit 2 and 4 of the bivalve-rich sandstone.  - 49 -  Othonella araguaiana (Bivalvia, Megadesmidae) from the Corumbataí... Ferraz Railway Station (Figure 1). According to the detailed stratigraphic and taphonomic analysis made by Simões and Kowalewski (1998), the Ferraz shell bed consists of four distinct microstratigraphic units. They include, from bottom to top (Figure 2), a lag concentration (Unit 1), a partly reworked storm deposit (Unit 2), a rapidly deposited sandstone unit with three thin horizons recording episodes of reworking (Unit 3), and a shell-rich horizon generated by reworking/winnowing, which was subsequently buried by storm-induced obrution deposit (Unit 4). The studied specimens of O. araguaiana  were recorded in the microstratigraphic unit 2 (DZP-17928) and 4 (DZP-2764) (Figure 2).Finally, the suprageneric systematics is based on Morris, Dickins and Asta fi eva-Urbaitis (1991), and, at the family level, on Runnegar and Newell (1971), Runnegar (1974) and Simões et al. (1997). RESULTSSystematic Paleontology Family MEGADESMIDAE Vokes, 1967Subfamily PLESIOCYPRINELLINAE Simões et al., 1997 Othonella Mendes, 1963 Diagnosis. Shell elongated, sub-triangular, non-gaped, with faintly marked umbonal carina. Hinge of right valve with an elongated, non-bi fi d, megadesmid tooth. Pallial line non-sinuate, striated and anteriorly dotted, not crossing adductor muscle scars; umbonal elevator and accessory muscle scars (a, b, ava) absent; mantle muscle (pits) present. Anterior adductor, pedal and protractor muscle scars not fused. Type-species. Othonella araguaiana Mendes, 1963,  by monotypy and srcinal designation. Othonella araguaiana Mendes, 1963Figure 3 Type-material. The lectotype (here designated) is DGP 7-982 (Mendes, 1963, Figures 3a and 3b) from P. illusa  biozone, Estrada Nova Formation (= Corumbataí Formation), was found in a coquina from Alto Garças, Mato Grosso, near the old powerhouse unit. The paratypes (here designated) are DZP-2764 and DZP-17928 from the P. illusa  biozone, Corumbataí Formation, found in a sandstone  bed (Ferraz shell bed) from Rio Claro, São Paulo. Diagnosis. Shell small (~3cm in length, see Table 1), elongated, sub-triangular, with a well de fi ned non-bi fi d,  blunt tooth in the right valve, and a corresponding socket in the left valve. Anterior adductor muscle scar subquadrate;  posterior adductor muscle scar elongated, placed behind umbonal carina. Anterior adductor, pedal and protractor muscle scars not fused. Pallial line relatively wide, without  pallial sinus, striated and anteriorly dotted, not crossing adductor muscle scars. Accessory muscle scars absent. Description. Shell sub-triangular, thick, equivalved, inequilateral, moderately elongated, in fl ated (Figures 3A, 3B, 3D, 3I). Umbonal region low, with slightly  prosogyrous beaks (Figures 3G and 3I), slightly elevated above hinge area; posterior umbonal carina rounded, faintly marked (Figure 3H). Lunule and escutcheon absent. Shell surface ornamented with well-marked growth lines of varying width, irregularly spaced (Figure 3H, J). Ligament opisthodetic, parivincular, external, attached by short nymphs (Figure 3I). Anterior dorsal margin straight, anterior extremity rounded; ventral margin convex,  posterior extremity truncated with poorly de fi ned respiratory margin; posterior dorsal margin straight. Hinge of right valve with anterior triangular socket, followed posteriorly  by triangular blunt tooth (Figures 3A, 3D, 3I, 3K). Hinge of left valve with anterior triangular blunt tooth followed  posteriorly by deep triangular socket (Figure 3G). Lateral teeth absent. Adductor muscle scars anisomyarian (Figure 3C). Anterior adductor scar dorsoventrally elongated, close to the anterior margin; anterior pedal protractor and retractor scars small, positioned above adductor, not fused (Figure 3C). Posterior adductor scar larger than anterior, Table 1.  Measurements of silicified shells of Othonella araguaiana  .
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