The San Juan Formation of the Eastern and Central Precordillera is composed mainly of fossiliferous limestone and marly limestone. The conodont biostratigraphy of the very top of this unit has been studied on several localities such as: Don Braulio, La Pola, Cecilia and Del Aluvión creeks, recording the Y. crassus Zone for this levels (Mestre, 2010; Mestre, 2013, Heredia and Mestre, 2013). The microfossils associated to this conodont biozone are represented by spicules, foraminifers and protoconchs, which are reported and figured in this work.Fil: Mestre Garcia, Ana Isabel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico San Juan. Centro de Investigaciones de la Geosfera y Biosfera; ArgentinaFil: Beresi, Matilde Sylvia. Consejo Nacional de Investigaciones Científicas y Técnicas. Científico Tecnológico Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; ArgentinaFil: Heredia, Susana Emma. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico San Juan. Centro de Investigaciones de la Geosfera y Biosfera; ArgentinaFil: Nestell, G.. University Of Texas; Estados Unido

Beresi, Matilde Sylvia

Heredia, Susana Emma

Mestre Garcia, Ana Isabel

Nestell, G.

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Publicación Especial Nº 13BUENOS AIRES2013ASOCIACIÓN PALEONTOLÓGICA ARGENTINACONODONTS FROM THE ANDESProceedings of the 3rd International Conodont Symposium&Regional Field Meeting of the IGCP project 591Edited byGuillermo L. Albanesi and Gladys Ortega79CONODONTS FROM THE ANDES - Publicación Especial Nº 13 - Julio 2013 - PALEONTOLOGICAL NOTEMICROFOSSILS OF THE YANGTZEPLACOGNATHUSCRASSUS ZONE IN THE MIDDLE DARRIWILIAN OF THE ARGENTINE PRECORDILLERAMESTRE, A. ¹, BERESI, M. ², HEREDIA, S. ¹ AND NESTELL, G. 31 CONICET-CIGEOBIO-Instituto de Investigaciones Mineras. Universidad Nacional de San Juan. Av. Libertador y Urquiza. 5400. San Juan. Argentina;amestre@unsj.edu.ar, sheredia@unsj.edu.ar2 CONICET-IANIGLA, CCT Mendoza,  Av. Ruiz Leal s/n, CP:5500. Mendoza. Argentina; mberesi@mendoza-conicet.gob.ar3 Department of Earth and Environmental Sciences, University of Texas at Arlington, Arlington, Texas 76019, USA; Faculty of Geology, St. Petersburg StateUniversity, St. Petersburg, Russia; gnestell@uta.eduKeywords: Microfossils. San Juan Formation. Middle Darriwilian. Argentine Precordillera.THE San Juan Formation of the Eastern and Central Pre-cordillera is composed mainly of fossiliferous limestone andmarly limestone. The conodont biostratigraphy of the last me-ters of this unit has been studied on several localities such as:Don Braulio, La Pola, Cecilia (Villicúm range), Cerro LaChilca and Del Aluvión creeks, recording the Y. crassus and theE. pseudoplanus zones for this levels (Mestre, 2010; Mestre,2012; Mestre, 2014, Heredia and Mestre, 2014) (Fig. 1A). The microfossils associated to Y. crassus Zone are repre-sented by sponge spicules, foraminifers and gastropod proto-conchs, which are reported and illustrated in this work.STRATIGRAPHYThe Ordovician carbonates exposed in the Del Aluvióncreek (Huaco), Don Braulio, Cecila, La Pola creeks (Villicúmrange) and Cerro La Chilca section, are composed of grey todark grey limestone, marls and mixed carbonate/siliciclasticsediments deposited in a ramp setting (Mestre, 2010).In the Del Aluvión creek the very top of San Juan Forma-tion composed by light grey to reddish brown bioclastic wacke-stone and bioclastic dolowackestone/dolopackstone (Fig. 1B-2).In the La Pola, Cecilia and Don Braulio creeks, top of this unitare represented by red ochre grainstone and burrowed greenishdark gray bioclastic wackestone (Fig.1B-1). In the Cerro LaChilca section the Y.crassus Zone occurring between 1.5 and 9m from the top of San Juan Formation are composed of green-ish light grey bioclastic packstone-grainstone and nodularwackestone-packstone (Fig. 1B-3).MICROFOSSILSForaminifersNestell et al. (2009) recovered the assemblage of agglutinateforaminifers from the upper most level of the San Juan Forma-tion in the Del Aluvión creek and assigned them to the Eo-placognathus pseudoplanus /Dzikodustablepointensis Zone. Thisassemblage of foraminifers consists of the monothalamous ag-glutinated genera Lakites, Amphitremoida, Lavella, Ordovicinaand Pelosina (Fig. 2, 1-3a,b).Recent studies on Darriwilian conodonts of this sectionrecorded the Yangtzeplacognathus crassus Zone for this level ofthe San Juan Formation, that enhances the biostratigraphicalsignificance of this foraminifer assemblage (Mestre, 2010;Heredia and Mestre, 2013; Mestre et al., 2013) proposing anolder age for these microfossils. Sponge spiculeThe spicule is a simple tripodal tricranoclad with a nodularbrachyome regularly preserved.It was recovered from the uppermost levels of the San Juan limestone in the Del Aluvión creek(Huaco locality) (Fig.1; Fig. 2, 4). This desma represents a typ-ical spicule of the family Hindiidae of the class Demospongea.The tricranocladine sponges assigned to the suborder Eutaxi-cladina Rauff, 1893, are a conservative group consisting of thesingle family Hindiidae.This desma represents the second mention of the familyHindiidae for the Middle Ordovician carbonates of the Pre-cordillera. Hindiid stratigraphical ranges appear in the Ordovi-cian to Middle Permian. CONODONTS FROM THE ANDES - Publicación Especial Nº 13 - Julio 2013 - PALEONTOLOGICAL NOTE80Figure 1. A, Location map of the study sections. B, The stratigraphical section of the upper part of the San Juan Formation and the lower part of theLos Azules Formation, with an indication of the beds of the microfossils species that were recovered, 1- Villicúm range; 2- Del Aluvión section; 3- CerroLa Chilca, section.This tricranoclad with a brachyome could belong to thewidespread Hindia Duncan 1879.  This genus has three-rayedspicules attach to convex shoulder of adjacent spicules firmlyunited by complex zigomes to the weakly knobby crest aroundthe neck of a brachyome, formed at the ray junctions. Carrera(2007) discovered Eoscheiella concava from the upper levels ofthe San Juan Formation at the Los Gatos creek in the CerroViejo section, located about 1 km to south of the Del Aluvióncreek. This hindiid demosponge is considered the oldest genusknown and displaced Hindia from the base of tricranocladineMESTRE ET AL.: MICROFOSSILS OF THE YANGTZEPLACOGNATHUS CRASSUS ZONE81Figure 2. Scanning Electron Microscope Microphotographs. All figured elements are referred to the Y. crassus Zone, uppermost San Juan Formation,middle Darriwiliano from Eastern and Central Precordillera. Scale bar = 100 μm. 1-3a-b, Foraminifers; 1, Pelosina teschenhagensis (Schallreuter 1983),sample DA5, Del Aluvión creek, INGEO-MP -306,.2, Lakites? sp. sample DA2, Del Aluvión creeks, INGEO-MP- 385; 3, 3a, Lavella? sp., 6b, view of the aper-tura, Scale bar= 10 μm, sample DA1, Del Aluvión creek, INGEO-MP no. 409. 4, Desma of Hindia Duncan 1879, sample DA1, Del Aluvión creek, INGEO-MP- 290 (1). 5-10, Gastropod protoconchs; 5, Open coiled (Cyrtoneritomorpha?), sample CHL3, Cerro La Chilca, INGEO-MP-1061 (1); 6, Opencoiled (Cyrtoneritomorpha?), sample DA1, Cerro Del Aluvión creek, INGEO-MP-291 (1); 7, Discoidal, sample DA1, Cerro Del Aluvión creek, INGEO-MP-292 (1); 8, Discoidal, sample TLP3, La Pola creek (Villicúm range), INGEO-MP-2568 (1); 9, Turriculate, sample TLP3, La Pola creek (Villicúm range),INGEO-MP-2569 (1); 10, Turriculate, sample DA5, Cerro Del Aluvión creek, INGEO-MP-293 (1).evolutionary history by Carrera (2007). However, Finks (2003)and Finks and Rigby (2004), identified Hindia as the basalgenus of the tricranocladines evolutionary record.The desma studied here is distinctly different from the sim-ple dipodal and tripodal tricranoclads without a brachyome,radial monaxons, and megarhizoclones of Eoscheiella concave.  The discovery of this tricanoclad with a brachyome whichpossibly represents the Hindia lineage among tricranocladinesponges extends the early history of this genus back to the Mid-dle Darriwilian. In this sense, two different lineages of triclan-oclads are present in the Middle Darriwilian of the ArgentinePrecordillera: Hindia and Eoscheiella–Scheiella lineages whichrepresent the oldest records of the triclanocladine sponges upto now known.Gastropod protoconchsEarly ontogenetic shells of gastropod (protoconchs) areminute (usually less than 1 mm), thin-shelled, and aragonitic.Thus, protoconchs can be destroyed by diagenetic processes(Dzik 1994). However, many occurrences of gastropod proto-conchs have been document in recent years even from the Or-dovician and the Darriwilian.Many juvenile gastropod shells of diverse morphologicaltypes were recovered from the upper levels of the San Juan For-mation at the Del Aluvión, La Pola creeks (Villicúm range) andCerro La Chilca creek. Thus, there are at least three distincttypes of Middle Ordovician morphotypes in the juvenile gas-tropod fauna: a) Type I: open coiled (Cyrtoneritomorpha?), inthe samples of the Del Aluvión creek (Huaco locality) andCerro La Chilca (Fig. 2, 5-6); b) Type  IV: Discoidal  in samplesfrom the Del Aluvión and La Pola creeks (Fig. 2, 7-8) and c)Type V: Turriculate with world  disjunct in samples from theDel Aluvión and La Pola creeks  (Fig. 2, 9-10). These morpho-types of protoconch morphology are described according toFrýda and Rohr (2003). Many of these gastropod protoconchsare smooth and openly coiled. The openly coiled initial whorlis known from early Paleozoic and was gradually lost duringthe Middle and Late Paleozoic probably due to planktic preda-tion (Nützel and Frýda, 2003). Early and Middle Paleozoic gas-tropod protoconchs generally differ strongly from theircorresponding adult morphologies (Seuss et al., 2012).REMARKS Microfossils associated to the Y. crassus Zone are representedby a demosponge spicule, foraminifers and gastropod proto-conchs.The foraminifer assemblage consists of the monothalamousagglutinated genera Lakites, Amphitremoida, Lavella, Ordovicinaand Pelosina (described in detail by Nestell et al. 2009). A largeassemblage of foraminifers was also recovered from beds whichrecord the Y. crassus Zone of Cerro La Chilca section, theseforaminifers will be described in further studies. However, inthe Villicúm´s sections foraminifers were not recovered in cor-relative beds suggesting this fossil group could be controlled bypaleoenvironmental conditions, preferring shallow or moreoxygenated waters of the ramp.The sponge spicule discovered in the Middle Ordovicianlimestone of the Del Aluvión creek represents the second men-tion of the family Hindiidae for the Middle Ordovician car-bonates of the Precordillera. It could belong to the hindiidlineage. This new triclanoclad and the sponge E. concava (Ca-rrera, 2007) from the Darriwilian of the Precordillera representthe oldest records of the triclanocladine sponges known to date.Three gastropod protoconch morphotypes from the studiedlocalities of the Eastern and Central Precordillera occur at thisstratigraphical level. REFERENCESCarrera, M. G., 2007. The oldest hindiid demosponge from the Darriwilian(Middle Ordovician) of the Argentine Precordillera: evolutionary implica-tions for the tricranocladines Journal of Paleontology 81(4): 754–759.Duncan, P. M.  1879. On some spheroidal lithistid Spongida form the UpperSilurian Formation of new Brunswick. Annals and Magazine of Natural His-tory (series 5) 4:84–91.Dzik, J. 1994: Evolution of ‘small shelly fossils’ assemblages of the Early Pale-ozoic. Acta Palaeontologica Polonica 39: 247–313.Finks, R., 2003. Paleozoic Demospongea: Morphology and Phylogeny, p. 63–80. In: R. Kaesler and J. K. Rigby (eds.), Treatise on Invertebrate Paleontology,Pt. E, Porifera (revised), 2: Introduction to the Porifera. The Geological So-ciety of America and the University of Kansas, Lawrence.Finks, R., and  J. K. Rigby. 2004. Paleozoic demosponges, p. 9–74. In: R.Kaesler and J. K. Rigby (eds.), Treatise on Invertebrate Paleontology, Pt. E.Porifera (revised), 3: Porifera (Demospongea, Hexactinellida, Heteractinida,Calcarea). Geological Society of America and the University of Kansas,Frýda, J. and Rohr, D. M. 2003. Gastropoda. In: The Great Ordovician Bio-diversification Event. Columbia University Press. New York: 184-195.Heredia, S. and Mestre, A., 2011. Middle Darriwilian conodont biostratigra-phy in the Argentine Precordillera. In: Gutiérrez Marco, J.C., Rábano, I.,and García Bellido, D. (Eds.) “Ordovician of the World”. Cuadernos delMuseo Geominero14: 229-234. Heredia, S. and Mestre, A., 2013. El conodonte Darriwiliano Lenodus variabilis(Sergeeva) en la Precordillera Central de San Juan, Argentina. Serie Cor-relación Geológica, 29 (1): 89-100. Mestre, A. 2010. [Estratigrafía y bioestratigrafía de conodontes de la “TransiciónCuspidal” de la Formación San Juan al sur del paralelo 30°, Precordillera deSan Juan. Facultad de Ciencias Exactas, Físicas y Naturales de la UniversidadNacional de San Juan. Tesis Doctoral: 330 pp. Inédito]Mestre, A., 2012. Bioestratigrafía de conodontes del techo de la FormaciónSan Juan y el miembro inferior de la Formación Los Azules, Cerro LaChilca, Precordillera Central. Ameghiniana 49 (2): 185-197.CONODONTS FROM THE ANDES - Publicación Especial Nº 13 - Julio 2013 - PALEONTOLOGICAL NOTE82MESTRE ET AL.: MICROFOSSILS OF THE YANGTZEPLACOGNATHUS CRASSUS ZONE83Mestre, A. and Heredia, S., 2013. Biostratigraphic significance of Darriwilianconodonts from Sierra de La Trampa (Central Precordillera, San Juan, Ar-gentina). Geosciences Journal 17(1): 3 – 53.Heredia, S. and Mestre, A., 2013. Advances in the Middle Darriwilian con-odont biostratigraphy of the Argentine Precordillera. This volume. Mestre, A, Beresi, M., and Heredia, S., 2013. Nautiloid cephalopod concen-tration beds in the San Juan Formation (Middle Darriwilian: Yangtzeplacog-nathus crassus Zone) of the Argentine Precordillera. This volume. Mestre, A., 2013. Middle Darriwilian conodont biostratigraphy of the Vil-licúm range, Eastern Precordillera. This volume.Nestell, G., Mestre, A., y Heredia, S., 2009.  First Ordovician Foraminiferafrom South America, San Juan Formation, Argentina. Micropaleontology 55(4): 329-344. Nützel, A. and  Frýda, J. 2003. Paleozoic plankton revolution: evidence fromearly gastropod ontogeny. Geology 31, 829–831.Rauff, H. 1893. Palaeospongiologie, Erster oder allgemeiner Theil, undZweiter Theil, erste Hälfte. Palaeontographica 40, 232 p.Seuss, B., Nützel, A., Scholz, H. and Frýda, J. (2012). The Paleozoic evolutionof the gastropod larval shell: larval armor and tight coiling as a result of pre-dation-driven heterochronic character displacement. Evolution & Develop-ment 14: 212–228.

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Microfossils of the Yangtzeplacognathus crassus zone in the middle darriwilian of the argentine precordillera

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Microfossils of the Yangtzeplacognathus crassus zone in the middle darriwilian of the argentine precordillera

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