Trilobites from the Base of the Ordovician System in Northwestern Argentina

M. Franco TORTELLO 1, Susana B. ESTEBAN2 and Guillermo F. ACEÑOLAZA2

Abstract.- TRILOBITES FROM THE BASE OF THE ORDOVICIAN SYSTEM IN NORTHWESTERN ARGENTINA: The Cambrian-Ordovician boundary interval is well represented within the trilobite Parabolina (Neoparabolina) frequens argentina Biozone in northwestern Argentina. The lower part of the biozone is characterized by a diverse agnostoid-olenid assemblage assignable to the latest Cambrian. On the other hand, the upper part of the unit is diagnosed by the occurrence of Jujuyaspis keideli Kobayashi (early Tremadocian). Stratigraphic information on seven classical sections from the base of the Ordovician System in the Cordillera Oriental and Famatina Range is summarized.

The Jujuyaspis keideli Subzone is formally described on the basis of the stratotype of the río Volcancito section (upper part of the “Lower Member” of the Volcancito Formation), where J. keideli is recorded in association with early planktic graptolites. In addition, distinctive trilobites from the overlying Kainella meridionalis Biozone are here reillustrated.

Resumen: TRILOBITES DE LA BASE DEL SISTEMA ORDOVÍCICO EN LA REGIÓN NOROESTE DE ARGENTINA: El intervalo que incluye la transición entre los Sistemas Cámbrico y Ordovícico se encuentra bien representado en la región noroeste de Argentina, dentro de la Biozona de Parabolina (N.) frequens argentina. El sector inferior de la biozona se caracteriza por una diversa asociación de agnóstidos y olénidos asignable al Cámbrico tardío, mientras que la parte superior se diagnostica con el registro de Jujuyaspis keideli Kobayashi (Tremadociano temprano). En esta comunicación se resume la información estratigráfica de siete secciones clásicas donde se encuentra representada la base del Ordovícico en el Sistema de Famatina y Cordillera Oriental. Asimismo se define formalmente la Subzona de Jujuyaspis keideli en base a la asociación de J. keideli y Rhabdinopora flabelliformis en la sección de río Volcancito (sector superior del “Miembro Inferior” de la Formación Volcancito, Sistema de Famatina). Por último, se re-ilustran trilobites que caracterizan la suprayacente Biozona de Kainella meridionalis.

Key words: Trilobites. Early Tremadocian. Cordillera Oriental. Famatina. Argentina.

Palabras clave: Trilobites. Tremadociano temprano. Cordillera Oriental. Famatina. Argentina.

Introduction

After debating for 25 years, the International Working Group on the Cambrian-Ordovician Boundary (COBWG) has expressed its preference for Green Point (Cow Head Group, Green Point Formation, Western Newfoundland) as the Global Stratotype Section and Point (GSSP) for base of the Ordovician System (approved by ISOS, ICS and IUGS; Cooper et al., 2001). The Green Point section (latest Cambrian-Arenig) is mainly composed of alternating shale, limestone and grainstone representing a base-of-slope depositional environment. The Cambrian-Ordovician boundary interval is characterized by a complete record of conodonts and graptolites. The base of the Ordovician System is indicated by the first appearance of the conodont Iapetognathus fluctivagus Cooper et al., which lies 4.8 m below the FAD of planktic graptolites (Rhabdinopora praeparabola Zone) (Cooper et al., 2001). Unfortunately, shelly fossils are rare at Green Point. However, Fortey et al. (1982) described trilobites from adjacent, more upslope sections and discussed their biostratigraphic implications.

The lowermost Tremadocian correlates with the FAD of the olenid trilobite Jujuyaspis Kobayashi, a cosmopolitan genus originally described from Argentina. Many typical sections spanning the Cambrian-Ordovician boundary are characterized by conodonts, planktic graptolites, and endemic trilobites associated with Jujuyaspis [e.g. Lawson Cove (western Utah, U.S.A), Yongwol (centraleastern Korea) - Magezhuang (North-East China) and Naersnes (Oslo region, Norway)]. In North America, Miller and Stitt (1995) correlated the bases of the Symphysurina bulbosa Subzone and the Iapetognathus Zone with the base of the Ordovician, pointing out the occurrence of Jujuyaspis in these units (e.g. Norford, 1969; Winston and Nichols, 1967; Taylor and Repetski, 1995; Westrop, 1986; Stitt and Miller, 1987; Dean, 1989; Miller and Taylor, 1995; Loch et al., 1999). Besides, in North China and Korea the genus has been described in association with Yosimuraspis in the homonymous zone (early Ordovician) (e.g. Chen et al., 1980; Zhou and Zhang, 1985; Kim and Choi, 2000), whereas it occurs within the range of Cordylodus lindstromi, Iapetognathus and Rhabdinopora flabelliformis socialis in Norway, Colombia, Bolivia and Argentina (Kobayashi, 1936; Harrington and Leanza, 1957; Henningsmoen, 1957; Pribyl and Vanek, 1980; Bruton et al., 1982; Baldis et al., 1987; Aceñolaza and Aceñolaza, 1992; Tortello and Esteban, 1999; Tortello et al., 1999). Jujuyaspis is also represented in the lowermost Tremadocian of Kazakhstan (Balashova, 1961) and Western Australia (Shergold, 2000).

Classical lowest Ordovician localities of northwestern Argentina are summarized in this chapter.

Some trilobite assemblages have proved to have great value as guides for the base of the Tremadocian.

The dominance of olenids enables correlation with deep water successions. Besides, occurrences of planktic graptolites and conodonts provide further information on the stratigraphic range and correlation of some sections.

The Parabolina (N.) frequens argentina Zone and the position of the Cambrian- Ordovician boundary

The Parabolina (Neoparabolina) frequens argentina Biozone (Harrington and Leanza, 1957) was described on the basis of trilobite faunas dominated by agnostoids and olenids of cosmopolitan and atlantic aspect. Although the whole Biozone was originally assigned to the Lower Tremadocian, Late Cambrian affinities of some species were clearly indicated by Kayser (1876), Kobayashi (1936, 1937), Harrington and Leanza (1957) and Branisa (1965).

The lower part of the biozone is characterized by a distinctive latest Cambrian assemblage mainly composed of Lotagnostus (Lotagnostus), L. (Semagnostus), L. (Distagnostus), Micragnostus, Strictagnostus, Pseudorhaptagnostus (Machairagnostus), Gymnagnostus, Leiagnostus, Beltella, Onychopyge, Plicatolina, Parabolina (Neoparabolina), Parabolinella and Angelina. These agnostoid-polymeroid faunas are typically recorded in both the lower part of the Volcancito Formation at río Volcancito and the Lampazar Formation at sierra de Cajas (Aceñolaza, 1968; Tortello and Aceñolaza, 1993; Esteban, 1999; Tortello and Esteban, 1999), characterizing the Pseudorhaptagnostus (Machairagnostus) - Gymnagnostus Assemblage Subzone (Tortello, in press).

Based on a global evaluation of the Lower Paleozoic of South America, Aceñolaza (1983) pointed out that the Cambrian-Ordovician boundary can be defined at the first appearances of Jujuyaspis keideli and Rhabdinopora flabelliformis (Graptolithina), demonstrating that the upper part of the Parabolina (N.) frequens argentina Biozone is assignable to the earliest Tremadocian (see also Salfity

et al., 1984; Moya et al., 1994). This scheme coincides with the recent designation of the base of the Ordovician System by the COBWG (ISOS, ICS, IUGS), which supports that the FAD of planktic graptolites and Jujuyaspis borealis Kobayashi constitutes a reasonable proxy for the boundary in North America (Cooper et al., 2001).

Jujuyaspis keideli Kobayashi is the most common olenid of the upper part of the P. (N.) frequens argentina Biozone (Benedetto, 1977). This taxon is frequently found in association with Parabolinella and Angelina. On the other hand, the agnostoids are rather scarce in this part of the unit, being restricted to Trilobagnostus chiushuensis (Kobayashi; Tortello et al., 1999) and some forms described under open nomenclature (e.g. Gymnagnostus sp., Anglagnostus? sp., Micragnostus sp.; Tortello and Aceñolaza, 1999) (Tortello, in press).

Stratigraphy

Stratigraphic information on seven classical sections from the lowest Ordovician of the Cordillera Oriental and Famatina Range (northwestern Argentina; Figure 1) is briefly summarized below.

The corresponding correlation chart is shown in Figure 2.

Río Santa Victoria. The Santa Rosita Formation is well represented along Rio the Santa Victoria (norhern Salta Province). This 1.425-m-thick succession is mainly composed of dark grey to black shales with intercalations of greenish-grey sandstones and shaly sandstones. The lower 600 m are very fossiliferous, yielding a varied trilobite fauna assignable to the Parabolina (N.) frequens argentina Biozone. The Cambrian-Ordovician boundary is recognized by the first appearance of Jujuyaspis keideli 250 m above the base of the formation, a few meters below the lowest observed occurrence of Rhabdinopora flabelliformis. On the other hand, Kainella meridionalis occurs 880 m above the base of the section, in association with Angelina hyeronimi (Kayser), Leptoplastides marianus (Hoek) and Parabolinella argentinensis Kobayashi (see Harrington and Leanza, 1957).

Pintayoc. This 150-m-thick section is located at the quebrada de Sapagua (eastern flank of Azul Pampa Range, 25 km northwest Humahuaca City). It is mainly composed of greenish-grayish sandstones and shales assignable to the Chalhualmayoc Formation (Mesón Group) and the lower part of the Santa Rosita Formation (Santa Victoria Group). Strata of the upper part of the succession are tectonically disrupted. The Cambrian-Ordovician boundary is recognized by the occurrence of large sized, well preserved specimens of Rhabdinopora flabelliformis (Graptolithina). Associated fossils are represented by trilobites of the Parabolina (N.) frequens argentina Biozone, brachiopods and echinoderms (see Aceñolaza, 1996a; Aceñolaza, 1998).

Quebrada Amarilla. The section crops out at the quebrada Amarilla (northern part of the Cajas Range, Jujuy Province). The 72-m-thick succession is composed of quartzites, sandstones and marls with intercalations of shales of the Cardonal Formation. The basal part of the section is assignable to the latest Cambrian (middle part of the Parabolina (N.) frequens argentina Biozone; Cordylodus caboti and C. intermedius Biozones). The Cambrian-Ordovician boundary is recognized by the first appearance of Jujuyaspis keideli (upper part of the P. (N.) frequens argentina Biozone; Cordylodus lindstromi Biozone). This species occurs associated with Trilobagnostus chiushuensis (Kobayashi), Angelina hyeronimi (Kayser) and Pseudokainella sp. On the other hand, Kainella meridionalis characterizes the homonymous biozone in the uppermost levels of the section (Cordylodus angulatus Biozone; late Lower Tremadocian) (see Aceñolaza, 1968; Rao and Hunicken, 1995; Ortega and Rao, 1995; Rao, 1999; Tortello et al., 1999).

Quebrada de Huichaira. This Cambrian-Ordovician section is at Pocoia locality (quebrada de Huichaira, middle part of the quebrada of Humahuaca). The 232-m-thick succession is composed of highly fossiliferous green and black shales bearing trilobites, brachiopods and echinoderms, subordinate sandstones and quartzites. Beds are sliced by longitudinal faults of a general submeridional trend. The Cambrian-Ordovician boundary is indicated by the occurrence of Jujuyaspis keideli (Parabolina (N.) frequens argentina Biozone). The upper part of the section is assignable to the Kainella meridionalis Biozone (Aceñolaza, 1996b).

Purmamarca. Several fragmentary Cambrian-Ordovician sections display on the western flank of the Sierra Alta (Purmamarca region). This is the type area of the Lower Ordovician index fossil

Figure 2. Correlation of Cambro-Ordovician sections in NW Argentina.

Jujuyaspis keideli (Kobayashi, 1936; Aceñolaza and Aceñolaza, 1992; Miller and Taylor, 1995; Aceñolaza et al., 2001). Sections are characterized by anoxic black shales, subordinate sandstones and quartzites.

This area was strongly affected by tectonics. Shales contain Jujuyaspis keideli and associated trilobites of the upper part of the Parabolina (N.) frequens argentina Biozone (Angelina hyeronimi, Parabolinella argentinensis, Plicatolina sp., Micragnostus sp., Anglagnostus sp.). “Inarticulate” brachiopods were also described from these strata (Harrington and Leanza, 1957; Aceñolaza, 1996b; Mángano et al., 1993; Tortello and Aceñolaza, 1999).

Angosto de Lampazar. This succession is located in the Angosto de Lampazar area (eastern flank of the quebrada del Toro, Salta Province). It represents one of the most classic Lower Palaeozoic localities in northwestern Argentina. The lower 93 m are shales and fine-grained sandstones of the Lampazar Formation, which are transitionally overlain by sandstones, quartzites, marls, shales, and limestones of the Cardonal Formation (77 m). The Lampazar Fm. bears the trilobites Parabolina (N.) frequens argentina (Kayser), Leptoplastides marianus (Hoek) and Parabolinella sp. (P. (N.) frequens argentina Biozone), whereas the middle part of the Cardonal Fm. is characterized by Leptoplastides marianus (Hoek), Trilobagnostus chiushuensis (Kobayashi), Asaphellus catamarcensis Kobayashi, Kainella meridionalis Kobayashi and Pseudokainella conica (Kobayashi) (K. meridionalis Biozone). In this section, the upper part of the P. frequens Zone and the basal part of the K. meridionalis Zone correlate with the Cordylodus angulatus Zone (late Lower Tremadocian) (see Harrington and Leanza, 1957; Rao and Tortello, 1998; Tortello and Rao, 2000).

Río Volcancito This section is at the deep Río Volcancito creek (eastern flank of the Famatina Range, La Rioja Province). The 170 m thick succession is mainly composed of dark grey and green shales and marls interbedded with carbonate sandstones of the Volcancito Formation. The lower part of the unit contains distinctive trilobites (lower part of the Parabolina (N.) frequens argentina Biozone) and conodonts (Cordylodus proavus, C. intermedius and C. lindstromi Biozones) of latest Cambrian age. The Cambrian-Ordovician boundary is recognized by the first appearance of Iapetognathus aengensis at 85 m from the base of the section, a few meters below the first occurrences of Jujuyaspis keideli (upper part of the P. (N.) frequens argentina Biozone) and planktic graptolites (Rhabdinopora flabelliformis Biozone) (see Harrington and Leanza, 1957; Esteban 1999; Tortello and Esteban, 1997, 1998, 1999; Albanesi et al., 1999, 2000).

The Jujuyaspis keideli Kobayashi Subzone

As noted above, occurrences of Jujuyaspis keideli in the quebrada Amarilla / Huichaira / Purmamarca and río Volcancito sections provide valuable keys to characterize the Cambrian-Ordovician boundary in the Cordillera Oriental and the Famatina Range, respectively. In addition, this species has been described from many other localities of Argentina and Bolivia (e.g. Tarija, La Caldera, San José, quebrada de La Cruz; see Harrington and Leanza, 1957; Branisa, 1965; Suárez-Soruco, 1976; Pribyl and Vanek, 1980; Aceñolaza and Aceñolaza, 1992).

Following the biostratigraphic scheme proposed by Aceñolaza and Baldis (1987), the Jujuyaspis keideli Subzone is formally proposed here. This new biostratigraphic unit is based on the stratotype of the río Volcancito section, where the subzone ranges throughout the upper part of the “Lower Member” of the Volcancito Formation [upper part of the Parabolina (Neoparabolina) frequens argentina Biozone] (Esteban, 1999; Tortello and Esteban, 1999). In this succession, Jujuyaspis keideli is recorded in association with the graptolites Rhabdinopora flabelliformis cf. socialis (Salter) and Rhabdinopora flabelliformis cf. norvegica (Bulman) (Esteban, 1999; Tortello and Esteban, 1999).

The J. keideli Subzone is correlatable with the graptolite Rhabdinopora flabelliformis socialis Zone and the conodont Cordylodus lindstromi and C. angulatus (lower part) Zones (Moya et al., 1994; Ortega y Rao, 1995; Rao, 1999; Albanesi et al., 1999; Tortello and Esteban, 1999). Representative trilobites from the sections cited above are listed here with reference to original description:

Angelina hyeronimi (Kayser, 1876, p. 7, pl. 1, fig. 5).

Anglagnostus? sp. (Tortello and Aceñolaza, 1999, p. 586, figs. 2.A-B).

Bienvillia sp. (Aceñolaza and Aceñolaza, 1992, fig. 2.F).

Jujuyaspis keideli Kobayashi (1936, p. 90, pl. 16, figs. 5-9).

Leptoplastides marianus (Hoek, 1912, p. 226, pl. 7, figs. 1-3).

Micragnostus sp. (Tortello and Aceñolaza, 1999, p. 587, figs. 2.C-D).

Parabolinella argentinensis Kobayashi (1936, p. 88, pl. 15, figs. 1-5).

Pseudokainella sp. (Tortello et al., 1999, p.5-566, fig. 4.Q).

Trilobagnostus chiushuensis (Kobayashi, 1931, p. 173, pl. 22, figs. 1-5).

The Kainella meridionalis Kobayashi Zone

Based on the stratigraphic scheme by Harrington and Leanza (1957), the Parabolina (N.) frequens argentina Zone is overlain by the Kainella meridionalis Zone. The latter is widely represented in the Eastern Cordillera (Salta and Jujuy Provinces), including the classical sections of Santa Victoria, Cajas Range, and Angosto de Lampazar (Harrington and Leanza, 1957). The unit correlates with the Cordylodus angulatus and the Rossodus manitouensis Zones (Rao and Hunicken, 1995; Ortega and Rao, 1995; Rao and Tortello, 1998; Rao, 1999; Tortello and Rao, 2000). The most representative trilobites of this zone association include:

Angelina sp. (Tortello et al., 1999, p. 5-565, figs. 4.A-F).

Apatokephalus tibicen Pribyl and Vanek (1980, p. 23, pl. 12, figs. 3-4).

Asaphellus catamarcencis Kobayashi (1935, p. 65, pl. 11, figs. 11-15).

Brackebuschia acheila Harrington and Leanza (1957: 226, fig. 123.a-d).

Kainella meridionalis Kobayashi (1935, p. 64, pl. 11, figs. 5-9).

Leptoplastides marianus (Hoek, 1912, p. 226, pl. 7, figs. 1-3).

Pseudokainella conica (Kobayashi, 1935, p. 65, pl. 11, figs. 16-17).

Trilobagnostus hoeki (Kobayashi; see Harrington and Leanza, 1957, p. 68, figs. 11.3-6).

Aknowledgements: The participation of G.A. was possible with the finantial support of the Fundación Antorchas.

Authors aknowledge D. Ruiz Holgado for the line drawings.

References

Aceñolaza, F.G. 1968. Geología estratigráfica de la región de la Sierra de Cajas, Dpto. Humahuaca (Provincia de Jujuy). Revista de la Asociación Geológica Argentina 23 : 207-222.

Aceñolaza, F.G. 1983. The Tremadocian beds and the Cambrian-Ordovician boundary problems in Latin America. En: Papers for the Symposium on the Cambrian-Ordovician and Ordovician-Silurian Boundaries, Nanjing Institute of Geology and Palaeontology, Academia Sinica, pp. 88-93.

Aceñolaza, G.F., 1996a. Presencia de Rhabdinopora (Grpatolithina) en la Quebrada de Humahuaca, Provincia de Jujuy. Ameghiniana 33 (1): 111-128. Buenos Aires.

Aceñolaza, G.F. 1996b. [Bioestratigrafía del límite Cámbrico-Ordovícico y Ordovícico basal en la quebrada de Humahuaca, provincia de Jujuy, República Argentina. Doctoral. Thesis, Universidad Nacional de Tucumán, 245 pp].

Aceñolaza, G.F., 1998. Graptolites, conodonts and trilobites of uppermost Cambrian and Lower Ordovician age from Jujuy Province, northern Argentina. Temas Geológico-Mineros Instituto Tecnológico Geominero de España, 23: 145- 147. Madrid.

Aceñolaza, F.G. and Aceñolaza, G.F. 1992. The genus Jujuyaspis as a world reference fossil for the Cambrian- Ordovician boundary, p. 115-120. En: B.D. Webby and J.R. Laurie (eds.), Global Perspectives on Ordovician Geology, Balkema, Rotterdam.

Aceñolaza, F.G. and Baldis, B.A.J. 1987. The Ordovician System of South America. IUGS Publication 22: 1-68.

Aceñolaza, G.F., Tortello, M.F. and Rábano I., 2001. The eyes of the early Tremadoc Olenid trilobite Jujuyaspis keideli Kobayashi, 1936. Journal of Paleontology 75 (2): 346-350.

Albanesi, G.L.; Barnes, C.R. and Hünicken, M.A. 2000. Conodont paleobiogeography of the Iapetus ocean in the Cambrian-Ordovician boundary interval. XXXI International Geological Congress, Río de Janeiro, Artículo en CDroom.

Albanesi, G.L., Esteban, S.B. and Barnes, C.R. 1999. Conodontes del intervalo del límite Cámbrico-Ordovícico en la Formación Volcancito, Sistema de Famatina, Argentina. Temas Geológico-Mineros, Instituto Tecnológico Geominero de España 26: 521-526.

Balashova, E.A. 1961. Some Tremadoc trilobites of the Aktyubinsk region. Akademiya Nauk SSSR, Trudy Geologicheskogo Instituta 18: 102-145.

Baldis, B.A.J., González, S. and Pérez V. 1984. Trilobites tremadocianos de la Formación Negritos (Perforación La Hielera), Llanos de Colombia. III Congreso Latinoamericano de Paleontología (Méjico), Memorias: 28-41.

Benedetto, J.L. 1977. Algunas consideraciones acerca de la posición del límite Cambro-Ordovícico en América del Sur. Geos 23: 3-11.

Branisa, L. 1965. Los fósiles guías de Bolivia. Boletín Servicio Geológico de Bolivia (GEOBOL) 6: 1-82.

Bruton, D.L., Erdtmann, B.D. and Koch, L. 1982. The Naersnes section, Oslo Region, Norway: a candidate for the Cambrian-Ordovician boundary stratotype at the base of the Tremadoc Series. En: M.G. Basset and W.T. Dean (eds.), The Cambrian-Ordovician boundary: sections, fossils distributions, and correlations, National Museum of Wales, Geological Series 3, Cardiff, pp. 61-69.

Chen, J., Zhou, Z., Zou, X., Lin, Y., Yang, X., Wang, Q., Qi, J., Wang, Q. and Lu, X. 1980. Ordovician sediments and faunas in the Taihang Mountains, North China. Memoirs of Nanjing Institute of Geology and Palaeontology, Academia Sinica 16: 111-148.

Cooper, R.A., Nowlan, G.S. and Williams, S.H. 2001. Global Stratotype Section and Point for base of the Ordovician System. Episodes 24(1 : 19-28.

Dean, W.T. 1989. Trilobites from the Survey Peak, Outram and Skoki Formations (Upper Cambrian-Lower Ordovician) at Wilcox Pass, Jasper National Park, Alberta. Geological Survey of Canada Bulletin 389: 1-141.

Esteban, S.B. 1999. [Estratigrafía, geología sedimentaria y paleontología del Ordovícico basal del Sistema de Famatina. Unpublished Ph.D. thesis, Facultad de Ciencias Naturales e Instituto Miguel Lillo, Universidad Nacional de Tucumán, Argentina].

Fortey, R.A., Landing, E. and Skevington, D. 1982. Cambrian-Ordovician boundary sections in the Cow-Head Group, Western Newfoundland. En: M.G. Basset and W.T. Dean (eds.). The Cambrian-Ordovician boundary: sections, fossils distributions, and correlations, National Museum of Wales, Geological Series, 3, Cardiff, pp. 95-129.

Harrington, H.J. and Leanza, A.F. 1957. Ordovician trilobites of Argentina. Department of Geology, University of Kansas Special Publication 1: 1-276.

Henningsmoen, G. 1957. The trilobite family Olenidae, with description of Norwegian material and remarks on the Olenid and Tremadocian Series. Skrifter Utgitt av det Norske Videnskaps-Akademi i Oslo I Matematisk- Naturvidenskapelig Klasse 1957(1), pp. 303, 31 pl.

Kayser, E. 1876. Über primordiale und untersilurische Fossilien aus der Argentinischen Republik. Palaeontographica, Supplementum 3, Lieferung 2, Theil II.

Kim, D.H. and Choi, D.K. 2000. Jujuyaspis and associated trilobites from the Mungok Formation (Lower Ordovician), Yongwol, Korea. Journal of Paleontology 74: 1031-1042.

Kobayashi, T. 1931. Studies on the Stratigraphy and Palaeontology of the Cambro-Ordovician Formation of Hua- Lien-Chai and Niu-Hsin-Tai, south Manchuria. Japanese Journal of Geology and Geography 8 (3): 131-189, pls. 16-22.

Kobayahi, T. 1935. On the Kainella fauna of the Basal Ordovician age found in Argentina. Japanese Journal of Geology and Geography 12 (3-4): 59-67.

Kobayashi, T. 1936. On the Parabolinella Fauna from Province Jujuy, Argentina with a note on the Olenidae. Japanese Journal of Geology and Geography 13 (1-2): 85-102, pls. 15-16.

Kobayashi, T. 1937. The Cambro-Ordovician shelly faunas of South America. Journal of the Faculty of Science, Imperial University of Tokyo, Section 2, 4 (4): 369-522, 8 pl.

Loch, J.D., Stitt, J.H. and Miller, J.F. 1999. Trilobite biostratigraphy through the Cambrian-Ordovician boundary interval at Lawson Cove, Ibex, western Utah, U.S.A.. Acta Universitatis Carolinae Geologica 43: 13-16.

Miller, J.F. and Stitt, J.H. 1995. Stratigraphic position and significance of Jujuyaspis and Iapetognathus in the Wilberns Formation, Texas. Ordovician Odyssey: Short Papers for the seventh International Symposium on the Ordovician System (Las Vegas), pp. 105-108.

Miller, J.F. and Taylor, M.E. 1995. Biostratigraphic significance of Iapetognathus (Conodonta) and Jujuyaspis (Trilobita) in the House Limestone, Ibex area, Utah. Ordovician Odyssey: Short Papers for the seventh International Symposium on the Ordovician System (Las Vegas), pp. 109-112.

Moya, M.C., Malanca, S., Monteros, J.A. and Cuerda, A.J. 1994. Bioestratigrafía del Ordovícico Inferior en la Cordillera Oriental Argentina basada en graptolitos. Revista Española de Paleontología 9: 91-104.

Norford, B.S. 1969. The Early Canadian (Tremadocian) trilobites Clelandia and Jujuyaspis from the Southern Rocky Mountains of Canada. Geological Survey of Canada, Contributions to Canadian Paleontology, Bulletin 182: 1-15, 56-59.

Ortega, G. and Rao, R.I. 1995. Nuevos hallazgos de graptolitos (Ordovícico Inferior) en la sierra de Cajas, Departamento Humahuaca, Provincia de Jujuy, Argentina. Boletín de la Academia Nacional de Ciencias 60: 293-316.

Pribyl, A. and Vanek, J. 1980. Ordovician trilobites of Bolivia. Rozpravy Ceskoslovenské Akademie Ved 90(2), 90 pp.

Rao, R.I. 1999. Los conodontes Cambro-Ordovícicos de la sierra de Cajas y del Espinazo del Diablo, Cordillera Oriental, República Argentina. Revista Española de Micropaleontología 31: 23-51.

Rao, R.I. and Hünicken, M.A. 1995. Conodont biostratigraphy of the Cambrian-Ordovician boundary in northwestern Argentina. Ordovician Odyssey: Short Papers for the seventh International Symposium on the Ordovician System (Las Vegas), pp. 125-128.

Rao, R.I. and Tortello, M.F. 1998. Tremadoc conodonts and trilobites from the Cardonal Formation, Incamayo Creek, Salta Province, northwestern Argentina. Palaeontologia Polonica 58: 31-45.

Salfity, J.A., Malanca, S., Moya, M.C., Monaldi, C.R. and Brandán, E.M. 1984. El límite Cámbrico-Ordovícico en el norte de la Argentina. IX Congreso Geológico Argentino, Actas 1: 568-575.

Shergold, J.H. 2000. The Early Ordovician trilobite genus Jujuyaspis in Australia. En: G.F. Aceñolaza, and S. Peralta (Eds.), Cambrian from the southern edge, INSUGEO, Miscelánea 6: 128-130.

Steinmann, G. and Hoek, H. 1912. Das Silur und Cambriun des Hochlandes von Bolivia und ihre Fauna. Neues Jahrbuch für Mineralogie, Geologie und Paläontologie 34, 176-252.

Stitt, J.H. and Miller, J.F. 1987. Jujuyaspis borealis and associated trilobites and conodonts from the Lower Ordovician of Texas and Utah. Journal of Paleontology 61: 112-121.

Suárez Soruco, R. 1976. El Sistema Ordovícico en Bolivia. Revista Técnica de YPFB 5 (2): 1-223.

Taylor, J.F. and Repetski, J.E. 1995. High-resolution trilobite and conodont biostratigraphy across the Cambrian- Ordovician boundary in south-central New Mexico. Ordovician Odyssey: Short Papers for the seventh International Symposium on the Ordovician System (Las Vegas), pp. 133-136.

Tortello, M.F. (in press). Biostratigraphic significance of the uppermost Cambrian-lowest Ordovician agnostoid trilobites from northwestern Argentina. Acta Geológica Hispánica.

Tortello, M.F. and Aceñolaza, G.F. 1993. Trilobites agnóstidos del límite Cámbrico-Ordovícico de la Formación Lampazar, Sierra de Cajas, Provincia de Jujuy, Argentina. Revista Española de Paleontología 8 (2): 177-184.

Tortello, M.F. and Aceñolaza, G.F. 1999. Trilobites agnóstidos del Ordovícico basal en la localidad de Purmamarca, Provincia de Jujuy, Argentina. Temas Geológico-Mineros, Instituto Tecnológico Geominero de España 26: 585-588.

Tortello, M.F. and Esteban, S.B. 1997. Significado bioestratigráfico de una asociación de trilobites del tramo basal de la Formación Volcancito (Sistema de Famatina, La Rioja, Argentina). Ameghiniana 34 (3): 265-270.

Tortello, M.F. and Esteban, S.B. 1998. La transición Cámbrico-Ordovícica en la Formación Volcancito (Sistema de Famatina, La Rioja, Argentina). VII Congreso Argentino de Paleontología y Bioestratigrafía, Resúmenes: 93.

Tortello, M.F. and Esteban, S.B. 1999. La transición Cámbrico-Ordovícico en la Formación Volcancito (sierra de Famatina, La Rioja, Argentina). Ameghiniana 36: 371-387.

Tortello, M.F., Rábano, I., Rao, R.I. and Aceñolaza, F.G. 1999. Los trilobites de la transición Cámbrico-Ordovícico en la quebrada Amarilla (sierra de Cajas, Jujuy, Argentina). Boletín Geológico y Minero 110: 555-572.

Tortello, M.F. and Rao, R.I. 2000. Trilobites y conodontes del Ordovícico Temprano del Angosto de Lampazar (provincia de Salta, Argentina). Boletín Geológico y Minero 111: 61-84.

Westrop, S.R. 1986. Trilobites of the Upper Cambrian Sunwaptan Stage, southern Canadian Rocky Mountains, Alberta. Palaeontographica Canadiana 3: 1-179.

Winston, D. and Nicholls, H. 1967. Late Cambrian and Early Ordovician Faunas from the Wilberns Formation of Central Texas. Journal of Paleontology 41: 66-96.

Zhou, Z.Y. and Zhang, J.L. 1984. Uppermost Cambrian and lowest Ordovician trilobites of Northeast China. En: Nanjing Institute of Geology and Palaeontology, Stratigraphy and Palaeontology of Systemic Boundaries in China, Cambrian- Ordovician Boundary (2), Anhui Science and Technology Publishing House, Hefei, pp. 63-163.

Recibido: 12 de Octubre de 2002

Aceptado: 4 de Diciembre de 2002

Figure 4. A-C, Leptoplastides marianus (Hoek); A, PIL 13936, cerro Gólgota; B, PIL 14363, Angosto de Lampazar; C, PIL 14361, Angosto de Lampazar. D-K, Jujuyaspis keideli Kobayashi; D, MLP 28603, quebrada Amarilla; E, MLP 30292, Purmamarca; F, MLP 30269, Purmamarca; G, PIL 14092, Purmamarca; H, MLP 28603, quebrada Amarilla; I, MLP 28612, quebrada Amarilla; J, MLP28619, quebrada Amarilla; K, MLP 28622, quebrada Amarilla. L, Apatokephalus tibicen Pribyl and Vanek, MLP 28662, quebrada Amarilla. M, Plicatolina sp., PIL 14099, Purmamarca. PIL: Paleontología Invertebrados Lillo, Universidad Nacional de Tucumán. MLP: Museo de La Plata.

Figure 5: A-B, Jujuyaspis keideli Kobayashi, Purmamarca; A, MLP 30289; B, MLP 30351. C-D, Asaphellus catamarcencis Kobayashi, Angosto de Lampazar; C, MLP 28768; D, MLP 28769. E-H, Kainella meridionalis Kobayashi, Iruya; E, MLP 4965; F, MLP 4957; G, MLP 4929; H, MLP 4954. I-J, Pseudokainella conica (Kobayashi), Angosto de Lampazar; I, MLP 28849; J, MLP 28851. K, Brackebuschia acheila Harrington and Leanza, PIL 14087, cerro Gólgota. PIL: Paleontología Invertebrados Lillo, Universidad Nacional de Tucumán. MLP: Museo de La Plata.