Blanca A. Toro

Reevaluación de las Biozonas de Tetragraptus Phyllograptoides y T. akzharensis (Ordovícico Inferior, Floiano) de la Cordillera Oriental Argentina

Abstract. REEVALUATION OF TETRAGRAPTUS PHYLLOGRAPTOIDES AND T. AKZHARENSIS BIOZONES (LOWER ORDO-VICIAN, FLOIAN) FROM THE CORDILLERA ORIENTAL ARGENTINA. This paper presents new accuracies and correlations of the Lower Ordovician (Floian) biostratigraphic scheme of the Cordillera Oriental. The Tetragraptus phyllograptoides and T. akzharensis biozones are reevaluated based on graptolite records from the lower levels of the Acoite Formation, outcropping in the Quebrada de Quinilicán (Sierra de Aguilar). The stratigraphic range of the characteristic taxa is discussed and compared with that observed in Los Colorados area. In the Tetragraptus phyllograptoides Biozone, it is worth mentioning the presence of Tetragraptus approximatus (Nicholson), Tetragraptus sp., Baltograptus geometricus (Törnquist), Cymatograptus demissus (Törnquist) and C. rigoletto (Maletz, Rushton and Lindholm) in addition to the index species. Tetragraptus amii (Elles y Wood), Cymatograptus protobalticus (Monsen), C. balticus (Tullberg) and Paradelograptus sp. appear in the lower part of the T. akzharensis Biozone, whereas Baltograptus vacillans (Tullberg), Corymbograptus v-fractus tullbergi (Monsen), Trichograptus dilaceratus (Herrmann), Expansograptus latus (Hall), E. holmi (Törnquist), E. constrictus (Hall), Acrograptus gracilis (Törnquist), and A. filiformis (Tullberg) are recorded in the upper portion of the same biozone. The regional correlation of both biozones is corroborated on the western margin of the Cordillera Oriental. The possible subdivision of the T. akzharensis Biozone is commented, according to the biostratigraphic scheme recently proposed for Scandinavia, and a more accurate intercontinental correlation of Northwestern Argentina Floian levels is proposed. In addition, the paleoecological implications of some studied taxa and the paleobiogeographical relationships between Northwestern Argentina and other Gondwanan regions are discussed, and the strong affinity with Baltoscandinavia during the Lower Ordovician is validated.

GRAPTOLITE-TRILOBITE BIOSTRATIGRAPHY IN THE SANTA VICTORIA AREA, NORTHWESTERN ARGENTINA. A KEY FOR REGIONAL AND WORLDWIDE CORRELATION OF THE LOWER ORDOVICIAN (TREMADOCIAN–FLOIAN)

Abstract. New graptolite-trilobite records provide relevant information for the biostratigraphic framework of the lower Paleozoic in western Gondwana. Aorograptus victoriae (T. S. Hall), together with Adelogratus cf. A. altus Williams and Stevens and Ancoragraptus bulmani (Spjeldnaes) are described for the first time in the Santa Victoria area. These species come from the upper beds of the Santa Rosita Formation, expanding the distribution of the A. victoriae Biozone. The complete succession of the Aorograptus victoriae, Araneograptus murrayi, Hunnegraptus copiosus, Tetragraptus phyllograptoides, T. akzharensis, Baltograptus cf. B. deflexus and Didymograptellus bifidus biozones is confirmed for the first time at a single locality in the Cordillera Oriental, indicating a late Tremadocian—late Floian age for the upper deposits of the Santa Rosita Formation and the Acoite Formation in the studied area. The co-occurrence of trilobites with the records of these graptolite biozones enables a better resolution of the upper boundary of the Notopeltis orthometopa Biozone and the definition of associations partially filling gaps in the succession of traditional trilobite biozones (Asaphellus cf. A. stenorhachis/Leptoplastides and Megistaspis (Ekeraspis) associations). It also contributes towards clarifying the distribution of some members of the well-known Thysanopyge Fauna (T. taurinus, T. victoriensis and T. clavijoi). Together with the proposed framework, we also discuss previous records of conodonts and key palynomorphs (particularly chitinozoans) related to graptolite occurrences. Additionally, we provide new insights on regional and intercontinental correlation with classic Early Ordovician sections in northwestern Argentina (e.g., Pascha-Incamayo, Los Colorados, Mojotoro range and Humahuaca area), southern Bolivia and Balto-Scandinavia.

Silurian and Devonian molluscan bivalves from precordillera region, western Argentina

Silurian and Devonian bivalves recovered from the Los Espejos (Silurian to Lower Devonian) and Talacasto (Devonian) Formations of the Northern Precordillera Mountain belt are described and illustrated. The fauna comprises some new nuculoids Deceptrix (Devonodeceptrix) jachalensis n. subgen. and n. sp., Nuculites argentinum n. sp., and Anthracoleda (Pseudoleda) minuta n. subgen. and n. sp., as well as the new pterioid Actinopteria modesta n. sp. Additionally, many poorly known South American bivalves are redescribed and figured. Silurian–Devonian pelecypod faunas from some South American localities show differences both in diversity and in faunal composition. Causes for these differences are briefly discussed. Controls other than latitude are suggested in order to explain biogeographical affinities of the studied fauna.

Testing phylogenetic relationships of Floian (Ordovician) Graptolite records from Argentina through Cladistic analysis

Abstract. The analysis of two-stiped dichograptids, largely of baltograptid and expansograptid taxa of the family Didymograptidae from the Floian of northwestern Argentina, provides insight into the evolutionary relationships of this group. The important proximal end characters clearly cluster the baltograptids with the derived genera Didymograptus and Aulograptus. One of the analyses provides the information that the genera Cymatograptus and Expansograptus share a common ancestor but are not closely related to the two-stiped Kiaerograptus and related taxa. Likewise, with the exception of some controversial taxa, the families Tetragraptidae and Didymograptidae can be readily differentiated, supporting previous interpretations that they represent monophyletic clades. Although the present analyses are poorly supported, they provide hypotheses that could be further tested based on new data on isolated dichograptid taxa based on isolated or relief-preserved material.

Análisis Morfométrico de Especies del Género Baltograptus (Graptolithina) en el Ordovícico Temprano de la Cordillera Oriental, Argentina

Resumen. La morfometría aplicada a graptolitos constituye una herramienta poco desarrollada en la actualidad. Este tipo de análisis se realiza por primera vez en Argentina, con el objetivo de brindar un soporte adicional para la clasificación sistemática a nivel de especie, de ejemplares problemáticos del género Baltograptus. El material proviene de la Cordillera Oriental, asignado previamente a las especies B. turgidus (Lee) y B. kunmingensis (Ni), e incluido sucesivamente en grupos informales. Estas especies fueron definidas originalmente en China, junto a muchas otras difícilmente diferenciables, que se caracterizan por presentar tubarios robustos de carácter deflexo, con rápido ensanchamiento de sus estipes. Material adicional de características similares, que fuera asignado anteriormente a Baltograptus bolivianus (Finney y Branisa), se incluye también en este análisis. A partir de la cuantificación de las variables morfométricas se llevó a cabo un análisis de tipo exploratorio (análisis de componentes principales), seguido del análisis de agrupamiento y del posterior análisis discriminante. Como resultado, el material se separa en tres agrupamientos diferentes, que corresponden claramente a las tres especies en discusión. Si bien las variables correspondientes al extremo proximal del tubario influyen en la discriminación de los grupos resultantes, aquellas correspondientes al resto de la colonia ejercen mayor preponderancia en la separación a nivel de especie. El análisis de los parámetros morfométricos seleccionados constituye una base necesaria de comparación con trabajos similares que se encuentran en desarrollo actualmente en China, donde estas y otras especies semejantes fueron originalmente descriptas. Abstract. MORPHOMETRIC ANALYSIS OF EARLY ORDOVICIAN BALTOGRAPTUS SPECIES FROM THE CORDILLERA ORIENTAL, ARGENTINA. The application of morphometric techniques to graptolites is still under development. This kind of analysis is carried out for the first time in Argentina to give additional support to the systematic classification at species level for problematic specimens of the genus Baltograptus. The material comes from the Cordillera Oriental, and was previously assigned to B. turgidus (Lee) and B. kunmingensis (Ni) and subsequently included in informal groups. These species were originally described in China, along with many other species of difficult differentiation, which are characterized by a robust and deflexed tubarium with a rapid widening of the stipes. Additional similar material previously assigned to Baltograptus bolivianus (Finney and Branisa), is included in the analysis. An exploratory analysis (principal component analysis) was carried out, based on the quantification of the morphometric variables. It was followed by a cluster analysis with a subsequent discriminant analysis. The results showed that the specimens are separated into three groups, clearly corresponding to the discussed species. The variables of the distal part of the tubarium appear to have more influence in the separation of species. However, some variables corresponding to the proximal end also influence the resulting group discrimination. The analysis of the selected morphometric variables constitutes a foundation for comparison with similar studies, currently in progress in China, where the analyzed species as well as other related species, were originally described.

ANÁLISIS MORFOMÉTRICO DE ESPECIES DEL GÉNERO BALTOGRAPTUS, ORDOVÍCICO TEMPRANO, CORDILLERA ORIENTAL, ARGENTINA

La morfometria aplicada a graptolitos, constituye una herramienta poco desarrollada en la actualidad. Este tipo de analisis se realiza por primera vez en Argentina, con el objetivo de brindar un soporte adicional para la clasificacion sistematica a nivel de especie, de ejemplares problematicos del genero Baltograptus. El material proviene de la Cordillera Oriental Argentina y fue asignado previamente a las especies B. turgidus (Lee) y B. kunmingensis (Ni), e incluido sucesivamente en grupos informales, como el de B. turgidus. Estas especies fueron definidas originalmente en China, junto a muchas otras dificilmente diferenciables, que se caracterizan por presentar tubarios robustos de caracter deflexo, con rapido ensanchamiento de sus estipes. Material adicional de caracteristicas similares, que fuera asignado anteriormente a Baltograptus bolivianus (Finney y Branisa), debido a la presencia de pliegues protecales, se incluye tambien en este analisis. A partir de la cuantificacion de las variables morfometricas (longitud de la sicula, ancho de la sicula, ancho proximal del estipe, ancho distal del estipe, ancho maximo del estipe y numero de tecas en 10 mm) se llevo a cabo un analisis de tipo exploratorio (analisis de componentes principales), seguido del analisis de conglomerados y del posterior analisis discriminante. Como resultado, el material se separa en tres conglomerados diferentes, que corresponden claramente a las tres especies en discusion. Si bien, las variables correspondientes al extremo proximal del tubario influyen en la discriminacion de los grupos resultantes, aquellas correspondientes al resto de la colonia ejercen mayor preponderancia en la separacion a nivel de especie. El analisis de los parametros morfometricos seleccionados constituye una base necesaria de comparacion con trabajos similares que se encuentran en desarrollo actualmente en China, donde estas y otras especies semejantes fueron originalmente descriptas.