Diego Balseiro

Infaunal molting in Trilobita and escalatory responses against predation

The outstanding fossil record of trilobites contrasts with our limited knowledge of their lifestyles and strategies. Aspects such as infaunalism and behavioral defensive skills in this group have yet to be demonstrated conclusively. We report new insights based on a striking sclerite configuration exhibited by three phacopid trilobite species, part of a late Silurian–Early Devonian Paciphacops ( Paciphacops ) Maksimova, 1972, lineage. An unusual molt pattern provides compelling evidence of infaunal behavior, which accounts for a hiding, antipredatory adaptation. In addition, strengthening of the exoskeleton and acquisition of spines indicate an evolutionary trend toward morphological defensive strategies. Both trends in active and passive traits are considered escalatory in nature, thus providing unequivocal support for understanding the ecological role of trilobites as a main prey group in the context of the global diversification of predators recorded during middle Paleozoic time.

Paleoecological dynamics of Furongian (late Cambrian) trilobite-dominated communities from northwestern Argentina

Abstract The Cambrian–Ordovician boundary interval is a critical moment in the ecology of trilobite communities. To understand this transition, we studied—at three different spatial scales—changes in the structure of olenid-dominated communities included in the Parabolina fauna, which flourished in the latest Cambrian, largely storm-dominated, successions of northwestern Argentina. At the local (∼meter) scale, species-poor communities occur in shoreface deposits. Relatively flat species-abundance distributions (SADs) and high evenness characterize upper offshore to offshore transition settings of the early highstand systems tract (HST), whereas uneven SADs in species-poor communities are typical of the lower offshore and shelf environments of the transgressive systems tract (TST). This pattern is unlikely to be caused by a change in time averaging and is consistent with the intermediate disturbance hypothesis predicting unimodal diversity gradients. The pattern is thus interpreted to be related to a trend in intensity and frequency of storm disturbance along local shallowing-upward gradients. At the regional scale (∼100 km), the diversity trend across the sampled west-east transect is rather variable and does not match the depth or oxygen-related gradients. At the biogeographic scale, the patterns of abundance of two key taxa (Parabolina and Asaphellus) show contrasting abundance and occupancy patterns between the Cordillera Oriental siliciclastic settings and the more carbonate-rich settings of Famatina (Argentina) and Oaxaca (Mexico). The presence of these genera in settings spatially adjacent, but environmentally different from their preferred habitats can represent a signature of source-sink dynamics. Low sample evenness values for the Cordillera Oriental contrast with those of coeval Laurentian communities, implying that a secular increase in evenness took place earlier in Laurentia than in Gondwana.

Compositional turnover and ecological changes related to the waxing and waning of glaciers during the late Paleozoic ice age in ice-proximal regions (Pennsylvanian, western Argentina)

Abstract. The late Paleozoic ice age (LPIA) had a profound effect on the biota. Despitemuch research having been focused on paleotropical regions or global-scale analyses, regional ecological changes have seldom been studied in ice-proximal basins. Here, I study the compositional turnover and diversity structure across the main Carboniferous glacial event recorded in western Argentina and the subsequent nonglacial interval. Brachiopod and bivalve data from western Argentina suggest that the transition from glacial to nonglacial climates caused major compositional changes. Turnover, however, was not uniform across the bathymetric gradient, being higher in deep environments. Because extirpation was concentrated in brachiopods, but immigration was similar in both clades, the taxonomic structure of the region was significantly modified. Although regional hierarchical diversity structure and occupancy distributions remained stable, dissecting the analysis in brachiopods and bivalves underscores that both clades had different responses to climate change. Brachiopods, on the one hand, show stability in the diversity structure and a very slight decrease in occupancies of intermediate genera, while bivalves showan important rise in diversity, both at the environment and regional scale, and an increase in genera with intermediate occupancies. The bathymetric diversity gradient was also modified from hump shaped with maximum diversity in the deep subtidal to a linear gradient with maximum values toward the offshore. However, relative compositional differences within environments remained stable, with maximum values at intermediate depths both in glacial and nonglacial intervals. Moreover, local-scale coexistence between brachiopods and bivalves changed in the nonglacial interval, showing significant segregation, which indicates relevant modifications in community assembly dynamics. Results from western Argentina highlight the magnitude of regional-scale ecological changes during the LPIA in ice-proximal regions, suggesting that the waxing and waning of glaciers was able to cause regional taxonomic turnover and mediumscale ecological changes even during intervals of relative macroevolutionary quiescence.

The phacopid trilobite Echidnops taphomimus n. sp. from the Lower Devonian of Argentina; insights into infaunal molting, eye architecture and geographic distribution

Abstract. Some Silurian-Devonian Argentinian trilobites characterized by infaunal behavior during molting are considered. After a taxonomic reappraisal, a species previously referred to a lineage of the phacopid Paciphacops is proposed as Echidnops taphomimus new species, from the Lower Devonian (probably late Lochkovian) of the Talacasto Formation, Argentine Precordillera. The visual surface of E. taphomimus indicates that a irregular pattern of lens arrangement, typical of early phacopids such as the Ordovician Ormathops, can also be recognized in more derived Devonian relatives, providing new insights on some evolutionary aspects of visual development. Echidnops is recognized in Australia and Argentina, recording an unusual distribution pattern in trilobites from the Lower Devonian of southern South America, otherwise mostly linked to faunas of related Gondwanan, austral circum-polar Devonian basins of the Malvinokaffric Realm. In accordance with proposed increasing predation pressure in the context of the Mid Paleozoic Marine Revolution, evidence indicates that the infaunal molting behavior, as defensive strategy, arose in several trilobite groups during Silurian-Devonian times, rather than in a single endemic lineage of Paciphacops species.

Diversification of Asaphellus Callaway, 1877 (Asaphidae: Trilobita) during the Tremadocian in South West Gondwana (Cordillera Oriental, Argentina)

ABSTRACT Asaphellus Callaway, 1877 is the earliest widespread genus of the family Asaphidae Burmeister, 1843, and although its systematics bears several problematic issues, an input to overcome these difficulties has been attempted. Our main focus is a group of strongly effaced Asaphellus species from the Cordillera Oriental (NW Argentina), including new material from the Santa Rosita Formation (Furongian-Tremadocian), mainly belonging to Alfarcito and Rupasca members (Quebrada del Arenal, Perchel, Jujuy Province). Asaphellus is reconsidered herein in order to restrict some features and discuss others not taken in account before. Moreover, several problematic Argentinean species are revised. The poorly known, but extensively cited, A. catamarcensis Kobayashi, 1935 is assessed here upon the type series in order to clarify its taxonomy and previous assignments. A reappraisal of A. stenorhachis (Harrington, 1938), another enduring problematic taxon, is proposed. Besides, A. isabelae n. sp., a unique species within the genus mainly due to its extreme effacement is defined herein. Other species left in open nomenclature are also described. Regarding local random occurrences in the Furongian, the genus radiated locally from the early Tremadocian (Tr1) onwards, however, taxonomic diversification was not mirrored by the abundance of Asaphellus in local communities. From the middle Tremadocian (Tr2) this diversification is associated with an increase in abundance, and even dominance of the taxon in numerous assemblages, further expanding its environmental range into shallower settings. The increase of Asaphellus diversity (Tr1- early Tr2) is decoupled from the records of local increase in Asaphidae diversity since the late Tremadocian (Tr3) and Floian, underscoring the complex pattern of diversification at different taxonomic levels.

Factors shaping community assemblages and species co‐occurrence of different trophic levels

Abstract Species assemblages are the results of various processes, including dispersion and habitat filtering. Disentangling the effects of these different processes is challenging for statistical analysis, especially when biotic interactions should be considered. In this study, we used plants (producers) and leafhoppers (phytophagous) as model organisms, and we investigated the relative importance of abiotic versus biotic factors that shape community assemblages, and we infer on their biotic interactions by applying three‐step statistical analysis. We applied a novel statistical analysis, that is, multiblock Redundancy Analysis (mbRA, step 1) and showed that 51.8% and 54.1% of the overall variation in plant and leafhopper assemblages are, respectively, explained by the two multiblock models. The most important blocks of variables to explain the variations in plant and leafhopper assemblages were local topography and biotic factors. Variation partitioning analysis (step 2) showed that pure abiotic filtering and pure biotic processes were relatively less important than their combinations, suggesting that biotic relationships are strongly structured by abiotic conditions. Pairwise co‐occurrence analysis (step 3) on generalist leafhoppers and the most common plants identified 40 segregated species pairs (mainly between plant species) and 16 aggregated pairs (mainly between leafhopper species). Pairwise analysis on specialist leafhoppers and potential host plants clearly revealed aggregated patterns. Plant segregation suggests heterogeneous resource availability and competitive interactions, while leafhopper aggregation suggests host feeding differentiation at the local level, different feeding microhabitats on host plants, and similar environmental requirements of the species. Using the novel mbRA, we disentangle for the first time the relative importance of more than five distinct groups of variables shaping local species communities. We highlighted the important role of abiotic processes mediated by bottom‐up effects of plants on leafhopper communities. Our results revealed that in‐field structure diversification and trophic interactions are the main factors causing the co‐occurrence patterns observed.

Teresa María Sánchez (1945–2011)

Teresa nació el 25 de enero de 1945 en Pehuajó (Provincia de Buenos Aires). En 1969 se graduó como Licenciada en Zoología de la Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata y en 1984 recibió el título de Doctora en Paleontología de la Facultad de Ciencias, Universidad de Bretaña Occidental (Brest, Francia). Era Investigadora Principal del CONICET y Profesora Titular en la carrera de Ciencias Biológicas, de la Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba, a cargo de las cátedras de Paleontología e Introducción a la Biología. Inició sus investigaciones en Paleontología con el estudio de vertebrados cretácicos de Argentina. Las circunstancias políticas de la década del 70 impulsaron su exilio a Venezuela. Allí, llevo a cabo un cambio radical en su tema de investigación, comenzando a desarrollar su doctorado acerca de la paleoecología de las comunidades marinas y faunas de moluscos bivalvos del Paleozoico Superior de la Sierra de Perijá (Venezuela). Desde su regreso al país en 1983 y junto a su esposo, el Dr. Juan Luis Benedetto, gestó un equipo de trabajo dedicado a la investigación de los más diversos aspectos de la biota marina del Paleozoico Inferior. En este marco Teresa realizó una importante contribución a la formación de recursos humanos, dirigiendo numerosas tesis doctorales y becas del CONICET, CONICOR y SECyT-UNC. Sus investigaciones estuvieron centradas en el estudio taxonómico y paleoecológico de los bivalvos del Paleozoico Inferior de Argentina. Fue una verdadera pionera en la investigación de la radiación temprana del grupo en nuestro país y de las relaciones de estas formas con representantes más jóvenes. Paralelamente, fue precursora de otra línea de investigación vinculada a la reconstrucción de los ecosistemas del Paleozoico Inferior argentino y a eventos bióticos de gran escala. Sus contribuciones en ambas temáticas, publicadas en las más prestigiosas revistas científicas, han merecido un importante reconocimiento internacional, constituyendo referencias obligadas para los especialistas. En los últimos meses estuvo abocada a la redacción de tres capítulos para el nuevo volumen Bivalvia y Rostroconchia del Treatise on Invertebrate Paleontology. La docencia también fue su vocación y su pasión. Se desempeñó como docente en la Facultad de Ciencias Naturales de la Universidad Nacional de Salta, en la Facultad de Ciencias Puras y Naturales de la Universidad Mayor de San Andrés (La Paz, Bolivia), en la Escuela de Geología, Minas y Geofísica, Facultad de Ingeniería de la Universidad Central de Caracas (Venezuela), en la Facultad de Ciencias Exactas, FísicoQuímicas y Naturales de la Universidad Nacional de Río Cuarto, y en la Facultad de Ciencias Exactas, Físicas y Naturales de la Universidad Nacional de Córdoba. Es importante destacar también la publicación de una importante obra de difusión: “La historia de la vida en pocas palabras” un libro ameno y atractivo para el público general que, además, se ha tornado en libro de consulta para estudiantes de grado. Asimismo, desempeñó una labor activa en la gestión científico-tecnológica, tanto en el ámbito universitario como en organismos científicos provinciales y nacionales. Teresa ha sido la gestora del Centro de Investigaciones Paleobiológicas (CIPAL) dependiente de la Facultad de Ciencias Exactas, Físicas y Naturales de la Universidad Nacional de Córdoba que alberga numerosos investigadores y becarios. Como directora del mencionado Centro siempre propició un ámbito de trabajo, de respeto y de libertad. En todos sus roles, como investigadora, docente, paleontóloga, divulgadora, colega, compañera y amiga, Teresa ha dejado huellas. Tenía una personalidad abierta, leal y afectuosa, era espontánea. Tenía la virtud de saber escuchar y ponerse rápidamente en el lugar del otro, con una palabra adecuada y especial para cada uno siempre a mano, poniendo el afecto y la comprensión por encima de todas las diferencias. Quienes firmamos esta nota, discípulos, compañeros, colegas y amigos, tuvimos el placer de compartir con ella el trabajo cotidiano. Hoy tenemos la satisfacción y la responsabilidad de haber recibido un enorme legado: su entusiasmo genuino por el conocimiento, la dedicación y el compromiso con el trabajo científico, el valor y la fortaleza para enfrentar las adversidades de la vida, la honestidad para apreciar los logros ajenos, la defensa de las convicciones, la pasión por la Paleontología.