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The dalmanitid trilobite Pachimocaspis n. gen. and new brachiopod-based insights into the Silurian-Devonian transition in southern South America

Published online by Cambridge University Press:  23 September 2025

Juan Jose Rustán Open the ORCID record for Juan Jose Rustán [Opens in a new window] ,
Juan Luis Benedetto ,
Enrique Alberto Randolfe Open the ORCID record for Enrique Alberto Randolfe [Opens in a new window] ,
Fernando Enrique Lopez ,
Alejandro Braeckman  and
Víctor Contreras
Juan Jose Rustán
Affiliation:
Centro de Investigaciones en Ciencias de la Tierra (CICTERRA), CONICET-Universidad Nacional de Córdoba, Av. Vélez Sarsfield 1611, X5016GCA, Córdoba, Córdoba, Argentina Cátedra de Paleontología, Carrera de Geología, Departamento de Ciencias y Tecnologías Aplicadas a la Producción, al Ambiente y al Urbanismo (DACyTAPAU), Universidad Nacional de La Rioja, Av. Luis M. de la Fuente s/n, 5300, La Rioja, Argentina
Juan Luis Benedetto
Affiliation:
Centro de Investigaciones en Ciencias de la Tierra (CICTERRA), CONICET-Universidad Nacional de Córdoba, Av. Vélez Sarsfield 1611, X5016GCA, Córdoba, Córdoba, Argentina
Enrique Alberto Randolfe*
Affiliation:
Centro de Investigaciones en Ciencias de la Tierra (CICTERRA), CONICET-Universidad Nacional de Córdoba, Av. Vélez Sarsfield 1611, X5016GCA, Córdoba, Córdoba, Argentina
Fernando Enrique Lopez
Affiliation:
Consejo de Investigaciones Científicas y Técnicas (CONICET) and Facultad de Ciencias Exactas, Físicas y Naturales (FCEFyN), Universidad Nacional de San Juan, Av. Ignacio de la Roza 590 (O), 5400, San Juan, Argentina
Alejandro Braeckman
Affiliation:
Facultad de Ciencias Exactas, Físicas y Naturales (FCEFyN) and Universidad Nacional de San Juan, Av. Ignacio de la Roza 590 (O), 5400, Rivadavia, San Juan, Argentina
Víctor Contreras
Affiliation:
Instituto de Geología Dr. Emiliano P. Aparicio, Facultad de Ciencias Exactas, Físicas y Naturales (FCEFyN), Universidad Nacional de San Juan, Av. Ignacio de la Roza 590 (O), 5400, Rivadavia, San Juan, Argentina
*
Corresponding author: Enrique Alberto Randolfe; Email: enrique.randolfe@gmail.com
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Abstract

We report a new stratigraphic section in the Argentine Precordillera (Zanja Honda, west of Pachimoco, San Juan Province), recording the Silurian-Devonian transition. It corresponds to particular siliciclastics of the uppermost 75 m of the Los Espejos Formation (LEF) exhibiting coquines at the base, noduliferous dark siltstones above, then greenish-brown sandstones, and subsequently, a reddish, massive, fine-grained sandstone interval. The overlying shaly lower interval of the Talacasto Formation records the homalonotid trilobite Burmeisteria notica (Clarke, 1913a), indicating the Lochkovian-Pragian interval. Brachiopods and trilobites of the basal coquinites are typical of the upper Silurian of the LEF elsewhere. However, Slovinograptus Urbanek, 1997, the youngest graptolite from southwestern Gondwana, indicates the Silurian-Devonian transition in the basal coquine. The dalmanitid Pachimocaspis pachimocensis new genus new species comes from this and other undoubted Silurian underlying coquinites. The brachiopod and trilobite associations disappear in the overlying dark nodular siltstone interval, replaced by an earliest Lochkovian Orthostrophia meridionalis Benedetto in Benedetto et al., 1992 brachiopod association and a monospecific Pachimocaspis pachimocensis n. gen. n. sp. Thus, we recognize a neat faunal turnover around the Silurian-Devonian boundary as in other southern South American localities. We refer to Pachimocaspis pachimocensis n. gen. n. sp. pygidia from the Silurian-Devonian of Bolivia and the lowest Pragian of the Talacasto Formation from Las Aguaditas locality in the Precordillera Basin. Pachimocaspis pachimocensis n. gen. n. sp. lacks the typical pygidial dalmanitid morphology, exhibiting instead a subelliptical shape with no caudal spine. Also, thoracic pleural tips are variably blunt along the thorax in contrast with the evenly spinose dalmanitid morphology. The morphology of this taxon challenges its systematic position in regarding Silurian-Devonian subfamilies from high paleolatitudes, resembling instead extra-Gondwanic, early Silurian synphoriines.

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Journal of Paleontology , Volume 99 , Issue 5 , September 2025 , pp. 1090 - 1106
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© The Author(s), 2025. Published by Cambridge University Press on behalf of Paleontological Society

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Handling Editor: Thomas Hegna

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