Science and Technology Production
Article
Authorship
RUSTAN, JUAN JOSE
;
Benedetto, Juan Luis
;
Randolfe, Enrique A.
;
Lopez, Fernando E.
;
Braeckman, Alejandro
;
Contreras, Victor
Date
2025
Publishing House and Editing Place
PALEONTOLOGICAL SOC INC
Magazine
JOURNAL OF PALEONTOLOGY
PALEONTOLOGICAL SOC INC
Summary
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We report a new stratigraphic section in the Argentine Precordillera (Zanja Honda, west to 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 re...
We report a new stratigraphic section in the Argentine Precordillera (Zanja Honda, west to 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, indicating the Lochkovian-Pragian. Brachiopods and trilobites of the basal coquinites are typical of the upper Silurian of the LEF elsewhere. However, Slovinograptus, the youngest graptolite from SW Gondwana, indicates the Silurian-Devonian transition in the basal coquine. The dalmanitid Pachimocaspis pachimocensis new genus and 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 brachiopod association and a monospecific P. pachimocensis one. Thus, we recognize a neat faunal turnover around the Silurian-Devonian boundary as in other southern Southamerican localities. We refer to P. pachimocensis pygidia from the Silurian-Devonian of Bolivia and the lowest Pragian of the Talacasto Formation from Las Aguaditas locality in Precordillera. P. pachimocensis 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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Key Words
BrachiopodsSilurian-DevonianSouth AmericaTrilobites