Early Ordovician Trilobites from the Nazareno Area, Northwestern Argentina

Abstract

The upper part of the Santa Rosita Formation (Ordovician, Tremadocian) in the Nazareno area, Cordillera Oriental, northwestern Argentina, records the vertical passage of high-energy, shallow water platform environments to offshore settings. Eighteen trilobite species are described from this locality for the first time. Although the taxa from the lower part of the succession (Leptoplastidessp.,Asaphellussp.) are scarce and non-age diagnostic, those from the upper part include diverse assemblages partially assigned to the late TremadocianNotopeltis orthometopaZone. Systematic descriptions of several species (Geragnostus nesossiiHarrington and Leanza,G. callaveiformisHarrington and Leanza,Asaphellus jujuanusHarrington,Notopeltis orthometopa[Harrington],Mekynophrys nannaHarrington,Ceratopyge forficuloidesHarrington and Leanza,Apatokephalus tibicenPřibyl and Vanĕk) are improved, the genusNileusDalman (includingN. australisn. sp.) is first reported from the Tremadocian of western Gondwana, and new species ofAsaphellusCallaway (A. nazarenensisn. sp.),ConophrysCallaway, andApatokephalusBrøgger are described. The trilobites have their closest affinities with faunas from Norway and Sweden.Notopeltis orthometopaandMekynophrys nannaare restricted to the uppermost part of the succession, well above the first records of most other trilobites recognized.

BroeggeriaWalcott and other upper Cambrian and Tremadocian linguloid brachiopods from NW Argentina

“…stenorhachis / Leptoplastides Association (Toro et al, ). This interval can confidently be correlated with the Broeggeria‐ bearing strata exposed at Nazareno, which according to Tortello and Esteban () lie immediately below the FAD of N. orthometopa (the ‘unnamed zone’ sensu Zeballo and Albanesi, ).…”

“…The upper part of the Santa Rosita Formation is made up mainly by black shales and grey mudstones intercalated with thin fine‐grained sandstone beds bearing symmetric ripples on the top (Tortello and Esteban, ). The mudstone intervals yielded diverse trilobite and rhynchonelliform brachiopod faunas (Benedetto, ; Tortello and Esteban, ) and contain the new species B. omaguaca (this paper). On the basis of the presence of trilobites of the N. orthometopa Zone and conodonts of the Paltodus deltifer Zone towards the upper third of the succession, the fauna is referable to the upper Tremadocian (Manca et al, ; Giuliano et al, ).…”

Diversity patterns in upper Cambrian to Lower Ordovician trilobite communities of north‐western Argentina

“…, ), and compiled from the literature (Harrington & Leanza ; Tortello et al . ; Tortello & Rao ; Zeballo & Tortello ; Esteban & Tortello ; Tortello & Esteban , ) (Serra et al . ).…”

“…We need to remark that the loTr2 and upTr2 time intervals have shorter durations compared to the other studied intervals. Although this issue could be solved simply by analysing the Tr2 as a whole, we prefer to study them separately because a major compositional turnover has been recognized between the two intervals (Tortello & Esteban ; Toro et al . ; Meroi Arcerito et al .…”

“…). Trilobites are the most abundant elements in these upper Cambrian and Lower Ordovician successions, and their taxonomy (Harrington & Leanza ; Tortello & Esteban , , ; Waisfeld & Vaccari , ; Tortello et al . ), biogeography (Vaccari & Waisfeld ; Benedetto et al .…”