Graptolite and conodont faunas of Upper Ordovician (Sandbian) successions of the Argentine Precordillera: Biostratigraphic implications

Abstract

Graptolites and conodonts are frequent fossils in Sandbian (Late Ordovician) outcrops of the Argentine Precordillera, where they present continuous and abundant records that enable detailed biostratigraphic studies. This work deals with the integration of previously available information on early Sandbian graptolites and conodonts, together with new paleontological data from the Las Aguaditas Creek (middle and upper members of the Las Aguaditas Formation), Las Chacritas River (Las Aguaditas Formation), and Cerro La Chilca (Los Azules Formation) sections from the Central Precordillera of the San Juan Province. The presence of the key graptolite Nemagraptus gracilis and the associated graptolite taxa allow for the documentation of the N. gracilis Zone. Specimens of N. gracilis and key conodont species, such as Pygodus anserinus coexist, along with a rich conodont fauna including Baltoniodus variabilis, whose range straddles the boundary between the Pygodus anserinus and the Amorphognathus tvaerensis zones. Detailed biostratigraphic ranges of the rich graptolite and conodont associations documented in the three study areas are provided. These records and an accurate taxonomic review allow a thorough description of the recognized zones. The use of conodont and graptolite zones from different areas of the Precordillera enables the verification of a diachronous contact, representing a hiatus that spans the middle to upper Darriwilian. Moreover, a brief taxonomic description of the identified graptolite species is provided.

Emended Sandbian (Ordovician) conodont biostratigraphy in Baltoscandia and a new species ofAmorphognathus

“…The fact that the Fågelsång Phosphate in the Sandbian stratotype section is considered to mark a sedimentary cap in the succession (Goldman et al 2020) would make the use of A. tvarensis CZ as the marker of lower boundary of the Sandbian Stage rather arbitrary. In several sections of Argentina the appearance of B. variabilis coincides with the appearance of Nemagrapthus gracilis , but again right above a hiatus in the succession (Serra et al 2015; Feltes et al 2018).…”

Biostratigraphy and palaeoecology of Middle and Late Ordovician conodonts from the Hulo and Yenwashan formations, western Zhejiang, south‐eastern China

“…Furthermore, the P. anserinus Biozone has been documented from the North China Craton (Jing et al, ; Z. H. Wang et al, ), the Precordillera of Argentina (Albanesi et al, ; Albanesi & Ortega, ； Feltes et al, ; Feltes, Serra, Ortega, & Albanesi, ; Heredia & Rosales, ; Lehnert, Bergström, Keller, & Bordonaro, ; Serra et al, ; Figure ), Newfoundland (Bergström, Riva, & Kay, ), North America (Bergström, ; Harris, Bergström, Ethington, & Ross, ), Kazakhstan (Tolmacheva, Degtyarev, Ryazantsev, & Nikitina, ), and Southern Asia (Agematsu, Sashida, Salyapongse, & Sardsud, ).…”