Cephalopod diversification and palaeobiogeographic connectivity during the middle Tremadocian (Early Ordovician): evidence from the southern Central Andean Basin (Jujuy, Argentina)

 

Authors: Cichowolski M, Evans DH, Vaccari NE, Voldman GG, Waisfeld BG

Article in press: Received 13 February 2026; Accepted in revised form 13 July 2026; Online 30 August 2026

Keywords: Cephalopoda, middle Tremadocian, Central Andean Basin, Northwestern Argentina, Coquena Formation, Santa Rosita Formation, diversification, peri-Gondwana,

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Abstract

Middle Tremadocian (Lower Ordovician) cephalopods are described from the Central Andean Basin of Jujuy and Salta Provinces, NW Argentina. The cephalopods consist of approximately 60 specimens from the Coquena Formation (Purmamarca Region), the Rupasca Member of the Santa Rosita Formation (Huacalera Region and Quebrada Yacoraite), and the Santa Rosita Formation of the Santa Victoria Region. They range in age from early to late middle Tremadocian (Tr2). The following taxa are recorded: Ellesmeroceras humahuacaensis Cichowolski et al., Saloceras cf. sericeum (Salter), Bactroceras sp., Rioceras escandei (Thoral), Slemmestadoceras? aff. attavus (Brogger), plus three indeterminate forms. Previously described Tremadocian cephalopods from the Central Andean Basin are reviewed and shown to be Floian in age. The assemblage is significant in demonstrating the presence and persistence of Ellesmeroceras at a time when the Ellesmeroceratidae had undergone sharp declines in low latitude regions and may reflect their survival at the margins of their ranges. Although the number of taxa is limited, the remainder of the assemblage is represented by forms that characterise part of the early cephalopod radiation beyond the palaeotropical realm during the middle Tremadocian. While the number of taxa involved is small, the links of these taxa to other palaeocontinents, as well as other parts of peri-Gondwana, may indicate that the Central Andean Basin acted both as a refugium, a dispersal and/or origination centre for cephalopods during the middle Tremadocian.