Redefining the Huayquerian Stage (Upper Miocene to Lower Pliocene) of the South American chronostratigraphic scale based on biostratigraphical analyses and geochronological dating

The Huayquerian Stage of the South American chronostratigraphic scheme (named for the Huayquerías del Este, Argentina) was originally based on a poorly known mammal association of six taxa from the Huayquerías Formation. We studied the geology, age and fauna of the Neogene sequence in this area, including the Huayquerías, Tunuyán and Bajada Grande formations. The sequence comprises a monotonous succession of synorogenic epiclastic sediments deposited under arid to semi-arid conditions. Zircon U–Pb dates from 10 tuffaceous levels (7.2–1.6 Ma) place deposition of the Huayquerías Formation during the late Tortonian or Messinian to early Zanclean, the Tunuyán Formation during the Zanclean–Piacenzian, and the Bajada Grande Formation during the Piacenzian–Calabrian. We present 43 and 31 new mammal taxon records for the Huayquerías and Tunuyán formations, respectively. Progressive faunal change was observed along the sequence. The first records of the Chaco tortoise Chelonoidis chilensis and the notoungulate Xotodon major, and the latest records of Interatheriidae and Typotheriopsis (notoungulates), Metacaremys calfucalel, Phtoramys hidalguense and Lagostomus pretrichodactyla (rodents), Chasicotatus ameghinoi and Macroeuphractus morenoi (xenarthrans) are reported. The faunal associations of the Huayquerías and lower Tunuyán formations are highly similar to each other, and to other coeval localities in Argentina. The Macroeuphractus morenoi Assemblage Biozone is proposed as the basis for redefining the Huayquerian Stage, due to the co-occurrence of three taxa with wide geographical distribution in southern South America: Macroeuphractus morenoi, Pseudotypotherium subinsigne and Lagostomus pretrichodactyla. The age of this biozone is constrained at c. 8–5 Ma in its type area.

Methods
Zircon geochronology

We sampled ten tuffaceous levels for dating (T), collecting c. 3 kg of rock. The tuff levels were georeferenced using Global Positioning System (GPS).

For samples other than T3, zircon aliquots were acquired through conventional density and magnetic separation procedures conducted in the School of the Earth, Ocean and Environment at the University of South Carolina (USA). U-Pb zircon geochronology was achieved by laser-ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS) at the University of South Carolina’s Center for Elemental Mass Spectrometry. Sample preparation, analysis, reduction, and filtering methods are detailed in Appendix S1. Age, isotopic ratio, and related data can be found in Romano et al. (2023, data 1).

Sample T3 was prepared and analysed by laser-ablation inductively coupled plasma mass-spectrometry by La.Te. Andes laboratory, Salta Province (Argentina). Zircon concentrates were obtained from samples using gravimetric, magnetic, and optical techniques (Fig. S5). Suitability for analysis was evaluated based on the mineralogy of each sample in the heavy mineral fraction and the morphological characteristics of the mineral phases, especially regarding the quantity and quality of zircons. The laboratory provided details of sample preparation, analysis, reduction, and filtering, as well as the age, isotopic ratio, and related data, and are included in Romano et al. (2023, data 2).

Fossil sample

More than 1100 vertebrate specimens, now housed at IANIGLA-PV, were collected in situ over a ten-year period (2013–2019, 2022) from 21 different fossiliferous sites and stratigraphic levels in the Huayquerías and Tunuyán fms. The collected specimens were georeferenced and their levels of origin were identified. Most specimens were determined by comparison with materials in Argentinean collections or bibliographic data, while others represent new taxa that will be described in future contributions (detailed in Romano et al. 2023, data 3). 

Similarity analysis

We compared the taxonomic composition of the Huayquerías and Tunuyán fms by calculating their overall similarity, as well as between each pair of sections defined in both units. The similarity analysis is based on presence-absence matrices (Romano et al. 2023, data 4) analysed using the Corrected Forbes Coefficient (CFC). Different possible settings were considered in the analysis, including the raw data taxa, taxa under open nomenclature, and ‘Lazarus taxa’. In addition, to evaluate whether similarity is biased at the species level, we also employed this information at the genus level. For more details, see Supplementary Materials.

Similarity analyses were performed using an R function provided by J. Alroy’s website (https://bio.mq.edu.au/~jalroy/Forbes.R). These analyses were computed in R (v4.2.2; R Core Team 2013)