New radiometric 40Ar-39Ar dates and faunistic analyses refine evolutionary dynamics of Neogene vertebrate assemblages in southern South America

Abstract

The vertebrate fossil record of the Pampean Region of Argentina occupies an important place in South American vertebrate paleontology. An abundance of localities has long been the main basis for constructing the chronostratigraphical/geochronological scale for the late Neogene-Quaternary of South America, as well as for understanding major patterns of vertebrate evolution, including the Great American Biotic Interchange. However, few independently-derived dates are available for constraining this record. In this contribution, we present new ⁴⁰Ar/³⁹Ar dates on escorias (likely the product of meteoric impacts) from the Argentinean Atlantic coast and statistically-based biochronological analyses that help to calibrate Late Miocene-Pliocene Pampean faunal successions. For the type areas of the Montehermosan and Chapadmalalan Ages/Stages, our results delimit their age ranges to 4.7-3.7 Ma and ca. 3.74-3.04 Ma, respectively. Additionally, from Buenos Aires Province, dates of 5.17 Ma and 4.33 Ma were recovered for "Huayquerian" and Montehermosan faunas. This information helps to better calibrate important first appearances of allochthonous taxa in South America, including one of the oldest records for procyonids (7.24-5.95 Ma), cricetids (6.95-5.46 Ma), and tayassuids (> 3.74 Ma, oldest high-confidence record). These results also constrain to ca. 3 Ma the last appearances of the autochthonous sparassodonts, as well as terror birds of large/middle body size in South America. South American faunal turnover during the late Neogene, including Late Pliocene extinctions, is interpreted as a consequence of knock-on effects from global climatic changes and initiation of the icehouse climate regime.

Figure 1

Map showing referenced localities and faunas. 1: Valle del Tonco (PPint); 2: Villavil (AndUp, AndLo, Jar); 3: Puerta de Corral Quemado (AndUp, AndLo, Jar); 4: San Fernando (AndUp, AndLo); 5: El Degolladito (Sal); 6: Quebrada de La Troya (TN); 7: Loma de las Tapias (LTA); 8: Caleufú (CACal); 9: Estancia Ré (CAHLo); 10: Puesto Colorado (CAHLo); 11: Loventué (CAHLo); 12: Telén (CAHLo); 13: El Guanaco (CAHUp); 14: Algarrobo del Águila (CAHLo); 15: Barrancas Coloradas (CAHUp); 16: Bajo Giuliani (CAHUp); 17: Don Mariano (CAHUp); 18: Quehué (CAHLo); 19: Salinas Grandes de Hidalgo (CAHLo); 20: Laguna Chillhué (CAHLo); 21: Cerro La Bota (CAChas); 22: Guatraché (CAHLo); 23: Chapadmalal (Chap); 24: Punta San Andrés (SAnd); 25: Punta Vorohué (Voro); 26: Arroyo Chasicó (Chas); 27: Cascada Grande (QSUp); 28: Paso del Halcón (QSLo); 29: Farola de Monte Hermoso (MH). Localities at La Pampa and Buenos Aires comprise the Pampean Region. The map was generated with QGIS 3.18 (freely available online: https://www.qgis.org/en/site/).

Figure 2

Temporal distribution of investigated faunas and their diversity patterns through time, in correlation with different relevant environmental and faunistic events. Richness, extPC per capita extinction, oriPC per capita origination. Yellow background: coolhouse climate regime; blue background: icehouse climate regime. Black silhouette: first appearance datum (FAD) of North American immigrants in southern South America; white silhouette: last appearance datum (LAD) of autochthonous lineages in South America. New dates and chronological age inferences are in pink. Temperature curve from ref.. Atom icon was taken and modified from commons.wikimedia.org, designed by Indolences; magnet, snowflake, iceberg and mountains icons were taken and modified from pixabay.com, designed by OpenClipart-Vectors (first two), madartzgraphics, and Radoan_tanvir respectively; meteorite icon was taken and modified from thenounproject.com, designed by icon 54; and representative silhouettes of camelids, equids, procyonids, cricetids, tayassuids, sparassodonts, terror birds, mustelids and canids were taken and modified from philopic.org; and were designed by Steven Traver (first two), RS, uncredited designer, An Ignorant Atheist, Zimices, and Ferran Sayol (last three) respectively. All icons and silhouettes are licensed under Creative Commons Attribution 4.0 Unported license (http://creativecommons.org/licenses/by/4.0/).

Figure 3

Faunal ordination, similarities and regionalism of the Neogene of Argentina. (a) Biplot of the first two axes of Correspondence Analysis (CA) using the complete sample; (b) biplot of the first two axes of Correspondence Analysis (CA) using the Pampean Region sample; (c) Parsimony Analysis of Endemicity (PAE), reduced strict consensus of four most parsimonious trees of 638 steps, excluding Sal and TN, under “standard” parsimony. Numbers below branches correspond to branch support (frequencies/difference of frequencies, respectively). Yellow triangles (a, b) and lines (c): faunas from Buenos Aires Province; orange circles (a, b) and lines (c): faunas from La Pampa Province (both within Pampean Region); blue squares (a) and lines (c): faunas from Cuyo and Northwestern regions of Argentina. Abbreviations in Table 1. Tree in (c) was obtained with TNT 1.5 (freely available online: http://www.lillo.org.ar/phylogeny/tnt/) and edited with LibreOffice Draw 7.0.3.1 (freely available online: http://www.libreoffice.org/).

Figure 4

Change in structural parameters of investigated faunas through time (approach 2). (a) Proportion of taxonomic groups across the Neogene faunas; (b) changes in proportion of body size classes (I, less than 100 g; II, between 100 g and 1 kg; III, 1–10 kg; IV, 10–100 kg; V, 100–500 kg; VI, 500–1000 kg; and VII, more than 1000 kg); (c) proportion of dietary classes across the Neogene faunas; (d) Log-Likelihood difference between successive faunas (significant cutoff [2] is indicated with a dashed line). Abbreviations in Table 1.