Cretaceous–Paleogene plant extinction and recovery in Patagonia

Abstract

The Cretaceous–Paleogene (K/Pg) extinction appears to have been geographically heterogeneous for some organismal groups. Southern Hemisphere K/Pg palynological records have shown lower extinction and faster recovery than in the Northern Hemisphere, but no comparable, well-constrained Southern Hemisphere macrofloras spanning this interval had been available. Here, macrofloral turnover patterns are addressed for the first time in the Southern Hemisphere, using more than 3500 dicot leaves from the latest Cretaceous (Maastrichtian) and the earliest Paleocene (Danian) of Argentine Patagonia. A maximum ca. 90% macrofloral extinction and ca. 45% drop in rarefied species richness is estimated across the K/Pg, consistent with substantial species-level extinction and previously observed extirpation of host-specialized leaf mines. However, prior palynological and taxonomic studies indicate low turnover of higher taxa and persistence of general floral composition in the same sections. High species extinction, decreased species richness, and homogeneous Danian macrofloras across time and facies resemble patterns often observed in North America, but there are several notable differences. When compared with boundary-spanning macrofloras at similar absolute paleolatitudes (ca. 50°S or 50°N) from the Williston Basin (WB) in the Dakotas, both Maastrichtian and Danian Patagonian species richnesses are higher, extending a history of elevated South American diversity into the Maastrichtian. Despite high species turnover, our analyses also reveal continuity and expansion of leaf morphospace, including an increase in lobed and toothed species unlike the Danian WB. Thus, both Patagonian and WB K/Pg macrofloras support a significant extinction event, but they may also reflect geographically heterogeneous diversity, extinction, and recovery patterns warranting future study.

Seed traits linked to differential survival of plants during the Cretaceous/Paleogene impact winter

“…This distinct pattern implies that contemporary asteroid impact winter models, which predict an impact winter lasting longer than a year, coupled with extreme desiccation (~85% reduction in precipitation according to Chiarenza et al, 2020) and a transient episode of permafrost conditions (Chiarenza et al, 2020;Tabor et al, 2020), probably are correct. Such an interpretation is bolstered by the perspective that high-latitude regions in the Southern Hemisphere, such as Patagonia, Argentina, were once regarded as refugia for plants during the K/Pg mass-extinction event (Barreda et al, 2012), although this changed when Stiles et al (2020) discovered evidence of a megafloral mass-extinction event of approximately the same magnitude (~90% of species) and nature (destruction of broadleaved evergreen forests, resulting in leaf physiognomicbased drops in mean annual temperatures of −5°C) as that described from western North America by Johnson (1996Johnson ( , 2002, Wilf et al (2003) and Wilf and Johnson (2004). Although these discoveries are new, they were nonetheless predicted by Iglesias et al (2011) following the description of an early Paleocene megafloral assemblage (Iglesias et al, 2007).…”

“…This presumably explains many of the more enigmatic aspects of the patterns of plant succession across the K/Pg boundary, such as why Agathis characteristically replaced other araucarian conifers with a stronger resemblance to Araucaria Sections Bunya, Intermedia and Araucaria or Wollemia in both New Zealand and South America (Pole and Vajda, 2009;Escapa et al, 2018), although podocarps, which apparently have a high incidence of recalcitrant seeds (Wyse and Dickie, 2017), eventually rose to dominance (Pole and Vajda, 2009). Similarly, lauraceous species along with non-magnoliid "dicot" angiosperms suffered considerable extinction in Patagonia, although they quickly returned to dominance (Stiles et al, 2020), a pattern also observed about ten thousand kilometers to the north in the Raton Basin of the southern Western Interior of North America (Berry, 2019a). Because there was obviously little or no biogeographic continuity between these regions, these similar biogeographic patterns must be due to fundamental rules of tropical plant community assembly (i.e., ecological succession following an environmental catastrophe).…”

“…2). Because some plant groups predicted to be among the hardest hit by the K/Pg impact event, such as Laurales (Wolfe, 1997), are known to have diversified shortly after the K/Pg event in both North America and South America (Northern and Southern Hemispheres) (Berry, 2019a(Berry, , 2020Stiles et al, 2020), these statistical analyses do not include species recognized from the early Paleocene recolonization flora unless these are known to have survived the impact event.…”

“…It is difficult to assign fossil species to Lauraceae exclusively; that is why most often these species are conservatively assigned to Laurales only (Upchurch, 1995;Pole and Vajda, 2009). Lauraceous or lauraceous-like plants have been reported from multiple K/Pg boundary localities across the Northern and Southern Hemispheres (Wolfe and Upchurch, 1987;Kauffman et al, 1990;Upchurch and Askin, 1990;Upchurch, 1995;Wolfe, 1997;Johnson, 2002;Barclay et al, 2003;Johnson et al, 2003;Poole and Cantrill, 2006;Pole and Vajda, 2009;Stiles et al, 2020). Hence, the global distribution of this clade during the Late Cretaceous can be used to test the hypothesis that the K/Pg boundary impact winter had global effects.…”

“…However, Wolfe's (1987) hypothesis cannot account for certain aspects of the plant fossil record, particularly the survival of slow-growing conifers (Brodribb et al, 2012), a pattern which Wolfe (1987) argued indicated that the Southern Hemisphere was not affected by the impact event. Furthermore, Wolfe (1987) argued that the K/Pg impact winter affected only North America because of the rapid settling of dust aerosols before these had time to encircle the globe, a perspective that is now known to be incorrect (Vajda et al, 2001(Vajda et al, , 2015McLoughlin, 2004, 2007;Bercovici, 2012, 2014;Stiles, et al 2020). Otherwise, Wolfe (1987: 220) reasoned, the K/Pg impact winter would have to have lasted "several years."…”