Macroecological responses of terrestrial vegetation to climatic and atmospheric change across the Triassic/Jurassic boundary in East Greenland

Abstract

The magnitude and pace of terrestrial plant extinction and macroecological change as sociated with the Triassic/Jurassic (Tr/J) mass extinction boundary have not been quantified using paleoecological data. However, tracking the diversity and ecology of primary producers provides an ideal surrogate with which to explore patterns of ecosystem stability, collapse, and recovery and to explicitly test for gradual versus catastrophic causal mechanisms of extinction. We present an analysis of the vegetation dynamics in the Jameson Land Basin, East Greenland, spanning the Tr/J extinction event, from a census collected paleoecological data set of 4303 fossil leaf specimens, in an attempt to better constrain our understanding of the causes and consequences of the fourth greatest extinction event in earth history. Our analyses reveal (1) regional turnover of ecological dominants between Triassic and Jurassic plant communities, (2) marked structural changes in the vegetation as reflected by potential loss of a mid-canopy habit, and (3) decline in generic-level richness and evenness and change in ecological composition prior to the Tr/J bound ary; all of these findings argue against a single catastrophic causal mechanism, such as a meteorite impact for Tr/J extinctions. We identify various key ecological and biological traits that increased extinction risk at the Tr/J boundary and corroborate predictions of meta-population theory or plant ecophysiological models. These include ecological rarity, complex reproductive biology, and large leaf size. Recovery in terms of generic-level richness was quite rapid following Tr/J extinctions; however, species-level turnover in earliest Jurassic plant communities remained an order of magnitude high er than observed for the Triassic. We hypothesize, on the basis of evidence for geographically ex tensive macrofossil and palynological turnover across the entire Jameson Land Basin, that the na ture and magnitude of paleoecological changes recorded in this study reflect wider vegetation change across the whole region. How exactly these changes in dominance patterns of plant primary production affected the entire ecosystem remains an important avenue of future research.

On the reconstruction of plant photosynthetic and stress physiology across the Triassic–Jurassic boundary

“…As the TJB is considered to represent an interval where plant thermal stress caused by excess leaf energy drove biodiversity loss (McElwain et al, 1999(McElwain et al, , 2007, this approach may assist in understanding the underlying physiological mechanisms that led to the plant responses evident in the fossil record. Ginkgoales adaxial epidermal cell densities show a general trend of increased plant stress and reduced photosynthetic performance prior to the TJB, concomitant with the start of severe biodiversity loss within fossil plant assemblages from the section (Figure 1).…”

“…Ginkgoales in the Astartekløft flora formed the upper canopy (McElwain et al, 2007). As plants from higher in the canopy are preferentially preserved as fossils (Spicer, 1980;Kürschner, 1997), the proportion of "sun" and "shade" leaves preserved as plant fossils should be consistent throughout the section, thus giving a signal of plant performance and stress physiology related to the environment, though not an indication of the relative numbers of leaf morphotypes in the standing vegetation, nor a change in canopy structure.…”

“…The stomatal reconstruction of paleo- [CO 2 ] is based on stomata of Ginkgoales and Stenopteris specimens collected by Thomas Harris during the 1920s from TJB sections in East Greenland compiled into a composite stratigraphy (Harris, 1937), while those specimens analyzed in this study were collected in 2002 from a single site in East Greenland (McElwain et al, 2007). In addition, the range of adaxial epidermal cell density values in extant G. biloba (Figure 3) was observed in leaves grown under current ambient atmospheric [CO 2 ] levels of approximately 390 ppm.…”

“…As part of this global warming trend, the Astartekløft region would have likely experienced a much greater increase in temperature (Huynh and Poulsen, 2005;McElwain et al, 2007), enhancing leaf thermal stress caused by reduced convective heat loss (McElwain et al, 1999). In addition to elevated atmospheric [CO 2 ] (McElwain et al, 1999;Berner and Beerling, 2007) and proposed emissions of methane (Palfy et al, 2001;Beerling and Berner, 2002), tropospheric SO 2 is also a powerful greenhouse gas (Grattan and Sadler, 1999).…”

A triász és a jura időszak határán lezajlott globális krízis és annak nyomai magyarországi rétegsorokban

“…MCELWAIN et al (1999) levél maradványok ezreinek vizsgálata alapján érveltek az ökoszisz téma összeomlása mellett. A relatív gyakoriság-eloszlások megvál tozása alapján valószínűsíthető a diverzi tás csökke nése a triász-jura határon, amely egybeesik a sztómasűrűség alapján számolt légköri CO 2 -koncentráció hirtelen növeke désé -vel (MCELWAIN et al 2007(MCELWAIN et al , 2009STEINTHORTSDOTTIR et al 2011), ami megerősíti a globális fel melegedés szerepét a kihalási eseménysorozatban. Ezzel szemben az európai kontinentális területekről származó palinológiai és paleobotanikai adatok nem utalnak az öko szisztéma jelentős érintettségére (BARBACKA et al 2017).…”