An explanation for conflicting records of Triassic–Jurassic plant diversity

Abstract

Macrofossils (mostly leaves) and sporomorphs (pollen and spores) preserve conflicting records of plant biodiversity during the endPermian (P-Tr), Triassic-Jurassic (Tr-J), and end-Cretaceous (K-T) mass extinctions. Estimates of diversity loss based on macrofossils are typically much higher than estimates of diversity loss based on sporomorphs. Macrofossils from the Tr-J of East Greenland indicate that standing species richness declined by as much as 85% in the Late Triassic, whereas sporomorph records from the same region, and from elsewhere in Europe, reveal little evidence of such catastrophic diversity loss. To understand this major discrepancy, we have used a new high-resolution dataset of sporomorph assemblages from Astartekløft, East Greenland, to directly compare the macrofossil and sporomorph records of Tr-J plant biodiversity. Our results show that sporomorph assemblages from the Tr-J boundary interval are 10-12% less taxonomically diverse than sporomorph assemblages from the Late Triassic, and that vegetation composition changed rapidly in the boundary interval as a result of emigration and/or extirpation of taxa rather than immigration and/or origination of taxa. An analysis of the representation of different plant groups in the macrofossil and sporomorph records at Astartekløft reveals that reproductively specialized plants, including cycads, bennettites and the seed-fern Lepidopteris are almost absent from the sporomorph record. These results provide a means of reconciling the macrofossil and sporomorph records of Tr-J vegetation change, and may help to understand vegetation change during the P-Tr and K-T mass extinctions and around the PaleoceneEocene Thermal Maximum.ompilations of stratigraphic ranges of land plants through geological time do not show abrupt declines in taxonomic diversity (1-3). This contrasts sharply with the history of animal life, which is marked by five geologically rapid decreases in global taxonomic diversity, known as mass extinctions (4, 5). This fundamental difference between the evolutionary histories of plants and animals may be due to the persistence of higher plant taxa, and has led to the suggestion that plants are more resistant to mass extinction than animals (1, 6-9). Despite this, studies of fossil plants during times of faunal mass extinction have revealed extensive ecological disruption and decreased plant genus/species diversity on local and regional scales (8, 9), suggesting that plants are not immune to the myriad environmental changes accompanying mass extinctions.However, plant fossils preserve conflicting records of diversity loss during these critical intervals in Earth history. Estimates of diversity loss based on macrofossils (mostly leaves) are typically much higher than estimates of diversity loss based on sporomorphs (pollen and spores). The end-Permian mass extinction [P-Tr; ∼251 million years ago (Ma)] in Australia saw a 97% regional diversity loss of macrofossils but a 19% loss of sporomorph diversity (8, 10), and the end-Cretaceous mass extinct...

On the Taxonomic Resolution of Pollen and Spore Records of Earth’s Vegetation

“…Similarly, certain plants are absent or poorly represented as dispersed sporomorphs, which creates marked gaps in the sporomorph record. The causes of these gaps are now fairly well understood in both Quaternary and pre-Quaternary time, and it is thought that they result from the low production of sporomorphs by certain plants and the poor preservation of certain sporomorph taxa (e.g., Davis 1963;Chaloner 1968;Birks 1980, 2000;Prentice and Webb 1986;Dunwiddie 1987;Jackson and Booth 2007;Traverse 2007;Mander et al 2010). Plant macrofossils (and other fossil groups, such as phytoliths; e.g., Piperno 2006; Strö mberg 2011) can be extremely valuable as a source of additional information on vegetation composition (e.g., Birks and Birks 2000), but some plants, such as Carya and Corylus, are poorly represented as macrofossils (Jackson and Booth 2007).…”

“…However, low taxonomic resolution presents a major barrier to the investigation of the dynamics of plant extinction using sporomorphs (Jackson and Weng 1999;Mander et al 2010;Mander 2011). For example, the Peltaspermaceae (a clade of seed ferns) suffered global extinction during the Triassic-Jurassic mass extinction, which coincided with major environmental change around 200 Ma and is recognized as the third-greatest extinction event in the history of life (McElwain and Punyasena 2007).…”

“…The extinction of this plant family is marked by the disappearance of the fossil leaf morphospecies Lepidopteris ottonis at the end of the Triassic period ). However, this extinction event is masked in the sporomorph record because the pollen type produced by the Peltaspermaceae (Cycadopites) was also produced by at least three other orders of coexisting plants (Cycadales, Ginkgoales, and Bennettitales; Mander et al 2010;Mander 2011). In addition, the local extinction or emigration of four cycad leaf genera during the TriassicJurassic mass extinction in East Greenland (Doratophyllum, Ctenis, Pseudoctenis, and Nilssonia; is not recorded by fossil pollen grains in this region because the classification of the pollen grains produced by these plants is currently too coarse to document vegetation change at the genus level (Mander 2011).…”

“…Low taxonomic resolution hampers the investigation of several critical issues in vegetation science. Some of these issues are based around evolutionary questions, including the nature and timing of plant species extinction (Jackson and Weng 1999;Mander et al 2010), the diversification history of major clades, such as grasses (Mander et al 2013), and the reconstruction of large-scale biogeographic patterns, such as the latitudinal diversity gradient (Qian and Ricklefs 2007;Jardine et al 2012). Others are concerned with the reconstruction of paleoecological conditions, which is confounded when congeneric species have widely different autecological preferences, such as spruce, alder, and oak (Lindbladh et al 2002;Lantz et al 2010;May and Lacourse 2012;Punyasena et al 2012).…”

The Triassic to Early Jurassic palynological record of the Tarim Basin, China

“…Rouse 1957;Delcourt et al 1963;Couper 1958;Balme 1995;van Konijnenburg-van Cittert 1993;Batten and Dutta 1997;Song et al 2000;Mander et al 2010;Mander 2011;Hermann et al 2011;Raine et al 2011;Bonis and Kürschner 2012) to assess broad temporal vegetation changes (Supplementary Table 2). Floras comprise a mixture of bryophytes, lycophytes, sphenophytes, ferns, pteridosperms, conifers and monosulcate pollen-producers (Fig.…”