Environmental mutagenesis during the end-Permian ecological crisis

Abstract

During the end-Permian ecological crisis, terrestrial ecosystems experienced preferential dieback of woody vegetation. Across the world, surviving herbaceous lycopsids played a pioneering role in repopulating deforested terrain. We document that the microspores of these lycopsids were regularly released in unseparated tetrads indicative of failure to complete the normal process of spore development. Although involvement of mutation has long been hinted at or proposed in theory, this finding provides concrete evidence for chronic environmental mutagenesis at the time of global ecological crisis. Prolonged exposure to enhanced UV radiation could account satisfactorily for a worldwide increase in land plant mutation. At the end of the Permian, a period of raised UV stress may have been the consequence of severe disruption of the stratospheric ozone balance by excessive emission of hydrothermal organohalogens in the vast area of Siberian Traps volcanism.

Fig. 1.

Selection of the latest Permian microspores of heterosporous lycopsids in tetrads from the Wordie Creek Formation, southern Jamesonland, East Greenland. Specimens can be assigned to various species of microspore form-genera Lundbladispora (a, c, e, and f), Densoisporites (b), and Uvaesporites (d). (Scale bar = 50 μm.)

Fig. 2.

Carbon-isotope (δ¹³C) profile for carbonates, spore/pollen ratio, and distribution of lycopsid microspores preserved in tetrads in the P-Tr transition sequence of southern Jamesonland, East Greenland. Assuming uniform sedimentation rates, 1 m of lower Wordie Creek Formation represents between 20 and 60 kyr (28). Although the conodont element Hindeodus parvus is the first unquestionable indication of earliest Triassic age, first occurrences of the bivalve Claraia approximate the P-Tr boundary. Small horizontal lines represent position of palynological samples. The sample gap is due to lack of accessible exposures. Relative abundance of the microspores is expressed as a percentage of the total spore/pollen assemblage content. The spore/pollen ratio represents the counted number of spores of lycopsids, ferns, and bryophytes, divided by the total number of counted identifiable spores and pollen grains [n = 80–380 per sample (standard count 200), tetrads representing four spores]. Successive ratios depict two-step vegetation development from closed gymnosperm woodland to open shrubland dominated by herbaceous lycopsids. A, Collapse phase; B, delayed-extinction phase (for details, see ref. 2).

Fig. 3.

Known occurrences of lycopsid microspore tetrads in P-Tr transition sequences. 1, East Greenland; 2, Sverdrup Basin, Arctic Canada (30); 3, Barents Sea (31); 4, Pechora Basin/Urals, Russia (32); 5, Russian Platform (33); 6, Southern Alps, Italy (34); 7, Transdanubian Mountains, Hungary (35); 8, Jungar Basin, North China (36); 9, Meishan, South China (37); 10, Raniganj Basin, India (38); 11, Auranga Basin, India (39); 12, Sri Lanka (40); 13, Mombasa Basin, Kenya (41) (paleogeography after ref. 42).