A Late Paleozoic climate window of opportunity.

The highest rates of global organic carbon burial (up to 6.5 × 10 mol/Myr) over the past half billion years occurred during the Carboniferous–Permian (330–260 Myr), in large part because of the accumulation and burial of peat in broad tropical lowland basins (1). Atypical rates of organic carbon sequestration led to low atmospheric pCO2 and anomalously high pO2 (Fig. 1), which in turn triggered the longest-lived and perhaps most severe icehouse of the Phanerozoic (1, 2) and, possibly, unusual physiological innovations, such as insect flight and gigantism (3). This unique atmospheric composition arose with the radiation of the Earth’s most expansive wetland tropical forests (4), introducing into the terrestrial realm a vast supply of biodegradably resistant organic matter. A hypothesized temporal gap of ∼120 Myr between the emergence of lignin biosynthesis by plants and the evolution of lignindegrading fungi has been argued to have created a window of opportunity for substantially increased organic carbon sequestration in the Carboniferous (4–6). This long-standing evolutionary lag paradigm becomes even more compelling with a recent fungal genome study suggesting that Agaricomycetes fungi, with the enzymatic capacity to efficiently degrade lignin, did not evolve until the early Permian (7). In PNAS, Nelsen et al. (8) invoke multiple lines of evidence to refute this paradigm, documenting: (i) the low lignin content of some of the most important Carboniferous peat-forming plants, (ii) the lack of correspondence between peak coal accumulation rates in North America and anticipated periods of abundant lignin production in late Paleozoic tropical forests, (iii) that lignin resistance to decay was not limited to Agaricomycetes fungi, and (iv) phylogenomic data that close the hypothesized evolutionary gap. There is a long-invoked but incorrect (9) perception that lycopsid-dominated forests were the main source vegetation of Carboniferous coals because of the abundance and resistance to decomposition of lycopsid periderm (bark) (e.g., refs. 1, 4, and 5). Thus, lycopsid dominance supposedly was essential for formation of the economically important Carboniferous coals. In what initially appears to support this perception, Nelsen et al. (8) use spatiotemporal trends in Phanerozoic organic-rich terrestrial sediments to demonstrate peak North American coal production during intervals when lycopsids dominated Euramerican tropical forests. Most dominant Carboniferous wetland plants, however, produced little wood, and the arborescent lycopsids were no exception (10). The towering lycopsid stature was made possible by unlignified periderm rather than lignin-rich wood. Therefore, they contributed less lignin to peat accumulations than Fig. 1. Late Paleozoic atmospheric pCO2 andpO2 (17–18), North American terrestrial organic sediment accumulation (8), and temporal distribution of tropical biomes (see text for details and references). Upper section is a detailed inset of the biomeglaciation history shown in the Lower section. Glaciation history from ref. 2. WB 1 owned, commissioned by, and courtesy of the City and County of Denver, K. Johnson, and J. Vriesen.WB 3 and 4, and Seasonally Dry Biome1 courtesy ofMary Parrish (National Museum of Natural History, Smithsonian Institution).