Ordovician-Devonian lichen canopies before evolution of woody trees

Ecological tiering is most inspiring in tropical rain forests. On Barro Colorado Island, Panama, for example, a canopy of almendro (Dipteris panamensis) about 30 m tall, with emergents to 40 m, rises above a tier of palms (Astrocaryum standlyeanum, Scheelea zonensis) 10–20 m tall, and shrubs of Psychotria spp. up to 2 m tall, but ground cover is limited by activity of leaf-cutter ants and termites (Leigh, 1999). Comparable reconstructions of fossil forests can be made from known allometric relationships between tree height and trunk diameter at breast height (Niklas, 1994), checked against complete fossil trees (Stein et al., 2007, Stein et al., 2012, Retallack and Huang, 2011, Retallack and Landing, 2014). Tiering also is known below ground in soils, with buttress roots in surface layers, tap roots deeper (Leigh, 1999), and fungal rhizines reaching down to the water table and limits of soil oxygenation (Graham et al., 2010a, Graham et al., 2010b). Depth of bioturbation and the declining acidity of soil water toward calcic horizons in soils reflect both productivity (Breecker and Retallack, 2014), and stature of vegetation (Retallack and Huang, 2011). Thus, paleosols provide an additional line of evidence for vegetation height and productivity in deep time.

Comparable marine tiering in deep time has been reconstructed using similar methods. Stalked crinoids and tall sponges exploit the water column a meter or so above bottom, with bryozoans, corals, brachiopods and clams below them. The height above the sea floor exploited by filter feeders increased from Cambrian to Ordovician (Bottjer and Ausich, 1986). Below the sea floor, Early Cambrian bioturbation was shallow (Jensen et al., 2007), but deep burrows evolved by Ordovician time (Sheehan and Schiefelbein, 1984). Just as Cambrian-Carboniferous marine communities evolved both higher and deeper tiers and corresponding rise in productivity (Bottjer and Ausich, 1986), so too did life on land.

In contrast with early Paleozoic fossil plants, which are rare and usually removed from growth position (Lang, 1937, Edwards and Axe, 2012, Edwards et al., 2013), paleosols provide evidence of Paleozoic vegetation in growth position on many stratigraphic horizons from root traces and depth to carbonate nodules (Edmunds, 1997, Driese et al., 1997, Driese and Mora, 2001, Hillier et al., 2008, Retallack, 2009a, Retallack, 2009b, Retallack, 2015a, Retallack, 2015b). Many fossil plant localities yield nematophytes much larger than associated vascular land plants (Hueber, 2001, Hillier et al., 2008, Retallack, 2015a, Retallack, 2015b, Retallack, 2020). This study reviews such evidence, and provides new evidence of nematophyte traces within paleosols of Ordovician and Silurian age to reconstruct the long-term evolution of tiers below and above ground from Cambrian microbial earths to Carboniferous forest ecosystems.

Material and methods

This study is a compilation of data on the size of fossil plants and soils. Paleosol data includes depth to carbonate (calcic or Bk horizon), corrected for burial compaction scaled to thickness of overburden (Sheldon and Retallack, 2001), from comprehensive accounts of Cambrian to Devonian paleosols in Australia (Retallack, 2009a, Retallack, 2009b), Ordovician paleosols of Pennsylvania and Tennessee (Retallack, 2015a), Silurian paleosols of Pennsylvania (Retallack, 2015b), and

Early Palaeozoic root traces and calcic horizons

Two especially important indices of vegetation height preserved in paleosols are drab-haloed root-traces and depth to carbonate nodules (calcic or Bk horizon). Pedogenic carbonate can be distinguished from marine carbonate nodules by the following criteria: nodules arranged in diffuse horizons below the upper surface of the bed at a distance that is similar within the same formation, micritic calciasepic and replacive fabric in thin section, growth both truncating and treuncated by burrows and

Early Paleozoic fungi

The once puzzling Paleozoic nematophytes, such as Prototaxites, are now known from their cellular anatomy to have been fungi rather than tracheophytes, despite their great size (Hueber, 2001, Edwards and Axe, 2012, Edwards et al., 2013, Retallack, 2020). Hyphal anatomy of Prototaxites rules out previous interpretations as yew trees or giant algae (Hueber, 2001), and continuity of tissues in massive trunks (Taylor et al., 2010) rules out interpretation as rolled up liverwort mats (Graham et al.,

In situ nematophtyes of Ordovician Juniata Formation

A buried stand of nematophytes is preserved in the Late Ordovician (Hirnantian), upper Juniata Formation at Beans Gap Tennessee (Fig. 7A: Retallack, 2015a). The trunks are natural casts of stems in a 24-cm-thick bed of sandstone, and each trunk can be traced downward to drab mottled, rooting structures in the purple-red Bedford clay paleosol, which is a weakly developed Fluvent, immediately below (Fig. 7B-C). Poorly preserved parallel hyphae, arranged into concentric growth rings characteristic

In situ nematophtyes of Silurian Bloomsburg Formation

Nematophytes have been found in the Late Silurian (Ludlovian) Bloomsburg Formation as trunks (Fig. 8F; Prototaxites) and leaf-like structures (Fig. 8E; Nematothallus: Strother, 1988) in gray sandstone paleochannels (Retallack, 2015b). Paleosols of the Bloomburg Formation also have drab mottles with numerous parallel filaments (Fig. 8C-D), like rooting structures of nematophytes (Driese and Mora, 2001, Hillier et al., 2008, Retallack, 2015a, Retallack, 2015b, Retallack, 2015c). As in other cases

Comparison with sedimentary record

This survey of early Paleozoic tiering above and below ground can be compared with the published record of tiering in early Paleozoic marine communities (Bottjer and Ausich, 1986), and the increase in nutritional quality and productivity of marine invertebrates through time (Bambach, 1993, Servais and Harper, 2018). The height of marine tiers above the seafloor increased with the height of fungal tiers on land, and the depth of marine burrowing below the seafloor increased with depth of roots

Mycotrophic hypothesis

A Silurian-Devonian canopy of lichens fed by extensive mycelia supports the mycotrophic hypothesis of Pirozynski and Malloch (1975), that colonization of land by plants required nutrition from fungal mycorrhizae. These essential symbionts of most land plants are mainly glomalean fungi of the phylum Glomeromycota (Hibbett et al., 2007), and not Oomycota as originally envisaged (Pirozynski and Malloch, 1975). The mycotrophic hypothesis was also originally linked to the idea that multicellular

Conclusions

This new view of terrestrial ecosystems reaching more deeply into their substrate and higher in their fluid medium than communities of the sea floor (Fig. 9, Fig. 10) supports the general argument of Bambach, 1993, Servais et al., 2019 that increased biomass, diversity and muscularity of early Palaeozoic marine communities, and increased diversity of marine phytoplankton were fueled by increased primary productivity and chemical weathering on land. Advances in bulk and reach of Ordovician life

Declaration of Competing Interest

The author declares that he has no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Dick Beerbower and Stephen Driese showed me critical sections in Pennsylvania and Tennessee. Ria Mitchell, Tim Lenton, Paul Strother, and Scott Redhead are thanked for useful discussion.