1 Back to the Beginnings: The Silurian-­ 2 Devonian as a Time of Major Innovation 15 3 in Plants and Their Communities 4 Patricia G. Gensel, Ian Glasspool, Robert A. Gastaldo, 5 Milan Libertin, and Jiří Kvaček 6 Abstract Silurian, with the Early Silurian Cooksonia barrandei 31 7 Massive changes in terrestrial paleoecology occurred dur- from central Europe representing the earliest vascular 32 8 ing the Devonian. This period saw the evolution of both plant known, to date. This plant had minute bifurcating 33 9 seed plants (e.g., Elkinsia and Moresnetia), fully lami- aerial axes terminating in expanded sporangia. Dispersed 34 10 nate∗ leaves and wood. Wood evolved independently in microfossils (spores and phytodebris) in continental and 35AU2 11 different plant groups during the Middle Devonian (arbo- coastal marine sediments provide the earliest evidence for 36 12 rescent lycopsids, cladoxylopsids, and progymnosperms) land plants, which are first reported from the Early 37 13 resulting in the evolution of the tree habit at this time Ordovician. 38 14 (Givetian, Gilboa forest, USA) and of various growth and 15 architectural configurations. By the end of the Devonian, 16 30-m-tall trees were distributed worldwide. Prior to the 17 appearance of a tree canopy habit, other early plant groups 15.1 Introduction 39 18 (trimerophytes) that colonized the planet’s landscapes 19 were of smaller stature attaining heights of a few meters Patricia G. Gensel and Milan Libertin 40 20 with a dense, three-dimensional array of thin lateral 21 branches functioning as “leaves”. Laminate leaves, as we We are now approaching the end of our journey to vegetated 41 AU3 22 now know them today, appeared, independently, at differ- landscapes that certainly are unfamiliar even to paleontolo- 42 23 ent times in the Devonian. In the Lower Devonian, trees gists. As we delve deeper and deeper in time, we will visit a 43 24 were not present and plants were shrubby (e.g., succession of some of the earliest plant life to cover Earth. 44 25 Aglaophyton major), preserved in a fossilized community Until this point, our time has been spent in various woodland 45 26 at the Rhynie chert locality in Scotland and other places. settings and, without a doubt, there were wooded topogra- 46 27 Many of these stem-group plants (i.e., preceding the dif- phies where we begin our “hike” [U1501]. But, unlike the 47 28 ferentiation of most modern lineages) were leafless and past 390 million years, trees will not remain a part of the 48 29 rootless, anchored to the substrate by rhizoids. The earli- countryside as we step back further and further in time. Trees 49 30 est land plant macrofossil remains date back to the and tree-like architectures will disappear from the fossil 50 record. We will see a significant decrease in the heights of the 51 plants anchored to more primitive soils, and we will pass 52 Electronic supplementary material The online version of this chapter (https://doi.org/10.1007/978-3-030-35058-1_15) contains supplemen- through the oldest shrub- or meadow-like areas where the 53 tary material, which is available to authorized users. tallest forms may have been only head high. As we continue 54 further, plants become diminutive, barely brushing against 55 P. G. Gensel (*) our ankles but appearing, seemingly, like a carpet covering all 56 Department of Biology, University of North Carolina, Chapel Hill, NC, USA available moist surfaces. And, finally, we reach a point where 57 e-mail: [email protected] we no longer encounter evidence of the very earliest plants in 58 I. Glasspool · R. A. Gastaldo sediments of a continental nature. Rather, minute and scat- 59 Department of Geology, Colby College, Waterville, ME, USA tered remains attributed to land plants, including their micro- 60 e-mail: [email protected]; [email protected] scopic parts, are now found preserved in ocean basins along 61 M. Libertin · J. Kvaček with members of the Paleozoic fauna (see Chap. 14). Before 62 AU1 National Museum, Prague, Czech Republic e-mail: [email protected]; [email protected] © Springer Nature Switzerland AG 2020 E. Martinetto et al. (eds.), Nature through Time, Springer Textbooks in Earth Sciences, Geography and Environment, https://doi.org/10.1007/978-3-030-35058-1_15 P. G. Gensel et al. Fig. 15.1 Archaeopteris has been described as the earliest known “modern tree”, having a woody trunk, growing to heights in excess of 30 m and bearing near-horizontal, helically arranged deciduous branches. (a) Artist reconstruction. Despite this, Archaeopteris has many features far removed from those of trees today. This progymnosperm had a pteridophytic method of reproduction and bore some of the earliest planate leaves, though they are remarkably fern-like in appearance. (b) Lateral branch with alternately arranged ultimate branches bearing spirally arranged, planar leaves. (a) © The Field Museum, GEO86500_125d and Karen Carr, Artist. With permission. (b) Image courtesy of Walter Cressler AU5 63 we enter the unknown, let us begin with an overview of what Protolepidodendropsis, which were persistent into the lat- 91 64 is familiar and what is unfamiliar. est Paleozoic. The second forest is stranger. These puz- 92 65 Traveling up or down any major Late Devonian river by zling forests were comprised of medium to very tall trees 93 66 “dugout” canoe, we first encounter the riparian forests lining that bore no leaves. Each tree had a sort of crown made up 94 67 the riverside in which the major tree, Archaeopteris, is known of branches that branched and branched and branched and 95 AU6 68 (Fig. 15.1). The name of this woody tree is a misnomer terminated in a branch. These plants, the cladoxylopsids 96 69 because it implies that the plant is an “early (archaeo) fern Calamophyton or Eospermatopteris, first appear in the 97 70 (pteris)”. Rather, this canopy-forming tree possessed a weird Middle Devonian and are successful cohabitants until the 98 71 combination, or mosaic, of gymnosperm and fern features, early Late Devonian. Lianescent (vines) and herbaceous 99 72 and is placed in a plant group distinctive from the others plants are known to occur in some of these early forests, as 100 73AU4 we’ve encountered thus far (see Sect. 15.1.1). When well as ancestors to the lycopsids, the zosterophylls (see 101 74 Archaeopteris colonized the landscape, the land’s surface Box 15.1), comparable to Early Devonian taxa. Plants get 102 75 was covered by a well-established tiered community. Gallery smaller the further we regress in time, with short-stature 103 76 forests grew adjacent to swampy areas populated by sprawl- riparian and coastal marsh-like vegetation expanding in 104 77 ing shrubs (Rhacophyton) and smaller groundcover plants, the late Early Devonian. These replaced open areas cov- 105 78 such as Protobarinophyton. Other low-lying environments ered by a “green fuzz” of the earliest vegetation in moist 106 79 were colonized by the earliest tree lycopsids, including settings adjacent to river or ocean margins (Silurian to ear- 107 80 Lepidosigillaria or Cyclostigma, similar to those that domi- liest Devonian) [U1503]. Descendants of the early colo- 108 81 nated wetland settings in the Carboniferous (see Chap. 13). nizers are still found in the mosses and liverworts. Yet, we 109 82 By the latest Devonian, plants had evolved reproductive have no evidence that any other earliest plant group 110 83 strategies to conquer seasonally dry regions outside of the remained relatively static and survived until today. 111 84 wetlands. Here, several types of early seed plants, such as Fossils representing the earliest evidence of plants, con- 112 85 Elkinsia, were abundant. These forests that might seem sisting of small to “large” macrofossils (large being a relative 113 86 familiar, in a general sense, become less familiar as we term of only several centimeters in length) and dispersed 114 87 approach the Middle Devonian. spores, take us into an even stranger world. While most of 115 88 Middle Devonian forests can be separated into two these fossils are found in Silurian and Devonian rocks, the 116 89 types [U1502]. The forest structure that retains a familiar earliest evidence of plants is known from the Ordovician 117 90 feeling is dominated by tree lycopsids such as (Katian or possibly earlier; Wellman 2010). Marine rocks 118 15 Back to the Beginnings: The Silurian-Devonian as a Time of Major Innovation in Plants and Their Communities 119 preserve small sporangia with spore tetrads, an evolutionary in our site visits to the Siluro-Devonian, however, the terms 169 120 feature ascribed to all higher plants (Steemans et al. 2009), rhyniophytoid, rhyniophyte, zosterophyll, trimerophyte, and 170 121 as well as a myriad of dispersed spore assemblages contain- progymnosperm, accompanied by diagnostic characters, will 171 122 ing similar types of spores from various parts of the globe. be used in the postcard descriptions to follow. 172 123 Before we machete our way through these unknown terranes, 124 we need to gain an appreciation for the plant groups that 125 occupied Silurian-Devonian landscapes. Box 15.1: Relationships of Siluro-Devonian Plants: Banks (1968) to Kenrick and Crane (1997) to Now 126 15.1.1 Relationships Banks (1968) subdivided the Early Devonian “psilo- phytes” into several distinct lineages, which were rec- 127 When fossil plants first were recognized as more than just a ognized as subdivisions. These were the Rhyniophytina, 128 carbon smudge on a rock surface, all Early Devonian plants Zosterophyllophytina, and Trimerophytina, and he 129 were collectively referred to as the “psilophyte flora” due to clarified lineages such as Lycophytina, the cladoxylop- 130 a similarity in growth architecture to a living plant (Arber sids, and the progymnosperms.