Paleobiology, 34(2), 2008, pp. 179–194 How green was Cooksonia? The importance of size in understanding the early evolution of physiology in the vascular plant lineage C. Kevin Boyce Abstract.—Because of the fragmentary preservation of the earliest Cooksonia-like terrestrial plant macrofossils, younger Devonian fossils with complete anatomical preservation and documented gametophytes often have received greater attention concerning the early evolution of vascular plants and the alternation of generations. Despite preservational deficits, however, possible phys- iologies of Cooksonia-like fossils can be constrained by considering the overall axis size in con- junction with the potential range of cell types and sizes, because their lack of organ differentiation requires that all plant functions be performed by the same axis. Once desiccation resistance, sup- port, and transport functions are taken into account, smaller fossils do not have volume enough left over for an extensive aerated photosynthetic tissue, thus arguing for physiological dependence on an unpreserved gametophyte. As in many mosses, axial anatomy is more likely to have ensured continued spore dispersal despite desiccation of the sporophyte than to have provided photosyn- thetic independence. Suppositions concerning size constraints on physiology are supported by size comparisons with fossils of demonstrable physiological independence, by preserved anatomical detail, and by size correlations between axis, sporangia, and sporangial stalk in Silurian and Early Devonian taxa. Several Cooksonia-like taxa lump fossils with axial widths spanning over an order of magnitude—from necessary physiological dependence to potential photosynthetic compe- tence—informing understanding of the evolution of an independent sporophyte and the phylo- genetic relationships of early vascular plants. C. Kevin Boyce. Department of the Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637. E-mail: [email protected] Accepted: 22 January 2008 Introduction ler and Churchill 1985; Rothwell 1995). That Much is unknown about the ecology and cooksonioid fossils are small but relatively in- physiology of the oldest preserved macrofos- tact sporophytes is supported by their uni- sil land plants. These cooksonioid fossils first form size in beds containing many fossils and appear in the Wenlockian (Edwards and Fee- by spore morphologies distinct from the larg- han 1980; Edwards et al. 1983) and remain a er taxa of which they could conceivably be dis- common component of the paleoflora through tal fragments (Edwards 1996). Their most the rest of the Silurian and the Early Devonian common treatment is an intermediate one in (Edwards 1973, 1979a). These fossils consist of which the plants are considered small and de- unadorned axes that can branch isotomously, terminate but without implication of gameto- are terminated by sporangia, and are quite phyte dependence for rooting and substrate small—often of mesofossil proportions. The interaction (Edwards and Fanning 1985; Ed- small size of the axes and their limited num- wards 1996). (Originally reflecting equivocal ber of dichotomies has led to distinct inter- evidence for vasculature [Taylor 1988] and pretations of plant habit, ranging from the fos- similar to the more common ‘‘rhyniophytoid’’ sils representing determinate, aerial axes bro- [Edwards 1996], ‘‘cooksonioid’’ here denotes ken from indeterminately growing rhizomes only simple, smaller taxa usually less than 1 not preserved (Niklas 1997; Kenrick and Davis mm in width and lacking signs of indepen- 2004) to complete, determinate sporophytes dent growth described below, thereby includ- rooted in a gametophyte, although without ing Cooksonia, Salopella, Tarrantia, Tortilicaulis, discussion of whether sporophyte physiolog- but excluding larger fossils such as Aglaophy- ical dependence extended beyond water and ton.) nutrient acquisition from the substrate (Mish- Despite their obvious centrality for under- ᭧ 2008 The Paleontological Society. All rights reserved. 0094-8373/08/3402-0002/$1.00 180 C. KEVIN BOYCE standing the early evolution of the vascular was based upon axes of 1 mm diameter and plant lineage, the importance of these cook- then generalized to other taxa. Furthermore, sonioid fossils has in many ways been over- these calculations were ultimately limited by whelmed by the abundance of information a lack of anatomical detail: in modeling pho- from the Lower Devonian Rhynie Chert of tosynthetic assimilation, only the endpoints of Scotland, almost 50 Myr younger than the old- the CO2 diffusion pathway were considered, est cooksonioids. The Rhynie Chert provides whereas inclusion of progressive CO2 deple- the earliest detailed anatomy (Edwards 1993) tion with loss to cells intermediate between for a diverse flora (Kidston and Lang 1917, the stomata and the innermost chlorenchyma 1920a,b; Lyon and Edwards 1991; Powell et al. is crucial (Konrad et al. 2000; Roth-Nebelsick 2000; Kerp et al. 2001), which has in turn in- and Konrad 2003). Similar problems can be ex- formed phylogenetic understanding (Gensel pected with hydraulic calculations for which 1992; Kenrick and Crane 1997) and allowed proximal water loss can be a far greater con- well-constrained modeling of physiological cern (Zwieniecki et al. 2004, 2006) than pro- function (Konrad et al. 2000; Roth-Nebelsick vided for by the assumption in early studies 2001; Roth-Nebelsick and Konrad 2003), to- of linear water loss along the axis. gether indicating an ancestral condition with Because anatomical context has proven es- little tissue diversity or organ differentiation sential, recent modeling necessarily has been but general photosynthetic and physiological limited to permineralized fossils. In the fu- continuity with later vascular plants. The Rhy- ture, techniques such as X-ray computed to- nie Chert also has yielded axial gametophyte mography (Tafforeau et al. 2006) might pro- fossils that are predominantly of equivalent vide greater anatomical information from tissue complexity and size to the correspond- cooksonioid mesofossils than is provided by ing sporophyte (Remy 1982; Remy et al. 1993; the current reliance on surficial exposure and Kerp et al. 2004; Taylor et al. 2005), thereby im- intentional breakages (Edwards and Axe plying that all land plants (Remy 1982) or at 2000). However, even if the detailed physio- least vascular plants (Kenrick 2000) diverged logical consideration received by Rhynie fos- from a common ancestor with equal genera- sils and other early permineralized plants tions, as opposed to following the trend from (Edwards et al. 2006) currently cannot be rep- gametophyte to sporophyte dominance ex- licated for the anatomically incomplete cook- pected from the characteristics and relation- sonioid compressions and mesofossils, their ships of living land plants. preservation may yet be sufficient to constrain Although available anatomy has been de- what physiologies would not have been pos- scribed for individual cooksonioid specimens, sible. the preservation of that anatomy is partial and gametophyte fossils are unknown. As a result, Size Constraints on Tracheophyte in syntheses of early evolutionary patterns, Physiological Independence cooksonioids are often atomized and serve as Plant Function and Division of Labor. The mileposts for the accumulation of individual functions essential for nearly all independent- vascular plant characteristics (e.g., Banks ly growing terrestrial plants—desiccation re- 1981; Knoll et al. 1984; Niklas 1997; Edwards sistance, structural support, hydraulic and 2003), leaving unresolved the implication that solute transport, and photosynthesis—can the general continuity with modern sporo- have contradictory requirements and necessi- phyte photosynthetic function demonstrable tate segregation into distinct tissues. For ex- in the Rhynie Chert persists in the still deeper ample, photosynthesis is severely limited if past despite the greater morphological sim- photosynthetic cells do not have direct access plicity of the fossils. to airspaces because CO2 diffusion is much Such continuity was supported by early more rapid through air than water (Raven modeling efforts upholding photosynthetic 1996); however, the epidermis must have con- independence of early vascular plant sporo- tiguous, cuticle covered cells if it is to act as a phytes (Raven 1984, 1993). However, that work barrier to water loss. Mechanical and hydrau- HOW GREEN WAS COOKSONIA? 181 lic functions can be similarly compromised by intercellular airspaces. In all Silurian and Early Devonian fossil plants except lycophytes, these mutually ex- clusive functions must be performed by (and all discrete tissues contained within) the axis as the only vegetative organ (Edwards 1993), placing geometric constraints and potentially a minimum viable size upon that axis. Mini- mum size constraints are easy to overlook be- cause diverse, small structures are abundant in more complex vascular plants, but they rep- resent specializations for limited functions. For example, the rachis of a maidenhair fern frond can be less than 500 ␮ thick, but is not photosynthetic and does not require airspac- es. The axes of early fossil plants could not eas- ily circumvent these functions and resulting tissue requirements. For example, the mycor- rhizal fungi seen in the Rhynie Chert (Taylor et al. 1995, 2004) would simplify the addition- al requirement of nutrient acquisition from the substrate, but would not