Were Mesozoic Ginkgophytes Shrubby? Data on leaf morphology in the Mesozoic of North America shows a proportional increase of bifurcated, ginkgo-like leaves during the middle of the Jurassic. This ginkophyte acme is correlated with W. A. Green—Department of Geology—Yale University—P. O. Box 208109, Yale Station—New Haven, Connecticut 06520—[email protected] a decreased proportion of the leaf forms associated with herbaceous or shrubby pteridophytes, and with no substantial change in the proportion of leaf forms associated with canopy gymnosperms. The increase in ginkgo-like foliage at the same time as fern-like forms decreased in relative abundance suggests replacement of The conventional view sees all ginkgophytes as some part of the forest understory or early-successional habitats by early ginkgophytes. That is, early ginkgophytes may not have arborescent, by analogy with modern Ginkgo biloba: been competing for light or water in an established gymnosperm canopy. This suggests that most Mesozoic ginkgophytes were shrubs rather than being large trees like the surviving Ginkgo biloba. Such a result explains the absence of Mesozoid ginkgophyte wood and supports the argument that has already been made from sedimentological data, that to a much greater extent than do individuals of Ginkgo biloba now cultivated around the world, many ancestral ginkgophytes pursued early-successional strategies. 1: Competitive displacement alues) 2 v Records of Jurassic fossil occurrences in the Compendium Index of Mesozoic and Cenozoic Jurassic Records from the Compendium Index earson residuals (c otal numbers of records Type Fossils (Dorf 1940), a catalog of fossil Early (Lias) Middle (Dogger) Late (Malm) T Marginal root-sum-squares of P 2 meters occurrences organized by gross morphology, show Pteridophytes 37, 32%, c2 = -0.34 8, 18%, c2 = -1.2 20, 25%, c2 = 1.0 65 1.6 a negative correlation between the presence Conifers 11, 9%, c2 = 1.5 6, 13%, c2 = -0.07 16, 20%, c2 = -1.2 33 2.0 Cycadophytes 45, 39%, c2 = 0.88 18, 40%, c2 = -0.24 39, 49%, c2 = -0.58 102 1.1 of ginkgphytes and shrubby or herbaceous groups Ginkgophytes 6, 5%, c2 = -2.1 12, 27%, c2 = 4.5 1, 1%, c2 = -1.0 19 5.1 (pteridophytes), and little relationship with the Other 1, 1%, c2 = -0.81 1, 2%, c2 = 1.0 0, 0%, c2 = 0.04 2 1.3 presence of predominantly arborescent groups Incertae Sedis 16, 14%, c2 = -1.0 0, 0%, c2 = -1.9 4, 5%, c2 = 2.0 20 3.0 but there is in fact little evidence (other gymnosperms). Earlier examinations of Totals 116 45 80 241 evolutionary change through time, like for arborescent ginkgophytes Niklas et al. (1985) and Lidgard and Crane (1988), show no particular ginkgophyte expansion in Other before the Tertiary. On the the Middle Jurassic, but Tralau (1967, 1968) and other careful studies of ginkgophytes support the Incertae sedis Middle Jurassic diversification shown by the Ginkgophytes contrary, a number of indications #R log file Compendium Index data presented here. Since > j Pteridophytes Conifers Cycadophytes Ginkgophytes Angiosperms Other Incertae sedis Total seem to suggest the possibility these data also include information about other malm 37 11 45 6 0 1 16 116 dogger 8 6 18 12 0 1 0 45 major groups, it provides evidence of covariation lias 20 16 39 1 0 0 4 80 % Cycadophytes that, if interpreted as competitive interactions, > sqrt(rowSums((chisq.test(j[,c(1:4,6,7)])$res)^2)) that many early gingkophytes malm dogger lias would suggest the presence of ginkgophytes of 2.861739 5.137568 3.065681 Conifers > sqrt(colSums((chisq.test(j[,c(1:4,6,7)])$res)^2)) were small shrubs: low stature and shrubbier growth form in the Pteridophytes Conifers Cycadophytes Ginkgophytes Other Incertae sedes Pteridosperms 1.602743 1.957023 1.086118 5.067988 1.310180 3.000517 forest understrory or open, disturbed environments. > sum(sqrt(colSums((chisq.test(j[,c(1:4,6,7)])$res)^2))) Lias Dogger Malm [1] 14.02457 > sum(sqrt(rowSums((chisq.test(j[,c(1:4,6,7)])$res)^2))) [1] 11.06499 2 meters COMPETITION-LIMITED NUTRIENT-LIMITED DISTURBENCE-LIMITED 2: Sedimentological Context Note: Space limitations prevent the depiction of a ground 30 meters Royer et al. (2003) have shown that Tertiary ginkgphytes are found preferenctially in levee cover of ferns, herbaceous lycopods, and bryophytes. Though and proximal floodplain sediments. it is difficult to find direct supporting evidence, is seems reasonable to assume that established, unstressed Mesozoic communities could have developed as many as three weakly stratified layers. Canopy height up to 30 meters Canopy height e.g. 1–5 meters Araucarian conifers, Canopy open Metasequoia e.g. Araucarian conifer, or irregular silhouette traced from Cycads, Bennettitalians modern sp. of Araucaria Canopy height e.g. (closed canopy growth form) Water table 0–1 meters Pleuromeia, exposed; e.g. Taxodium no aerial Ferns, Equisetum Araucarian conifer, silhouette traced from canopy modern sp. of Araucaria Bennettitalian Williamsonia stewardiana, (open canopy growth form) e.g. as reconstructed by Sahni (1932) Hypothetical under- Taxodiacious conifer, story ginkgophyte Canopy height silhouette traced from Bijuvia simplex, Isoetes modern Taxodium sp. Cycad, Mesozoic lycopod as reconstructed by 0–1 meters Pleuromeia, as Florin (1933) reconstructed by Hirmer (1933) Conifer, silhouette e.g. traced from modern Metasequoia glyptostrobus Cycad, Leptocycas gracilis, Bennettitalian, as (open canopy growth form) Equisetum, Ferns as reconstructed by reconstructed by Delevoryas and Hope (1971) Delevoryas (1971) Second Terrace Equisetum; silhouette traced Osmunaceous fern, silhouette traced from from modern Equisetum telmeteia photograph of modern Osmunda cinnamome Marratalian fern; silhouette traced First Terrace from modern Angiopteris evecta Hypothetical early- successional ginkgophyte Levee Active Floodplain Isoetids; silhouettes traced from modern Isoetes sp. Point Bar Backswamp r Table Wate Hypothetical reconstruction of floodplain plant communities Channel from the Middle Jurassic of North America Distal floodplain; disturbance less frequent, canopy better developed, species longer lived, higher in stature and with greater investment in disseminules and non-photosynthetic Proximal floodplain; vegetative growth. Fossils found in fine sediments, often with ped structures or identifiable soil stratification. Note that the gradiant depicted disturbance frequent compared with generation above is frequently compressed into relatively short distances when flood disturbance is low. This representation time, canopy poorly developed, species likely to be short-lived, low of is intended to show all the possible habitats and communities any of which stature, and r-selected, investment in reproduction and vegetative growth comparatively low. Clonal may be missing in a given situation. reproduction and ast initial growth likely. Fossils found in coarse or well-sorted sediments showing evidence of alluvial deposition and lacking extensive soil formation. Note that the common occurrence in the fossil record of allochthonous (transported) ginkgophyte foliage in proximal facies must be discounted. 3: Absence of wood 4: Atypical characters Scott et al. (1962) could verify no occurrences of clear ginkgophyte wood earlier than the Eocene: “in marked contrast to the Ginkgo biloba has polymorphic foliage, an abundant leaf remains of the order, fossil woods of the Ginkgoales early bolting growth phase, and poor shade are rare” (Scott et al. 1962:1095). No other convincing candidates tolerance, all characteristics that are rare or Summary of Indications for Mesozoic ginkgophyte wood have been found, which is surprising absent in habitually canopy-forming trees. See Royer et al. (2003) for further discussion. 1. Ginkgophytes seem to replace (negatively co-vary with) given the amount of other well-preserved Mesozoic gymnosperm wood. herbaceous or shrubby rather than arborescent groups. 2. Fossil ginkgophytes are found predominantly in high-energy facies that are less likely to have supported an established Works Cited complex canopy. Delevoryas, T. (1971) Investigations of North American Cycadeoids: Weltrichia and Williamsonia from the Jurassic of Oaxaca, Mexico. American Journal of Botany 78:177–182. 3. Firmly identifiable ginkgophyte wood does not seem to Delevoryas, T. and R. C. Hope (1971) A new Triassic cycad and its phyletic implications. Postilla 150:1–14. appear in the fossil record until the late Cretaceous or Tertiary. Dorf, E. (1940) An illustrated catalogue of Mesozoic and Early Cenozoic plants of North America. Science 91(2368):1–2. Florin, R. (1933) Studien über die Cycadales des Mesozoikums. Kungl. Svenska Vetenskapsakademiens Handlinger III 12:1–134. Hirmer, M. (1933) Rekonstruktion von Pleuromeia sternbergi Corda nebst bemerkungen zur Morphologie der Lycopodiales. Paleontographica B 78:47–56. 4. Modern Ginkgo biloba has a suite of characters that are not Knoll, A. H., K. Niklas, and B. H. Tiffney (1979) Phanerozoic land plant diversity in North America. Science 206:1400–1402. typical of canopy trees including polymorphic foliage, an Lidgard, S. and P. R. Crane (1988) Quantitative analysis of early angiosperm radiation. Nature 331:344–346. Royer, D. L., L. J. Hickey, and S. L. Wing (2003) Ecological conservatism in the “living fossil” Ginkgo. Paleobiology 29(1):84–104. early bolting growth phase, and poor shade tolerance. Scott, R. A., E. S. Barghoorn, and U. Prakash (1962) Wood of Ginkgo in the Tertiary of western North America. American Journal of Botany 49(10):1095–1101. Sahni, B. (1932) A petrified Williamsonia (W. sewardiana, sp. nov.) from the Rajmahal hills, India. Palaeontologia Indica 20(N.S.):1–19. Tralau, H. (1967) The phytogeographic evolution of the genus Ginkgo L. Botaniska Notiser 120:409–422. Tralau, H. (1968) Evolutionary trends in the genus Ginkgo. Lethaia 1:63–101. Funding for this work was provied by a Yale University fellowship and by NSF Grant DEB–0235736.