Brigitte Meyer-Berthaud

Archaeopteris is the earliest known modern tree

Archaeopteris is an extinct plant which is of botanical interest for two reasons. It was the main component of the earliest forests until its extinction around the Devonian/Carboniferous boundary, and phylogenetically, it is the free-sporing taxon that shares the most characteristics with the seed plants. Here we describe the largest group of anatomically preserved Archaeopteris remains ever found, from the Famennian marine beds of south-eastern Morocco, and provide the first evidence that, in terms of development and branching strategies, these 370-million-year-old plants were the earliest known modern trees. This modernization involved the evolution of four characteristics: a lateral branching syndrome similar to the axillary branching of early seed plants; adventitious latent primordia similar to those produced by living trees, which eventually develop into roots on stem cuttings; nodal zones as important sites for the subsequent development of lateral organs; and wood anatomy strategies that minimize the mechanical stresses caused by perennial branch growth.

The development of Archaeopteris: new evolutionary characters from the structural analysis of an Early Famennian trunk from southeast Morocco

A 5 m long trunk of a young Archaeopteris/Callixylon erianum tree from the Late Devonian of Morocco shows new branching patterns for early lignophytes. This progymnosperm tree produces a helical pattern of traces that we infer belonged to reduced, short-lived, primary (apical) branches (type A) as well as two types of adventitious traces (types B and H). We infer that type-B traces supplied branches that initiate close to the site of attachment on the trunk of some, but not all type-A branches in an irregular but nonrandom pattern. Unlike ephemeral type-A branches, those of type B persist and become long-lived, potentially permanent units of the architecture of Archaeopteris trees. Type-H adventitious traces are also short-lived and occur singly or in serial groups, but differ from traces of either type A or B branches by lacking differentiation into a readily identifiable organ category. We interpret type-H traces as supplying latent primordia that could develop into either adventitious roots or shoots depending on extrinsic factors. Our new data suggest that Archaeopteris had a wide range of branch primordium amplitude. Type-B branches compare with axillary lateral branch buds of some Early Carboniferous spermatophytes (Calamopitys) and are a major developmental departure from the strictly apical, pseudomonopodial shoot branching of older aneurophyte progymnosperms. Type-H traces suggest that Archaeopteris trees had some potential for formation of adventitious roots or shoots in response to environmental factors, such as partial burial by overbank sedimentation. Collectively, these novel methods of tree branching may partly explain the extraordinary success and worldwide dominance of Archaeopteris forests on fluvially dominated, Late Devonian floodplains.

The diversification of early arborescent seed ferns

Abstract Galtier, J. and B. Meyer-Berthaud. (UMR Botanique et Bioinformatique, CIRAD, TA40/PS2, 34398 Montpellier, France). The diversification of early arborescent seed ferns. J. Torrey Bot. Soc. 133: 7–19. 2006.—Seed plants of the lowermost Carboniferous time (Tournaisian) display important differences in morphology and habit with at least two size classes. On one side are plants of modest stature with protostelic stems, manoxylic wood and large leaves, interpreted as calamopityan, buteoxylalean and lyginopteridalean seed ferns, and on the other side are arborescent plants, with trunks up to 2 metres in diameter, of which the systematic position remains unclear. We summarize present information on these trees characterized by a thick development of generally dense wood, a broad eustele consisting of a large number of discrete primary xylem strands, short internodes and deciduous medium-sized fronds. New data have been recently obtained on Pitus, the best known member of this group, on Eristophyton, Bilignea, Stanwoodia, Aporoxylon and on several new taxa which exhibit a broad circular parenchymatous pith surrounded by numerous sympodial xylem strands, but which differ in secondary xylem and leaf trace features. Of particular interest is a new plant that has leaf traces originating as a double strand and a petiole base of the Kalymma-type, therefore showing characteristics of calamopityan seed ferns but being quite distinct in features of the stele, secondary xylem and phloem. Emphasis is placed on the evolutionary dynamics of this important diversification of arborescent plants which included: i) a rather abrupt increase in overall diameter, ii) increase in primary xylem size, with regard to other contemporaneous early seed plants; iii) very different types of wood, instead of a single dense/pycnoxylic wood as generally assumed; iv) distinct periderm types; v) deciduous leaves. The last feature may be interpreted as an innovation, related to the tree habit, within the lignophytes. The origin and systematic position of these arborescent plants remain problematical.

The plant Leclercqia (Lycopsida) in Gondwana: implications for reconstructing Middle Devonian palaeogeography

Abundant and well-preserved material of the ligulate lycopsid genus Leclercqia is reported from a new Middle Devonian locality in northeastern Queensland (Australia). The plants occur in a chert horizon in the Storm Hill Sandstone of the Dosey-Craigie Platform. Lithological data and conodont analyses combined with information from in situ spores provide an age for the plant levels ranging from Eifelian, possibly Middle Eifelian, to Early Givetian. Plant taxonomic identification is based on vegetative and fertile stems that display both external morphology and anatomy. This material represents the best documented occurrence of Leclercqia outside Laurussia and possibly the earliest in Gondwana; it provides evidence that colonization of Gondwana by the species L. complexa was contemporaneous to that of Siberia and Kazakhstan. Analysis of the distribution patterns of L. complexa suggests that it was adapted to a wide range of environments, but within certain limits which we hypothesize to be those of a climatic belt. Such considerations support previous studies using other biological data, such as faunas and palynomorphs, for reconstructing Devonian palaeogeography. They favour a close proximity of Laurussia and Gondwana rather than the occurrence of a wide ocean separating the two palaeocontinents in Middle Devonian times.

Transitional changes in arborescent lignophytes at the Devonian–Carboniferous boundary

Abstract: It is usually considered that after the extinction of the Devonian tree Archaeopteris, no new arborescent lignophytes were established before the late Tournaisian. A reassessment of this pattern is presented here based on a three-fold approach: a re-evaluation of the taxic diversity of Tournaisian lignophyte trees based on descriptions of new plants from palaeotropical latitudes, a study of the patterns of phenotypic changes occurring among early lignophytes using a principal coordinate analysis and a phylogenetic analysis of the affinities of the arborescent taxa. The best supported results indicate that a substantial taxonomic and phenotypic diversity of arborescent lignophytes was already established in the first part of the Tournaisian, including some taxa that persisted until the Serpukhovian. Two genera may have originated in the Late Devonian and crossed the Devonian–Carboniferous boundary. Fewer originations and a decrease in phenotypic diversity occurred in the Viséan. The phenotypic distinctiveness of tree stems compared with those of other growth forms in the lignophytes is assessed. We propose a scenario in which the presence of lignophyte trees is continuous across the Devonian–Carboniferous boundary, with arborescent taxa distinct from Archaeopteris already present in the latest Devonian, possibly in upland floras, and diversifying significantly soon after the Devonian–Carboniferous boundary. Supplementary material: A list of taxa, characters and matrices used in the principal coordinate analysis and phylogenetic analysis is available at http://www.geolsoc.org.uk/SUP18447.

Studies on a new lower carboniferous flora from kingswood near pettycur, Scotland. I. Preliminary report

Abstract A new Asbian flora preserved as calcareous permineralizations and as fusian has been discovered closely associated with the famous Pettycur flora in Fife, Scotland. This new flora occurs in limestones within a volcanogenic sequence at Kingswood, near Pettycur. The limestones are tentatively interpreted as having been deposited in a lake possibly within a crater. The plant fossil assemblage is dominated by fusainized pteridospermous remains which include stems, rachides, distal parts of fronds and pollen organs. Associated with these are numerous fragments of gymnospermous wood. Permineralized axes of the lycopod Oxroadia are also abundant and occur with megasporophylls of Achlamydocarpon -type. Sphenopsids and ferns are rare and always fusainized. The assemblage contains new taxa still to be named, such as two pollen organs showing intermediate features between Lyginopteridaceae and Medullosaceae and at least two species of Lyginorachis . Some gymnospermous stems with pycnoxylic wood and triangular pith also represent new taxa of unknown affinities. The occurrence of growth rings in some woody specimens is noted. Whilst this flora is of the same age as the Pettycur assemblage, the most significantly striking feature is that there is only one genus, Archaeocalamites , in common. This almost certainly must reflect an ecological control within the volcanic terrain.

First record of a large Callixylon trunk from the late Devonian of Gondwana

A 5 m long permineralized trunk from the Upper Kellwasser member of southern Morocco represents the first record of a large trunk of an identifiable species of Callixylon , C. erianum , from Gondwana. This occurrence constitutes the most reliable evidence based on plant megafossils for a floral connection between Laurussia and Gondwana in late Devonian times and for a proximity of these continents in Famennian times. The potential of this trunk for studies of the architecture and growth patterns of the earliest trees with a gymnospermous type of arborescent habit is discussed.

Fossil plants from the Viséan of East Kirkton, West Lothian, Scotland

Plant fossils are a common and important element in the East Kirkton biota of Brigantian (late Visean age). The most important taxa are preserved as compressions or anatomically preserved as permineralisations. The basis of the quantitative study of the flora and the distribution of individual plant species was the trenched section excavated for the East Kirkton Project. The largest diversity of compressions have been recorded from loose blocks. In the trenched section, the uppermost ashes contain only lycopsid compressions including Stigmaria. Nodules in the underlying shales yield mainly lycopsid leaf and sporophyll compressions

A tree without leaves

The puzzle presented by the famous stumps of Gilboa, New York, finds a solution in the discovery of two fossil specimens that allow the entire structure of these early trees to be reconstructed.

Reconstructing the Architecture and Growth Habit of Pietzschia levis sp. nov. (Cladoxylopsida) from the Late Devonian of Southeastern Morocco

Pietzschia levis sp. nov. is represented by a 40‐cm‐long, anatomically preserved stem discovered in the locality of Mader el Mrakib, dated as Early Famennian by the associated marine fauna. Its assignment to the genus Pietzschia is based on the following characters: a dissected primary vascular system comprising a ring of discrete, radiating xylem plates encircling numerous smaller bundles; a large pith; several protoxylem poles in peripheral xylem plates that are not associated with parenchyma; sclerenchyma plates alternating with the peripheral xylem plates; absence of secondary tissues; and traces to lateral organs in groups, departing from more than two peripheral xylem plates. Anatomical characters that are unique to the new species are extensive aerenchymatous tissues occurring within the stem and bases of lateral organs; the small amount of vascular tissues; and the large number of peripheral xylem plates involved in emission of traces. The first two characters indicate ecological preferences of our new species for wetland habitats. Developmental studies of its anatomy indicate that P. levis was an erect plant that did not exceed 1 m in height. Its stem is conical and possibly oval in section. It has a determinate primary growth, and its lateral development is constrained by a lack of secondary tissues. There is no evidence for forked or digitate branching in distal parts. Lateral organs are inserted spirally and at acute angles. Beyond the fourth node, short and long internodes alternate. Bases of lateral organs are wide, bilaterally symmetrical, and decurrent over an extensive length. Small adventitious roots occur in the proximal part of the stem. Architecturally, P. levis resembles Lorophyton goense, a Middle Devonian taxon described from Belgium. Within the range of growth habits displayed by Devonian cladoxylopsids, P. levis is interpreted as a small form rather than as the juvenile stage of a larger arborescent type of plant.