Stephen E. Scheckler

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.

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.

Miospore lateral distribution in upper Famennian alluvial lagoonal to tidal facies from eastern United States and Belgium

Abstract Miospore assemblages from continental rocks of mid late Famennian age of the Hampshire Formation of Virginia and West Virginia, U.S.A., are compared with those from the nearly coeval marine facies of the Ourthe Valley, Belgium. Eighteen spore taxa from the type Famennian 2c section in Belgium are also found in the continental beds of Virginia and West Virginia. Nearshore sediments carry more uniform and richer miospore assemblages than any one continental facies. Offshore currents or barriers, however, can alter the mixture of miospores coming from various land source areas by different drainages. Onshore autochthonous and allochthonous beds generally show correspondence between the recovered megafossils and the miospore assemblages. But the miospore assemblages enable us to distinguish between deposits resulting from short-term flooding episodes, those of autochthonous upland backswamps and deltaic marshes. We can thus distinguish upstream from downstream environments in our continental samples. Near-shore sediments in Belgium show a dominance of upstream over downstream derived miospores that suggests frequent strong floods in their fluvial systems.

Devonian spore ultrastructure: Rhabdosporites

The fine structure of in situ Rhabdosporites langii miospores is described. The trilete spores are characterized by an irregular pseudosaccus that is conspicuous on the distal surface of the grain; ornamentation consists of narrow coni. The central body of the spore is constructed of superimposed lamellae, each up to 50 nm thick. Pseudosaccus formation occurs as a result of a separation of the outer one third of the sporoderm of the central body. As development continues small perforations are formed in the pseudosaccus wall where lamellae separate. The development of the saccus, protosaccus and pseudosaccus is compared, and the biological significance of camerate grains discussed.