Harlan P. Banks

Leclercqia complexa gen. et sp. nov., a new Lycopod from the late middle devonian of eastern New York

Abstract Leclercqia complexa gen. et. sp. nov. is described as a slender, herbaceous lycopod assigned to the family Protolepidodendraceae. Its laminar leaves are divided into an elongate, acuminate, often recurved central division and two opposite, divided, lateral divisions. Leaves are in a low spiral, eight to ten per gyre. Each has a single vein composed of tracheids. Lateral branches from this vein extend only to the base of each lateral division. The main vein extends to tip of leaf. Stomata are slightly sunken and scattered. The xylem strand is a solid rod with fourteen to eighteen protoxylem ridges; maturation is exarch. Protoxylem is composed of annular, spiral, and reticulate elements. Metaxylem is composed of scalariform elements and tracheids with elongate to oval and uni- to multiseriate bordered pits. Leaf traces rise from protoxylem ridges and extend obliquely to bases of leaves. Sporangia are globose to elliptical, attached to sporophylls by a pad of tissue just proximal to the lateral segments and have sporangia on their walls. Dehiscence occurs along the upper margin of the sporangium and is parallel to midline of sporophyll. Groups of sporophylls alternate with vegetative leaves. Spores in situ 60–85 μ in diameter exclusive of ornamentation, with curvaturae perfectae, spinae 5–9 μ long on an equatorial ridge, and biform ornamentation on distal and proximal faces. Plant is eligulate and probably homosporous.

The oldest vascular land plants: A note of caution

Abstract The oldest fossil proven to be a vascular land plant is Cooksonia , from the Pridolian Stage of the Silurian Period in Wales. Microfossils macerated out of older Silurian strata, such as sheets of cuticle, “tubes” usually called tracheid-like elements or even plant tracheids, and spores with a resistant exine and a trilete mark, have been used to suggest the occurrence of vascular plants prior to the Pridolian. It is pointed out that these microfossils may originate from algae, bryophytes, or groups now extinct or from animals. Hence they alone cannot demonstrate older vascular plants. The microfossils need extensive comparative study on a world-wide basis.

Serrulacaulis furcatus gen. sp. nov., a New zosterophyll from the lower upper devonian of New York state

A new zosterophyll is described from the lower Upper Devonian of New York. Serrulacaulis furcatus is characterized by two rows of emergences which are opposite in arrangement and attached on opposite margins of the axis. They are deltoid in side view and are contiguous vertically, giving the stem a scalloped or saw-toothed appearance. Epidermis over much of the plant consists of elongate cells and interspersed isodiametric cells. The vascular strand consists of spirally thickened tracheids. Rhizomes are densely covered by rhizoids that are more numerous on small papillae and at the apex of tooth-like emergences. Aerial axes terminate in circinate apices. Some axes bear short-stalked, reniform sporangia arranged alternately in two rows on one side of the axis only. The dehiscence zone is a thickened rim over the distal, convex margin of the sporangium. Dehiscence is basipetal, producing two equal valves. Spores are smooth to slightly granulate, circular to subtriangular, with a trilete mark that extends 13–12 the radius and is surrounded by a darkened triangular area. Sawdonia ornata, described earlier from the same quarry, and now Serrulacaulis, are the two genera of zosterophylls proven to occur in Late Devonian time. The world-wide distribution of zosterophylls and the fact that the group is a natural one are reviewed.

The role of psilophyton in the evolution of vascular plants

This paper lists the evolutionary advances that Psilophyton (Trimerophytina) shows over the simplest vascular plants like Cooksonia and Rhynia (Rhyniophytina). Both morphological and anatomical (using Psilophyton dawsonii, the only completely petrified example) advances are treated. Similarly Psilophyton is shown to be less advanced than some groups presumed to be derivatives of trimerophytes. Evolution within the genus Psilophyton and within Trimerophytina is discussed briefly.

Evolution — an Overview

We might continue from Carboniferous time to the present detailing the new types of plants that evolved. But it may be more instructive to stop, examine the whole picture, and pose another question. Has evolution proceeded at a uniform rate since the beginning or have there been some major bursts of activity?

Plants, Their Fossilization, and Techniques of Fossil Study

If you have ever watched moving water carry sand grains, silt particles, twigs, seeds, leaves, or in flood time large logs and even houses, you have watched what may be a preliminary stage in the formation of fossils. When the stream slows down as it meets a body of standing water, such as a pond, lake, or sea, its load begins to be dropped or to sediment out. The heavier particles are dropped first, closer to shore, and the lighter particles are dropped farther from shore. Changes in the rate of flow of the moving water produce variations in the size and type of particles deposited over any particular area. In this way a delta is built up (Fig. 1-1). A vertical section through the deltaic deposits may reveal layers of varying thickness and constitution, some sandy, some muddy, some with otherwise mixed particles. Fig. 1-2 shows rock strata that were built up in this way.

Comparative morphology and the rise of paleobotany

Abstract In the past half century paleobotany in North America has progressed from a subdiscipline of comparative morphology to a broadly based, analytical, interpretative, sometimes experimental subject. Major advances have been made in the area of early land plants, Carboniferous floras, palynology, Precambrian evolution and angiosperms. Important new directions have been initiated, e.g., the use of cladistics, geochemistry, broad surveys of changes in diversity, analyses of radiations and extinctions.

Angiosperms — the Culmination of Plant Evolution

The precursors of angiosperms are still unknown. All we know is that angiosperms appeared in Lower Cretaceous strata (see Fig. 2-2, page 19), became abundant and varied by Late Cretaceous time and evolved by Eocene time many modern genera. Reports of pre-Cretaceous angiosperms are usually based on fragmentary evidence such as a leaf, a pollen grain, or a bit of secondary wood. None of these specimens can be unequivocally called an angiosperm rather than a gymnosperm, and paleobotanists generally remain skeptical of them. Morphologists, on the other hand, are inclined to view the complexity of the flower and the embryo sac as well as the wide variety of early angiosperms in the Cretaceous Period, as evidence that the group must have had a long period of evolution prior to Cretaceous time. Axelrod suggested that the first angiosperms arose on highlands far back from the coastal plains and so were not preserved as fossils. But unless someone makes the fortunate discovery of an unequivocal pre-Cretaceous angiosperm we must use the available evidence to learn what we can of the first angiosperms and their subsequent evolution.

The Riddle of the Pine Cones

The male cones of modern pines appear to be simple structures composed of microsporophylls arranged spirally on an axis (Fig. 8-3). The axis of the female cone, in contrast, bears what appear to be megasporophylls, but each of them is subtended by a leaflike structure (Fig. 8-2). The female cone therefore appears to be a compound structure. Morphologists’ repeated attempts to understand the difference have been interesting and instructive. But the solution provided by paleobotanical studies deserves special attention here. The leafy twigs and the cones of modern pines should be viewed first, though, to introduce sufficient terminology for understanding the fossils. The modern plants have the added advantage of being readily available to supplement diagrams of extinct forms.

The Invasion of the Land

The early manifestations of life detailed in Chapter 2 and those plants described in Chapter 3 gave rise to organisms from which land plants might evolve. A detailed study of early vascular plants, then, should shed light on the speed and direction of their evolution. The patterns of early vascular plant evolution have long fascinated botanists, and there is voluminous literature on the subject. However, many conclusions that have been drawn suffer from two serious deficiencies. First, the number of plants proven to be vascular is small, and details of their structure are extremely limited. Second, the precise age of the rock strata whence they have been collected has often been unknown or unrecorded, or has been revised in accordance with new data provided by animal paleontology and stratigraphy. The viewpoint presented here relies on newer studies that have added markedly to our knowledge of the plants themselves and on more recent analyses of the age of the various subdivisions of Devonian time (Table 4-1). Underlying the various descriptions that follow are the ever-present questions: Do the early plants resemble those we know today? How rapidly did the first land plants become diversified? Are there some early plants which appeared and then became extinct? Can we find early plants that were successful on the new terrestrial environment and from which other forms may be descended?