Zosterophyllophyta

Categories: Evolution; paleobotany; seedless vascular plants; taxonomic groups

Today, scientists believe that zosterophyllophytes and lycopods arose at about the same time from a common but unknown ancestor. The zosterophyllophytes went extinct, but the lycopods have survived until today.

Characteristics

The Zosterophyllophyta evolved independently from the Rhyniophyta, but both groups shared a number of characteristics. Their aerial stems arose from a horizontal axis, the rhizome, when one of two branches formed by the forking rhizome turned and grew upward. Some of the zosterophyllophytes had no leaves, the common condition in the rhyniophytes. The two groups were distinguished by sporangial position and the presence or absence of a coiled stem that grew in length by unrolling like a New Year’s Eve noisemaker. In the rhyniophytes, the sporangia were borne at the branch tips. In the zosterophyllophytes, the sporangia were borne on short branches along the sides of the stem. Growth from a coiled stem is characteristic of most zosterophyllophytes but does not occur in the rhyniophytes (or lycopods).

The zosterophyllophytes formed dense stands that could cover hundreds of square meters. Most of the plants existed in a vegetative state (lacking reproductive organs) for most of their lives. Their ability to grow and reproduce asexually through K- and H-branching allowed them to spread and dominate an area (a characteristic known as turfing). H-branching occurred when the aerial stem forked twice in rapid succession. One of the two branches formed by the first forking was short and ran parallel to the ground. This short axis forked again, and one of the new branches turned upward, while the other returned to the ground. Viewed from the side, the pattern resembled a capital H. In K-branching, a short branch on the rhizome forked, giving rise to two new axes. The upper axis was coiled and became a stem. The lower axis was not coiled and became a root. Viewed from the side, the branching pattern resembled a capital K.

Taxonomy

Zosterophyllophyta contains two orders: the Zosterophyllales, whose members have completely naked stems, and the Sawdoniales (also called prelycopods), whose aerial stems bear flaps of photosynthetic tissue (enations) that lacked vascular tissue. Typical members of the Zosterophyllales are Zosterophyllum and Rebuchia. Members of the Sawdoniales include Bathurstia, Crenaticaulis, and Serrulacaulis.

Zosterophyllales

All members of this order have naked stems which bear clusters of oval to kidney-bean-shaped sporangia on short side branches at the stem tip. Depending on the species, the sporangia may be arranged either spirally, like the red line on a barber pole, or in two vertical rows. For most species of Zosterophyllum, only the aerial stems are known. The lower portions of these stems lack a cuticle (a surface covering secreted by the plant to retard water loss). Since a cuticle was absent, the lower portions of Zosterophyllum probably grew in standing water. Although the aerial stem normally branched by forking, occasionally H-branching also occurred. The oldest fertile specimens (Z. myretonianum and Z. fertile) are from 412 million to 406 million years old. In a specimen of Zosterophyllum, from Bathurst Island in Arctic Canada, the aerial stems grew from a horizontal rhizome that bore rootlike structures on its lower surface.

Rebuchia resembles Zosterophyllum in all features except sporangial position. The sporangia of Rebuchia are borne on short side branches at the stem tip. They arise in two rows on opposite sides of the stem, but the stalks on which they are borne curve so that the sporangia all lie on the same side of the stem when mature.

Sawdoniales

Members of the Sawdoniales have spherical sporangia that are located on short side branches at various places along the stem. Flaps of photosynthetic tissue (enations) are present. These lack vascular tissue and do not influence the growth of the adjacent vascular tissue in the stem. The enations are arranged either randomly or in one or two rows. The sporangia are not associated with the enations. In the lycopods, the sporangia sit on the upper surface of vascularized leaves (called sporophylls), which are spirally arranged. Bathurstia, Crenaticaulis, and Serrulacaulis are typical sawdonialeans whose wedge-shaped enations lacked vascular tissue.

The enations of Bathurstia are arranged randomly on the stem. The sporangia are grouped together in two rows on opposite sides of the stem tip. Bathurstia exhibited K-branching. In Crenaticaulis, two rows of triangular, rounded (that is, crenate) enations were located on opposite sides of the stem. Subordinate branches, which were once thought to be rhizophores similar to those of Selaginella, are found just below each fork of the main axis. These subordinate branches are clearly stems, while rhizophores are rootlike. Therefore, the subordinate branches do not indicate a close relationship with the lycopods, as scientists once thought. Sporangia arose in two rows on opposite sides of the stem. Rootlike structures were found on some specimens of Crenaticaulis, and root hairs may be present. The gametophyte of Crenaticaulis was similar in appearance to Sciadophyton. In Serrulacaulis, two rows of triangular, pointed (that is, serrate) enations were located on opposite sides of both the aerial stem and rhizome. Sporangia occur alternately in two rows on one side of the stem. Rhizoids were seen coming from the rhizome.

Herbivory

The spinelike enations found on sawdonialeans were not adaptations to prevent insects from eating the plant (herbivory). All the known contemporary insect herbivores were too small to be affected by them. Wounds found on fossil plants are consistent with sap-sucking and not chewing insects. Insect coprolites (fossil fecal material) containing a mixture of vegetative plant cell types and spore masses are known but were produced by detritivores (animals that eat dead plant material). No evidence for any herbivore that chewed and digested living plant material has been found prior to 345 million years ago.

Asteroxylon mackiei

For some researchers, the most important member of the Sawdoniales (formerly Asteroxylales) was Asteroxylon, a plant from the Rhynia Chert of Scotland. The naked rhizome of Asteroxylon grew along the ground and branched by forking into two new axes. Because Asteroxylon was a large plant, its water and anchorage needs could not be met with rhizoids alone. Enationless, rootlike structures (possibly adventitious roots) depart from the rhizome and penetrate the soil. The aerial stem that arose from the rhizome had a single central axis from which the lateral branches arose. The aerial stem was densely covered with spirally arranged enations, which superficially resembled leaves but lacked vascular tissue. A vascular trace did leave the central vascular cylinder of the stem and traveled to the base of the leaf but did not enter it. Sporangia were found scattered among the enations. Each sporangium was borne on short stalk containing vascular tissue.

The sporangial position and overall appearance of Asteroxylon is very similar to that of the living lycopod, Huperzia (Lycopodium) selago. The sporangia of Huperzia are also scattered on short axes among the leaves of the stem. The enations of Asteroxylon lacked vascular tissue, a characteristic of the leaves (microphylls) of the lycopods, which initially prevented researchers from classifying Asteroxylon as a lycopod. By redefining a microphyll as a stem outgrowth which influences the growth of the adjacent vascular tissue in the stem, Asteroxylon can be classified as a lycopod because a vascular trace did run up to the leaf base although it did not enter the leaf. Vascular tissue might never have formed in the microphyll of Asteroxylon, or the vascular tissue that was once present might have been lost. Asteroxylon is now placed in the Drepanophycales along with Drepanophycus and Baragwanathia.

Baragwanathia

Baragwanathia‘s stem is covered with long, thin microphylls, each with a single vein. The sporangia are borne on the stem among the microphylls and appear to be spirally arranged. Baragwanathia may be the oldest lycopod, although this statement does cause controversy. The age of oldest specimens from Australia has been disputed. Some researchers believe that they are more than 414 million years old, while others claim that the sediments in which Baragwanathia was found are less than 406 million years old. If the older age is accepted, Baragwanathia is older than the simpler rhyniophytes (Aglaophyton) and zosterophyllophytes (Zosterophyllum) and contemporaneous with the cooksonioids. An older Baragwanathia supports an evolutionary origin for the lycopods distinct from that of the rhyniophytes and prevents the zosterophyllophytes from being the direct ancestors of the lycopods. Therefore, the Sawdoniales are mistakenly called prelycopods. The evolutionary development of microphylls through the sequence of naked stems, enations, and finally microphylls using Asteroxylon as an intermediate is also not possible. The similarity between the Sawdoniales and the lycopods is an example of convergent evolution.

Bibliography

Gensel, Patricia G., and Henry N. Andrews. Plant Life in the Devonian. New York: Praeger, 1984. Good descriptions of the zosterophyllophytes with valuable illustrations. Illustrations, bibliographical references, index.

Gensel, Patricia G., and Dianne Edwards. Plants Invade the Land: Evolutionary and Environmental Perspectives. New York: Columbia University Press, 2001. Provides the most current information on the zosterophyllophytes. Illustrations, bibliographical references, index.

Kenrick, Paul, and Peter R. Crane. The Origin and Early Diversification of Land Plants: A Cladistic Study. Washington, D.C.: Smithsonian Institution Press, 1997. Detailed cladistical treatment of the zosterophyllophytes. Excellent line drawings, bibliographical references, index.