Ancient lycophytes included giant trees that dominated coal swamps about 320 million years ago.

Fossils show that ancient lycophytes evolved additional features convergently with seed plants, including a vascular cambium and cork cambium that enabled them to form trees up to 40 m tall (Fig. 33.9). Swamps that formed widely about 320 million years ago were dominated by tree-sized lycophytes. Unlike the vascular cambium of seed plants, the vascular cambium of these lycophytes (Fig. 33.10) produced relatively little secondary xylem and no secondary phloem. Instead, the giant lycophytes relied on thick bark for mechanical support. Thus, tree-sized lycophytes were markedly different from the trees familiar to us today.

image
FIG. 33.9 Giant lycophytes. (a) Artist’s depiction of giant lycophytes. (b) A fossil grove of lycophyte trees showing rooting structures and thick stems.

696

HOW DO WE KNOW?

FIG. 33.10

Did woody plants evolve more than once?

OBSERVATIONS Today, the vascular cambium occurs almost exclusively among seed plants, with only limited development of secondary xylem in some adder’s tongue ferns and quillwort lycophytes (Isoetes species). The fossil record, however, shows that other plants, now extinct, also had a vascular cambium.

HYPOTHESIS The vascular cambium evolved convergently in several different groups of vascular plants.

EXPERIMENT AND RESULTS The vascular cambium is recorded in fossils by xylem cells in rows oriented radially in the stem or root. Thus, the giant tree lycophytes of Carboniferous coal swamps (the top photo) had a vascular cambium, as did extinct tree-sized relatives of the horsetails (the bottom photo) and other extinct horsetail relatives. Phylogenetic trees generated from morphological features preserved in fossils unambiguously show that woody lycophytes, woody horsetail relatives, and the group of seed plants and progymnosperms did not share a common ancestor that had a vascular cambium. Moreover, anatomical research shows that the vascular cambium of extinct woody lycophytes and the giant horsetails Archaecalamites and Calamites generated secondary xylem but not secondary phloem, unlike the vascular cambium of seed plants. Living horsetails do not have a vascular cambium, but fossils show that they are descended from ancestors that did make secondary xylem and have lost this trait through evolution.

image
FIG. 33.10
Photos source: Andrew Knoll, Harvard University.

CONCLUSION Fossils and phylogeny support the hypothesis that the vascular cambium and, hence, wood evolved more than once, reflecting a strong and persistent selection for tall sporophytes among vascular plants.

SOURCE Taylor, T. N., E. L. Taylor, and M. Krings. 2009. Paleobotany. Amsterdam: Elsevier.

These forest giants were not outcompeted by seed plants, but persisted for millions of years in swampy environments alongside early seed plants. It was environmental change that spelled their doom. When changing climate dried out the swamps, the large lycophytes disappeared, along with their habitat. Little Isoetes was left as the only living reminder of a once-dominant group of plants.

These ancient forests left one other legacy: the coal deposits we mine today. As trees died and fell over into the swamp, their bodies decomposed slowly. Over time, as material accumulated and became buried, the combined action of high temperature and pressure converted the dead organic matter first into peat and then into the carbon-rich material we call coal. Thus, a major source of energy we use today derives originally from photosynthesis carried out by lycophyte trees.

697