First Wetlands

425–390 million years ago (Late Silurian–early Middle Devonian)

What plants were dominant during this time?

  • Around 440 million years ago, (early Silurian Period) the edges of streams and ponds were lined with small plants, like liverworts and mosses.

  • These earliest plants (bryophytes) do not have the ability to grow very tall, or branch in their spore-bearing stems.

  • From these lowly ancestors, plants evolve the ability to branch, which was a profound adaptation!

    • Branching of stems was fundamental for allowing plants to create a great diversity of shapes and forms.

  • This allowed plants to become specialized: some stems to be used for creeping along the ground, others for upright support, or reproduction, or photosynthesis

  • It also allowed plants to greatly increase in plant size, and allowed plants the ability to disperse many, many spores over great ranges.

  • The earliest branching plants were just green stems without leaves (e.g. Aglaophyton). These plants had equal-branching where the stems looked "Y"-shaped. At the tips of their stems were spore cases.

Above: A reconstruction of Aglaophyton

Above: A reconstruction of a rhyniophyte, Rhynia

Were these "vascular plants"?

  • There were several plant groups that looked similar (i.e. equal-forking branches, no leaves, and spore cases), but internally there were changes occurring.

  • The earliest branching plants were non-vascular, or lacked special cells to move water and sugars (i.e. xylem and phloem).

  • Some early plants, like Aglaophyton, show traces of simple water-conducting cells. These plants are called "pre-vascular plants"

  • Shortly after, true vascular plants appear on the landscape. They possess specialized cells (xylem) to efficiently move water from the "roots" to areas of photosynthesis.

  • These plants were known as the rhyniophytes, such as Rhynia, and they are the earliest vascular plants to appear on Earth.

How do leaves arise?

  • Around 420 million years ago, a group of plants called the lycophytes arose from some rhyniophyte-like ancestor.

  • The earliest Lycophytes were the zosterophylls, which displayed a different branching pattern and placement of the spore cases.

  • Zosterophylls, like Sawdonia, had unequal-branching, larger amount of xylem, and their spore cases were arranged along the stems, instead of at the tips.

  • These features probably allowed zosterophylls to get taller than earlier plants, such as Aglaophyton or the rhyniophytes.

  • Zosterophylls had small spines along their stems, which may have been used for support, or possibly photosynthesis, but these were not leaves.

  • Some researchers think that, over time, these spines became the earliest leaves


Left: A reconstruction of a zosterophyll, Sawdonia

Right: A reconstruction of zosterophylls Sawdonia and Drepanophycus

First leaves appear with the clubmosses

  • The lycophytes also include the clubmosses, which are still alive today

  • Early clubmosses evolved the first leaves; which were small, but numerous on the plant

  • These small leaves are arranged in a spiral fashion, giving them a moss-like appearance, but they frequently have club-like cones at the top, thus the name "club-moss".

  • The origin of leaves allowed plants to create efficient "solar collectors": leaves increase the amount of light that a plant could collect for photosynthesis, without increasing the bulk volume of plant material since the leaves are wide and flat.

  • Fossils of these plants look similar to modern clubmosses, such as Huperzia, that adorn northern forest floors.

Where did these plant live?

  • These earliest land plants (i.e. Aglaophyton, rhyniophytes, lycophytes) lived close to the edges of ponds and freshwater lakes.

  • In modern terms, we would call this early ecosystem a marsh: areas of saturated soil or shallow water covered by soft green (herbaceous) plants.

  • A marsh is just one type of wetland: areas where water meets land; these can be freshwater or saltwater areas.

  • Examples of wetlands are marshes (herbaceous-dominated wet areas), swamps (tree-dominated wet areas), bogs (peat-dominated wet areas), ponds/lakes (deeper waters), rivers/streams (flowing waters), reefs/beaches (coastal marine waters), and estuaries (brackish waters).

  • These plants set the stage for later plants that exhibited complex branching patterns and plants that grew to tree-size.

Above: The living clubmoss, Huperzia lucidula

When did the first trees evolve?

What was the Earth like before this time?