Earth's First Wetlands

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. 

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 that were not considered true vascular tissue. These plants are called "para-vascular plants."

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

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

Around 420 million years ago, a group of plants called the lycophytes arose from some rhyniophyte-like ancestor (e.g., Cooksonia cambrensis)

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

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.

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.