The First Trees Ripped Themselves Apart To Grow

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A new set of fossil discoveries show that the evolutionary arms race that are forests started with plants that literally had to rip themselves apart in their battle for the canopy. The first forests on this planet arose some 385 million years ago and were unlike anything we know today. They consisted of a clade of trees known scientifically as Cladoxylopsids, which have no living representatives in these modern times. How these trees lived and grew has remained a mystery since their fossilized trunks were first discovered but a new set of fossils from China reveals that these trees were unique in more ways than one.

Laying eyes on a full grown Cladoxylopsid would be a strange experience to say the least. Their oddly swollen base would gradually taper up a trunk that stretched some 10 to 12 meters (~30 - 40 feet) into a canopy of its relatives. They had no leaves either. Instead, their photosynthetic organs consisted of branch-like growths that were covered in twig-like projections. Whereas most fossils revealed great detail about their outward appearance, we have largely been in the dark on what their internal anatomy was like. Excitingly, a set of exquisitely preserved fossils from Xinjiang, China has changed that. What they reveal about these early trees is quite remarkable.

As it turns out, the trunks of these early trees were hollow. Unlike the trees we know today, whose xylem expands in concentric rings and forms a solid trunk, the trunk of Cladoxylopsid was made up of strands of xylem connected by a network of softer tissues. Each of these strands was like a mini tree in and of itself. Each strand formed its own concentric rings that gradually increased the size of the trunk. However, this gradual expansion did not appear to be a gentle process.

As these strands increased in size, the trunk would grow larger and larger. In doing so, the tissues connecting the strands were pulled tighter and tighter. Eventually they would tear under the strain. They would gradually repair themselves over time but the effect on the trunk was quite remarkable. In effect, the base of the tree would literally collapse in on itself in a controlled manner. You could say that older Cladoxylopsids developed a bit of a muffin top at their base. 

A cross section of a Cladoxylopsid trunk showing the hollow center, individual xylem strands, and the network of connective tissues.

A cross section of a Cladoxylopsid trunk showing the hollow center, individual xylem strands, and the network of connective tissues.

Although this seems quite detrimental, the overall structure of the tree would have been quite sturdy. The authors liken this to the design of the Eiffel tower. Indeed, a hollow cylinder is actually stronger than a solid one of the same dimensions. When looked at in the context of all other trees, this form of growth is pretty unique. No other trees are constructed in such a manner.

The authors speculate that this form of growth may be why these trees eventually went extinct. It would have taken a lot of energy to grow in that manner. It is possible that, as more efficient forms of growth were evolving, the Cladoxylopsids may not have been able to compete. It is anyone's guess at this point but this certainly offers a window back into the early days of tree growth. It also shows that there has always been more than one way to grow a tree.

Photo Credits: [1] [2]

Further Reading: [1]

Fossilized Forest

A fossilized forest discovered in Arctic Norway is shedding light on one of the earliest forests to have evolved on this planet. Preserved in situ, these fossils reveal what life was like for these plants some 380 million years ago.

This was the Devonian Era, a time in which plants were starting to conquer the land. During this time period, the land mass that is now Norway was located on the equator. The tropical climate of this time likely fostered the growth of these early forests, causing a race for the sky. For their time, these forests were monstrous in proportion.

The fossils are comprised of a long extinct species of plant known scientifically as Protolepidodendropsis pulchra. These "trees" were not any sort of tree that we would readily recognize. To see their closest living relatives today, you will have to take a knee. These were forebearers of the club mosses (Lycopodiaceae).

These forests stood around 4 meters (13 feet) in height. Even more peculiar, they grew densely packed with only about 20 centimeters separating each trunk. The trunks themselves were stunning having been covered in diamond-shaped plates. Like the club mosses, they reproduced by spores.

Another interesting thing about such discoveries is that it allows us to infer quite a bit about what was going on in the atmosphere as well. With such densely packed forests spreading over the land, the Devonian world was, for the first time, seeing a massive drawdown of atmospheric CO2 levels. Plants were changing the globe as they rose to prominence. Along the way, they were irrevocably changing the course of life on Earth.

Photo Credits: Cardiff University, Illustration by Dr. Chris Berry from Cardiff University

Further Reading:
http://geology.geoscienceworld.org/content/43/12/1043.full

Ancient Equisetum

Whenever you cross paths with an Equisetum, you are looking at a member of the sole surviving genus of a once great lineage. The horsetails, as they are commonly called, hit their peak during the Devonian Era, some 350 + million years ago. Back then, they comprised a considerable portion of those early forests. Much of the world's coal deposits are derived from these plants.

The horsetails once towered over the landscape, reaching heights of 30 meters or more. Today, however, they have been reduced to mostly small, lanky plants. The tallest of the extant horestails are the giant horsetail (Equisetum giganteum) and the Mexican giant horsetail (Equisetum myriochaetum) of Central and South America. These two species are known to reach heights of 16 ft. (4 m.) and 24 ft. (7 m.) respectively. Certainly an impressive site to see.

Equisetum giganteum (Chad Husby for scale.)

Equisetum giganteum (Chad Husby for scale.)

As a genus, Equisetum is composed of somewhere around 20 species, with many instances of hybridization known to occur. Most species tend to frequent wet areas, though dry, nutrient poor soils seem to suit some species just fine. The horsetails are known for their biomineralisation of silica, earning some the common name of "scouring rush." Settlers used to use these plants to clean their pots and pans. However, this is certainly not why this trait evolved. It is likely that the silicates have something to do with structural support as well as physical protection against pathogens. More work needs to be done looking at the benefits rather than the mechanisms involved.

Equisetum myriochaetum

Equisetum myriochaetum

Though they are not ferns, horsetails are frequently referred to as "fern allies." This is due to the fact that, like ferns, horsetails are not seed plants. Instead, they produce spores and exhibit a distinct alternation of generations between the small, gamete-producing gametophyte and the tall spore-producing sporophyte. Spores are produced from a cone-like structure at the top of the stem called a stobilus. This may be attached to the photosynthetic stem or it can arise as its own non-photosynthetic stem. Either way it is an interesting structure to encounter and well worth studying under some form of magnification.

Despite their diminutive appearance, many horsetails are quite hardy and thrive in human disturbance. For this reason, horsetails such as E. hyemale and E. arvense have come to be considered aggressive invasive species in many areas. They thrive in nutrient poor soils and their deep, wide-ranging rhizomes can make control difficult to impossible. There is something to be said for these little plants. Love them or hate them, they have stood the test of time. They were some of the first plants on land and it is likely that some will be here to stay, even if we go the way of the Devonian forests.

Photo Credits: [1] [2] [3] [4]

Further Reading: [1] [2] [3]