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]

Amber Fossils of Grain

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In what may be one of the most interesting fossil discoveries in recent years, scientists from Oregon State University have described the earliest fossil evidence of grasses. Encased in 100 million year old amber this ancient grass spikelet suggests grasses were already around in the early to mid Cretaceous period. This is some 20 to 30 million years earlier than previous estimates for grass evolution. If that isn't cool enough, the grass appears to have been infected by a fungus related to ergot (the darker portion at the top), showing that this parasitism may be as old as grasses themselves. 

We humans have a long history with ergot's fondness for grasses. It is best known for producing the chemical precursors of LSD (as well as many other useful drugs) and has been implicated in some major historical events throughout our short time on this planet. However, suggesting that dinosaurs were getting high off the stuff is pushing it. Ergot likely evolved its chemical cocktail to deter herbivores from eating the grasses that it parasitizes. It has a bitter taste and cattle are said to avoid grasses that have been infected by it. It is quite possible that dinosaurs probably did the same thing. 

Either way, this finding represents a major milestone in the understanding of one of the most important plant families on the planet. Following the mass extinction at the end of the Cretaceous, grasses quickly rose to dominate roughly 20% of global vegetation. This little piece of amber now suggests that dinosaurs and their neighbors likely had a role in shaping this plant family. 

Photo Credit: Oregon State University

Further Reading: [1]

An Abominable Mystery

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We all love flowers but for all the attention we pay them, their origin remains elusive. Darwin called their sudden appearance in the fossil record an “abominable mystery.” Since Darwin's time, we have been able to clarify that picture a little bit. Even so, our understanding of the origin of the angiosperm lineage is dubious at best. When and why did flowers evolve?

For millions of years the land was dominated first by ferns and their allies and then by gymnosperms like cycads and gingkos. It was not until the Cretaceous that angiosperms began to rise to their current place as the dominant and most diverse group of plants. Their sudden appearance on the scene has been largely shrouded in mystery. There is scant fossil evidence to illustrate the early evolutionary steps in this development of flowers. Many paleobotanists believed that flowers had their origin in shrub-like ancestors of gymnosperms. Others felt that the origin of flowers belonged with the seed ferns (http://bit.ly/1zKfriM).

Around 2001 a fossil discovery from Yixian Formation, Liaoning, China was believed to have changed all of that. A researcher by the name of Ge Sun had stumbled upon a very primitive looking fossil plant. To his surprise, the reproductive structures seemed to show stamens in pairs below carpels and a lack of petals and sepals. The formation in which the fossil was found dated back to the Jurassic period. Could this represent the remains of the earliest flowers?

The fossil has been coined Archaefructus and since its discovery at least two species have been identified. Archaefructus was an aquatic plant, likely living on the edge of freshwater lakes. These fossils (as one would expect) are quite contentious. Some argue that it is more derived than would be expected from the first flower. Recently it has been suggested that Archaefructus is a sister lineage to early flowering plants, not unlike Nymphaeales or Amborella living today. 

What Archaefructus does suggest is that flowers had their origin much earlier than the Cretaceous. Other discoveries from the same formation (ie. Archaeamphora longicervia) suggest that flowering plants were already diversifying at this time. So, if this is the case, when did flowers appear on the scene? Far from the smoking gun that a fossilized flower would represent, researchers are nonetheless finding tantalizing fossil evidence that places the origin of flowering plants all the way back to the Triassic. 

By examining Triassic microfossils, some researchers believe they have found fossilized pollen grains that are distinctly angiosperm in origin. I won't go into it here but extant examples show a major distinction between pollen from gymnosperms and pollen from angiosperms. If this is true, flowers may be way older than ever expected. For now, the jury is still out on this one. 

Flowers evolved for sex. We associate animals like bees, bats, and birds with flowers today but most of these lineages came much later in the game. Exactly what was around pollinating early flowers remains a bit of a mystery as well. Were the earliest flowers wind pollinated or was there some insect or even reptile that served the selection pressure necessary for their evolution? Only time and more fossil discoveries will tell. 

Photo Credit: Shizhao (Wikimedia Commons)

Further Reading:

http://www.sciencemag.org/content/296/5569/899.abstract?ck=nck&siteid=sci&ijkey=8dZ6zTqF606ps&keytype=ref

http://faculty.frostburg.edu/biol/hli/research/Eoflora.pdf

http://www.ohio.edu/people/braselto/readings/angiosperms.html

http://journal.frontiersin.org/Journal/10.3389/fpls.2013.00344/full

http://www.amjbot.org/content/96/1/5.abstract

The First Forests

Have you ever wondered when the first forests appeared on Earth? What were these first forests like? What kinds of trees made up these forests? A recent fossil discovery is helping to answer some of these questions.

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The story begins back in 1870's in a quarry located in the town of Gilboa, NY. Quarry workers dug up a literal "forest" of fossilized stumps. These stumps were in still-life position and they astounded the paleontological community. The rocks they were found in were of the middle Devonian era, which would make these stumps a relic of the oldest evidence of forests on the planet. The only problem was that the upper parts of these "trees" remained elusive. For over 130 years the true identity of these ancient trees remained a complete mystery. What were they? How did they live?

Fast forward to 2004. Linda VanAller Hernick and Frank Mannolini were searching another Gilboa quarry not too far from where the stumps were first discovered. There they uncovered the fossilized crown of a fern-like tree. The following year they extracted the trunk of a large tree from the same quarry. When they looked at what they had found they soon realized they had discovered the missing pieces to the Gilboa stumps! Putting the pieces together, the researchers realized that the crown, trunk, and stumps were all pieces of the same species. 

Coined "Wattieza," the trees are not trees as we know them today. These were relatives of the ferns. They reproduced with spores, not seeds. Forests of Wattieza grew during the Devonian, some 415 to 360 million years ago. This is a major milestone in the understanding of how plants and ecosystems evolved over time. The presence of forests most likely paved the way for more diversification because forests tend to create their own microclimates as well as foster new niches. 

Photo Credit: Frank Mannolini/New York State Museum and www.nature.com

Further Reading:
http://www.nature.com/nature/journal/v446/n7138/abs/nature05705.html

Cooksonia: A Step Into the Canopy

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For plants, the journey onto land did not happen over night. It began some 485.4–443.4 million years ago during the Ordovician. The best evidence we have for this comes in the form of fossilized spores. These spores resemble those of modern day liverworts. Under high powered microscopes, one can easily see that they were indeed adapted for life on land. These early plants were a lot like the hornworts, liverworts, and mosses we see today in having no vascular tissues for transporting water, an adaptation that would not come along for another few million years. 

Without vascular tissues, plants like liverworts and mosses cannot transport water very far. They instead rely on osmosis and diffusion to get water and nutrients to where they need to be, which severely limits the size of these types of plants to only a few centimeters. This growth pattern carried on well into the Silurian. Until then, the greening of our planet happened in miniature. 

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Around 415 million years ago, however, plants became vascularized. This changed everything. It set the stage for the botanical world we know and love today. Paleobotanists place the fossil remains of these newly evolved vascular plants in the genus Cooksonia. Based on what we would call a plant today, Cooksonia probably pushes the limits. However, in some species the branching structure is full of dark stripes, which have been interpreted as vascular tissues. It still wasn't a very tall plant with the tallest specimen standing only a few centimeters but it was a major step towards a much taller green world. 

Cooksonia did not have any leaves that we are aware of but some species certainly had stomata (another major innovation for water regulation in plants). Each branched tip ended in a sporangium or spore-bearing capsule. It has been suggested that Cooksonia may not represent an individual photosynthetic plant but rather a highly adapted sporophyte that may have relied on a gametophyte for photosynthesis. This hypothesis is supported by the diminutive size of many Cooksonia fossils. They simply do not have enough room within their tissues to support photosynthetic machinery. Because of this, some botanists believe that vascularization sprang from a dependent sporophyte that gradually became more and more independent from its gametophyte over time. Until an associated gametophyte fossil is found, we simply don't know. 

Photo Credits: Steel Wool (http://bit.ly/1AjLYh8) and Sabrina Setaro (http://bit.ly/16mdyxw)

Further Reading: [1] [2] [3]