The Evolution of a Helicopter

Stevenson, Robert A., Dennis Evangelista, and Cindy V. Looy. "When conifers took flight: a biomechanical evaluation of an imperfect evolutionary takeoff." Paleobiology 41.2 (2015): 205-225. [SOURCE]

The whirring helicopter seeds of modern day conifers (as well as a handful of other tree species) are truly marvels of evolution. We humans have yet to top the simple efficiency of this form of locomotion. It is easy to see how such seed anatomy benefits a tree. Instead of plummeting to the ground and struggling under the shade of its parents, winged seeds are often carried great distances by the breeze. Such a dispersal mechanism is so effective that multiple tree lineages have converged on a single asymmetrical wing design of their samaras.  

The key to this type of seed dispersal lies in the movement of the seed in the air. The whirring motion allows the seeds to stay airborne as they are carried away from their cones. It would be all too easy to argue that any intermediate must be doomed to failure. However, this is not the case. A rich collection of 270 million year old fossils discovered in Texas is shining light on how at least one lineage of conifers settled in on this wonderful adaptation for seed dispersal. 

Artists reconstruction of a seed-bearing shoot of Manifera talaris. Drawing by Ivo Duijnstee

Instead of producing one type of winged seed, an ancient species of conifer known scientifically as Manifera talaris produced multiple different samara designs. Some were symmetrical, others were double winged, and still others matched what we would readily recognize as a samara today. It would seem that early conifers were “trying out” many different forms of wind dispersed seed designs. Manifera talaris was alive during the early Permian. At that time, there were not many animals alive (that we are aware of) that could function as seed dispersers for conifers. Instead, these early trees relied on the wind to do the work for them. 

Though these fossils offer a unique window into the evolution of winged seeds, they do not give any indication as to how each seed designs would have performed. For paleobotanist Dr. Cindy Looy, this meant a chance to have a little fun with science. She and her colleagues built functional paper models of each of the samara types represented in the fossils. By attaching the paper wings to comparably sized seeds from an extant conifer, she was able to test the flight performance of each of these samara types. What she found was quite interesting. 

As it turns out, symmetric and asymmetric double-winged seeds performed quite poorly. They fluttered to the ground, barely achieving any rotation. Contrast this with the asymmetric single-winged seeds, which stayed airborne for twice as long as any other samara design. What this research shows is that early conifers were, in a sense, "experimenting" with different samara designs. Those designs that allowed for greater seed dispersal produced more trees that did the same. 

Photo Credit: Dr. Cindy Looy

Further Reading: [1]

Ancient Equisetum

Photo by Christian Ostrosky licensed under CC BY-NC-ND 2.0

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.) Photo by Chad Husby licensed under CC BY-NC-ND 2.0

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.

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 by born1945 licensed under CC BY 2.0

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

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

Amber Fossils of Grain

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

Photo by Shizhao licensed under CC BY-SA 2.5

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

Cooksonia: A Step Into the Canopy

Photo by Steel Wool licensed under CC BY-NC-ND 2.0

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. 

Photo by Sabrina Setaro licensed under CC BY 2.0

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]

Carboniferous Remains

My recent fossil hunting excursion has found me a bit obsessed with the paleobotanical world. As a child, I cut my teeth on biology through the lens of fossilized remains of ancient sea creatures. However, this trip turned up something quite special for a plant lover like myself. Among seams of coal we were finding a bewildering amount of fossilized plant life. However, these were no ordinary plants. What we were finding were the remains of some of the earliest forests on our planet. The rocks we were searching in date back to the Carboniferous Era, over 300 million years before the present. For most of the day I was quite excited by what looked like fossilized fern fronds. It was only after I got home and did some research that I realized how special these fossils really are. 

Illustration by Styracosaurus Rider

Though they resemble the ferns we find throughout the world today, these were not ferns as we know them. What we had found eternalized by mineralization were the remains of a group of plants that no longer exist. These, my friends, were the seed ferns. This lineage arose during the Devonian but really hit their peak abundance during the Carboniferous and Permian eras. Evolutionarily speaking, the seed ferns sit somewhere between true ferns and cycads on the tree of life. The clue to their position here are the presence of seeds, something ferns don't have but cycads do. The seed ferns are only distantly related to ferns and more closely related to cycads. Many believe they represent the earliest gymnosperms. 

Hanging off of the branches are what are referred to as "cupules."Inside the cupules sat the embryos. It is believed that these were wind pollinated as most fossilized cupules show an opening at one end where pollen would have entered. As it stands currently, the seed ferns are the first plants known to reproduce via seeds and therefore represent a major step in the evolution of plants as a whole. Seeds meant that unlike ferns, which require moist conditions to reproduce, the seed ferns were able to colonize a wider swath of habitat types, especially those that were much drier than the swamps in which terrestrial plants evolved.

Why the seed ferns went extinct while both ferns and cycads didn't is not entirely clear. It is likely that competition with more derived seed bearing plants certainly didn't help matters. Despite their disappearance, the legacy of seed ferns lives on today in the fossil fuels we are so hopelessly addicted to and the fossil evidence they leave behind. At the very least, pondering over their fossilized remains is something everyone should take the time to do. 

Photo Credit: [1] [2]

Further Reading: [1] [2] [3] [4]

Carnivores in Amber

Carnivorous leaves from Eocene Baltic amber. (A) Overview of the leaf enclosed in amber showing the adaxial tentacle-free side in slightly oblique view and stalked glands at the margin and on the abaxial side; arrowhead points to the exceptional long tentacle stalk with several branched oak trichomes attached. (B) Overview of the leaf enclosed in amber, showing abundant tentacles on the abaxial side. (C) Margin of abaxial leaf surface with tentacles of different size classes and nonglandular trichomes [SOURCE]

Carnivorous plants are marvels of evolution. Adapting to nutrient poor conditions, these botanical curiosities have evolved myriad ways of capturing and digesting prey. For all of their extant diversity, the fossil record of carnivorous plants over the eons is pretty much non existent save for some highly contentious fossils from China as well as some fossilized seeds of the aquatic carnivore, Aldrovanda. However, a recent discovery out of Russia changes everything. Beautifully preserved in amber, we now have the first conclusive fossil evidence of a carnivorous plant.

The amber was found in a mine in Russia and is estimated to be between 35 and 47 million years old, during an epoch known as the Eocene. Inside are beautifully preserved leaves of what seems to be a species of Roridula. The leaves clearly show specialized stalked glands with a pore at the tip. The researchers who discovered the amber also found evidence of the sticky secretions that were used to capture its prey.

Overviews showing the tentacle-free adaxial surface and tentacles along the leaf margins (B & C). (D) Partial leaf tip showing different size classes of stalked glands. [SOURCE]

The resemblance of these leaves to the leaves of extant Roridula is uncanny. Modern Roridula do not directly digest their prey. Instead, they rely on a symbiotic relationship between a species of bug, which lives on the leaves without getting stuck. The bugs hunt down and eat trapped insects. As they eat, the bugs defecate and it is their nitrogen-rich feces that the plants absorb for sustenance. It is quite possible that the fossilized Roridula also relied on these insects as well, though no direct evidence of this was found. 

The most interesting aspect of this discovery is its location. Today, Roridula is found only in South Africa. Its presence in Russia hints at a historic distribution that is much wider than previously thought. It has long been assumed that Roridula is a neoendemic to South Africa, with the family having arisen there and nowhere else. This discovery now shows Roridula to be a paleoendemic, once having a much wider distribution but currently restricted to South Africa. This discovery is an excitingly huge step in our understanding of carnivorous plant evolution. 

Morphological comparison of the carnivorous leaf fossils from Baltic amber (Left) and extant Roridula species (Right). (A and B) Leaf tip ending in a sole tentacle. (C and D) Stalked glands of different size classes. (E and F) Hyaline unicellular nonglandular trichomes. (G and H) Epidermal cells and stomata. (I–L) Multicellulartentacles. (A, C, E, and G) (I and J). (B, D, K, and L) R. gorgonias. [SOURCE]

Photo Credit: Alexander R. Schmidt, University of Göttingen

Further Reading: [1]