History of Grass Evolution Written in Dinosaur Poop

Photo by Sugeesh licensed by CC BY-SA 3.0

Grasses dominate our planet today but that has not always been the case. Because of both their ecological and cultural importance , the origin and diversification of grasses has long been a hot topic in biology. We know that grasses really hit their stride following the extinction of the dinosaurs, and that they changed herbivore anatomy in a big way, but their origins remain shrouded in mystery. Recently, a discovery made in fossilized dinosaur poop has shone a surprisingly bright light into the history of grasses on our planet.

Prior to this discovery, the earliest evidence of grasses came in the form of fossilized pollen and tiny pieces of silica called phytoliths. Phytoliths are essentially tiny pieces of glass that serve as a form of defense against herbivores. Because they are made of silica and fossilize well, phytoliths turn up frequently in the fossil record. This makes them extremely useful for finding evidence of grasses even where whole-plant fossilization is unlikely.

Illustration by Nobu Tamura (http://spinops.blogspot.com) licensed by CC BY-NC-ND 3.0

Whereas phytoliths are not unique to grasses, their form is often taxon-specific. With a good eye and a bit of training, one can look at a phytolith under a microscope and tell you what type of plant it came from. This is where the dinosaur poop comes into the picture. By examining fossilized dinosaur poop from India, paleontologists can get an idea of what dinosaurs were eating.

By examining the fossilized poop of a group of large herbivorous dinosaurs called Titanosaurs, paleontologists now have a better idea of grass diversity in the late Cretaceous. They have uncovered a surprising diversity of phytoliths, which demonstrate that at least 5 distinct grass taxa that we would recognize today were alive and well some 100.5 to 66 million years ago. These include extant groups like Oryzoideae (think rice and bamboo), Puelioideae, and Pooideae (think wheat, barley, oat, rye, and many lawn and pasture grasses). There were other lesser known lineages mixed in there as well.

Fossilized dinosaur poop or “coprolite.” USGS Public Domain

These findings are exciting for a variety of reasons. For one, it tells us that despite lacking teeth specialized for eating grasses, large herbivorous dinosaurs like the Titanosaurs were nonetheless incorporating these plants into their diet. It also tells us that grasses were already quite diverse by the late Cretaceous. The fact that modern clades of grass were around back then sets back grass evolution many millions of years. It also tells us something about grass biogeography. It suggests that grasses were already wide spread across the supercontinent of Gondwana long before India broke away. Finally, it tells us that grasses evolved silicate phytoliths long before more recognizable grass-eating herbivores came onto the scene.

I am always blown away by the details paleontologists are able to extract from such tiny fossils. Who knew dinosaur poop could tell us so much?

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

Further Reading: [1]

Sea Oats: Builder of Dunes & Guardian of the Coast

Photo by Sara Johnson

Coastal habitats can be really unforgiving to life. Anything that makes a living along the coast has to be tough and they don’t come much tougher than sea oats (Uniola paniculata). This stately grass can be found growing along much of the Atlantic coast of North America as well as along the Gulf of Mexico. What’s more, its range is expanding. Not only is this grass extremely good at living on the coast, it is a major reason coastal habitats like sand dunes exist in the first place. Its presence also serves to protect coastlines from the damaging effects of storm surges. What follows is a celebration of this amazing ecosystem engineer.

Sea oats is a dominant player in coastal plant communities. Few other species can hold a candle to its ability to survive and thrive in conditions that are lethal to most other plants. The ever-present winds that blow off the ocean bring with them plenty of sand and salt spray. Sea oats takes this in strides. Not only are its tissues extremely tough, they also help prevent too much water loss in a system defined by desiccation.

Photo © Don Henise licensed by CC BY 2.0

The life cycle of sea oats begins with seeds. Its all about numbers for this species and seat oats certainly produces a lot of seed. Surprisingly, many of the seeds produced are not viable. What’s more, most will never make it past the seedling stage. You see, sea oat seeds require just the right amount of burial in sand to germinate and establish successfully. Too shallow and they are either picked off by seed predators or the resulting seedlings quickly dry up. Too deep and the limited reserves within mean the seedling exhausts itself before it can ever reach the surface.

Still, enough seeds germinate from year to year that new colonies of sea oats are frequently established. Given the right amount of burial, seedlings focus much of their first few months on developing a complex, albeit shallow root system. Within two months of germination, a single sea oat can grow a root system that is as much as 10 times the size of the rest of the plant. This is because sand is not a forgiving growing medium. Sand is constantly shifting, it does not hold on to water very long, and it is usually extremely low in nutrients. By growing a large, shallow root system, sea oats are able to not only anchor themselves in place, they are also able to take advantage of what limited water and nutrients are available.

It is also this intense root growth that makes sea oats such an important ecosystem engineer in coastal habitats. All of those roots hold on to sand extremely well. Add to that some vast mychorrhizal fungi partnerships and you have yourself a recipe for serious erosion control. The interesting thing is that as sea oats grow larger, they trap more sand. As more sand builds up around the plants, they grow even larger to avoid burial. This process snowballs until an entire dune complex develops. As the dunes stabilize, more plants are able to establish, which in turn attracts more organisms into the community. A literal ecosystem is built from sand thanks to the establishment of a single species of grass.

Photo © Hans Hillewaert / CC BY-SA 4.0

As sea oats mature, they will begin to produce flowers, and the process repeats itself over and over again. As mentioned above, the sea oats seeds are subject to a lot of seed predation. This means that as sea oat populations grow, more and more animals can find food in and among the dunes. So, not only do sea oats build the habitat, they also supply it with plenty of resources for organisms to utilize.

The power of sea oats does not end there. Because they are so good at controlling erosion, they help stabilize the shoreline from the punishing blow of storm surges. Dune systems, especially those of barrier islands, help reduce the amount of erosion and the momentum of wave action reaching coastal communities. Many states here in North America are starting to realize this and are now protecting sea oat populations as a result.

Sea oats, though tough, are not indestructible. We humans can do a lot of damage to these plants and the communities they create simply by walking or driving on them. Pathways from foot and vehicle traffic kill off the dune vegetation and create a path of least resistance for wind, which quickly erodes the dunes. Apart from that, development and resulting runoff also destroy sensitive dune communities, making our coastlines that much more vulnerable to the inevitable storms that threaten their very existence.

As our climate continues to change at an unprecedented rate and storms grow ever stronger, it is very important that we recognize the role important species like sea oats play in not only providing habitat, but also protecting our coastlines. Dune stabilization and restoration projects are growing in popularity as a cost effective solution to some of the threats facing coastal communities. Among the many techniques for restoring dunes is the planting of native dune building species like sea oats. If you live near or simply like to enjoy the coast, please stay off the dunes. Foot and vehicle traffic make quick work of these habitats and we simply cannot afford less of them.

Watch our short film DUNES to learn more about these incredible ecosystems.

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

Further Reading: [1] [2] [3] [4] [5]

Life With Endophytic Fungi

Endophytic fungi living in the cells of a grass leaf. Photo by Nick Hill (Public Domain)

Talk about plants long enough and fungi eventually make their way into the conversation. These two walks of life are inextricably linked and probably have been since the earliest days. At this point we are well aware of beneficial fungal partners like mycorrhizae or pathogens like the cedar apple rust. Another type of relationship we are only starting to fully appreciate is that of plants and endophytic fungi living in their above ground tissues. 

Endophytic fungi have been discovered in many different types of plants, however, it is best studied in grasses. The closer we look at these symbiotic relationships, the more complex the picture becomes. There are many ways in which plants can benefit from the presence of these fungi in their tissues and it appears that some plants even stock their seeds with fungi, which appears to give their offspring a better chance at establishment. 

To start, the benefits to the fungi are rather straight forward. They get a relatively safe place to live within the tissues of a plant. They also gain access to all of the carbohydrates the plants produce via photosynthesis. This is not unlike what we see with mycorrhizae. But what about the plants? What could they gain from letting fungi live in and around their cells?

One amazing benefit endophytic fungi offer plants is protection. Fungi are famous for the chemical cocktails they produce and many of these can harm animals. Such benefits vary from plant to plant and fungi to fungi, however, the overall effect is largely the same. Herbivores feeding on plants like grasses that have been infected with endophytic fungi are deterred from doing so either because the fungi make the plant distasteful or downright toxic. It isn't just big herbivores that are deterred either. Evidence has shown that insects are also affected.

There is even some evidence to suggest that these anti-herbivore compounds might have influences farther up the food chain. It usually takes a lot of toxins to bring down a large herbivore, however, some of these toxins have the potential to build up in the tissues of some herbivores and therefore may influence their appeal to predators. Some have hypothesized that the endophytic fungal toxins may make herbivores more susceptible to predators. Perhaps the toxins make the herbivores less cautious or slow them down, making them more likely targets. Certainly more work is needed before anyone can say for sure.

Italian ryegrass (Lolium multiflorum) is one of the most studied grasses that host endophytic fungi. Photo by Matt Lavin licensed under CC BY-SA 2.0

Another amazing example deals with parasitoids like wasps that lay their eggs in other insects. Researchers found that female parasitoid wasps can discriminate between aphids that have been feeding on plants with endophytic fungi and those without endophytic fungi. Wasp larvae developed more slowly and had a shorter lifespan when raised in aphids that have fed on endophytic fungi plants. As such, the distribution of plants with endophytic symbionts may have serious ramifications for parasitoid abundance in any given habitat.

Another benefit these endophytic fungi offer plants is increased photosynthesis. Amazingly, some grasses appear to photosynthesize better with endophytic fungi living in their tissues than plants without fungi. There are many mechanisms by which this may work but to simplify the matter, it appears that by producing defense compounds, endophytic fungi allow the plant to redistribute their metabolic processes towards photosynthesis and growth. In return, the plants produce more carbohydrates that then feed the fungi living in their tissues. 

One of the most remarkable aspects about the relationship between endophytic fungi and plants is that the plants can pass these fungi on to their offspring. Fungi are able to infect the tissues of the host plants' seeds and therefore can be carried with the seeds wherever they go. As the seedlings grow, so do the fungi. Some evidence suggests this gives infected seedlings a leg up on the competition. Other studies have not found such pronounced effects.

Still other studies have shown that it may not be fungi in the seeds that make a big difference but rather the fungi present in the decaying tissues of plants growing around them. Endophytic fungi have been shown to produce allelopathic compounds that poison neighboring plants. Areas receiving lots of plant litter containing endophytic fungi produced fewer plants but these plants grew larger than areas without endophytic fungi litter. Perhaps this reduces competition in favor of plant species than can host said endophytes. Again, this has potentially huge ramifications for the diversity and abundance of plant species living in a given area.

We are only beginning to understand the role of endophytic fungi in the lives of plants and the communities they make up. To date, it would appear that endophytic fungi are potentially having huge impacts on ecosystems around the globe. It goes without saying that more research is needed.

Photo Credits: [1] [2]

Further Reading: [1] [2] [3] [4] [5] [6]