December 4. Rise of grasses and some coal

Everyone knows grass. Common worldwide today, grasses include the cereal grains such as rice and wheat, bamboo, swamp sedges, and of course lawns, but as common as they are today, grasses were the last major group of plants to evolve. They were certainly present but uncommon during the Cretaceous – dinosaur coprolites, or fossil excrement, are known with grass components. But grasses really began to expand during the early part of the Cenozoic, the Paleocene and Eocene epochs.

Grasses continued to evolve through the Cenozoic, inventing novel ways to fix carbon during photosynthesis. Grasses including maize or corn, sugar cane, and sorghum use a more efficient method of carbon fixation than many plants, a method that is thought to be a relatively recent development – meaning probably the past 40 million years. 

Grasses diversified a lot in the middle to late Miocene, around 6 to 10 million years ago. Their dominance in prairies and savannahs may be a result of their drought tolerance and ability to use carbon dioxide more efficiently than some other plants, even in low CO2 conditions such as were developing over the course of the Cenozoic. Another factor might be a co-evolution with hoofed animals that grazed on grasses, and helped spread them. 

Coal mining, Powder River Basin, Wyoming (USGS photo)

In the western United States during the Paleocene, as grasses were beginning to expand, swamps and lakes were forming in what are now northeastern Wyoming and southeastern Montana, the Powder River Basin. This basin was one of the low-lying areas between two of the late Cretaceous Laramide uplifts, the Black Hills and Big Horn Mountains. Those mountains were actively uplifting into the early Paleocene, and shedding sediment into the adjacent swampy basins, where extensive forests grew in Paleocene and Eocene time. The package of rocks including fluvial or river-borne sediments, lake deposits, and organic material deposited in swamps is called the Ft. Union Formation. The climate was warm temperate to subtropical but with alternating warm and cooler intervals during the Paleocene and Eocene, the first epochs of the Cenozoic. The plant matter in the Ft. Union Formation has produced thick coal beds. One individual bed, near Gillette, Wyoming, is about 110 feet thick.

Similar coals of Cenozoic age can be found throughout the Rocky Mountains, but the Wyoming-Montana coals are the most important economically. Wyoming is the leading state in the US for coal, with 338 million tons produced in 2013, 39% of the U.S. total. West Virginia, at #2, produced 11% of the total, which amounts to about 1.1 billion tons of coal for the whole United States.

In other plant-related news, just this week a new discovery was announced of a carnivorous plant in Baltic amber, dating to about 40 million years ago, the Eocene epoch. LINK

Today, December 4, is St. Barbara’s Day. She’s the patron saint of artillerymen, mathematicians, and miners. Geologists sometimes adopt her, usually as an excuse for a party.

—Richard I. Gibson

LINKS and References:
Recent evolution of grasses
Carnivorous plant

Evolution of grasses 

Cretaceous and Tertiary coals of the Rocky Mountains and Great Plains regions, by R. Flores and T. Cross, 1991, in GSA DNAG volume P-2, Economic Geology, U.S.

Photo from USGS 

November 9. Sequoia

By the end of the Cretaceous, flowering plants were reaching into most of the ecological niches across the globe, and were dominant in many . But obviously, the more primitive gymnosperms such as ferns and conifers survived, as they are with us today. While some of them, such as ginkgoes, have declined in diversity, they haven’t changed much in more than 100 million years.

The genus Sequoia appears in the fossil record during the Jurassic in China, but it seems to have expanded across at least the northern continents of North America, Europe, and Asia during the Cretaceous. The species known from the Cretaceous, including Sequoia dakotensis, found in South Dakota and others found in Greenland, are extinct, and the surviving coast redwoods, Sequoia sempervirens, are the only surviving members of the genus. The giant sequoias of the Sierra Nevada are technically members of the genus Sequoiadendron, closely related but distinct from Sequoia sempervirens, the coast redwood.

Sequoias were apparently not well adapted to the polar regions, even though the climate was mild enough for plants to grow there. The closely related Metasequoia, or dawn redwood, was deciduous and lost its leaves in winter, so it had a more extended range. There may have been some hybridization between early Sequoias and Metasequoia, resulting in the modern coast redwood.

Redwoods seem to have thrived in the greenhouse conditions of the Cretaceous, but during the Cenozoic and up to the present, their range became restricted, presumably because of overall cooling that led to the glacial period of the past two million years or so. Sequoias have high water content in their tissues which makes them more susceptible to freezing than some other trees.

Sequoia cones were and are tiny compared to the massive sizes of the trees, but they are some of the most common fossils of sequoia. The name sequoia honors Sequoyah, the Cherokee who invented a system for writing the Cherokee language in the 1820s. 

—Richard I. Gibson

November 8. Flowering plants

Today I’m going to tackle flowering plants – obviously a huge topic, so we’ll only touch the surface of this subject.

Plants grew on land from sometime during the Silurian, at least 425 million years ago, or perhaps even earlier. Forests thrived by Devonian time, 380 million years ago. Plants were so abundant that their remains in the form of coal give their name to the Carboniferous Period. But for close to 300 million years, plants were without flowers.

Historically, fossil flowers seemed to appear so suddenly in the fossil record that they were a problem for Charles Darwin’s evolutionary theory, and he called it an “abominable mystery.” Since then, enough transitional fossils have been found to demonstrate the episodic evolution of angiosperms, flowering plants, from gymnosperms like seed ferns. But the details of that evolution are still pretty sparse, and the question of why did it take so long to get there remains unanswered.

Magnolia photo by WibblyWibbly (public domain)

There are some possible flowering plants from the Jurassic, and there’s some fossil pollen from the Triassic that has similarities to angiosperms, but the oldest pretty much accepted flowering plant is called Archaefructus, and it dates to about 125 million years ago, the middle part of the early Cretaceous in northeastern China, found in part of the famous Jehol biota that includes well-preserved insects, dinosaurs, and mammals, as well as the typical floral assemblage of cycads, gingkoes, and conifers. Flowers were definitely rare, but they were present.

Whether or not Archaefructus is a basal ancestor to modern flowering plants is debated – it might be a sister group rather than a direct ancestor. There are some other early Cretaceous examples as well.

It took an additional 25 million years, until about 100 million years ago, at the middle of the Cretaceous, for flowering plants to really take off. It’s called the great angiosperm radiation, when a dramatic increase in flowering plant diversity took place. But even that is no longer seen as quite the sharp event it was formerly viewed as. Many modern groups, including magnolias, had developed millions of years before then. Nonetheless, geologically speaking, there was still a real ramping up of diversity and numbers of flowering plants in mid-Cretaceous time.

The radiation of flowering plants has traditionally been tied to a co-evolution with insects, with both evolving a symbiotic relationship that continues to this day. The relative sudden explosion of flowers may be an example of a highly successful trait that initially evolved in some isolated ecosystem, such as today’s Galapagos Islands, where Darwin’s finches famously evolved various traits to fill a diversity of ecological niches. It’s also been suggested that angiosperms, present but dominated by the conifers and cycads, were opportunistic plants that invaded niches where the competition from gymnosperms was less, establishing a foothold that simply grew and grew.

The attractiveness and food sources provided by flowers and fruits wasn’t lost on mammals, either. A typical view holds that the evolution of flowering plants was a boon to mammals that scrounged on forest floors or climbed into shrubs, bushes, and trees. The relationship between plants and mammals was symbiotic, like their connection to insects. Mammals ate plant fruits and helped spread their seeds. However, a 2013 study found that the mid-Cretaceous, when the great flowering plant radiation took place, was actually a time of decreasing diversity among mammals. This may seem counter-intuitive, but for whatever reason, it appears that insectivores, and not plant-eaters, were the mammals best suited for survival in the Cretaceous. Those insectivores basically gave rise to all modern mammals except the egg-laying platypuses and spiny anteaters.

There’s even been a suggestion that grazing by those huge, hungry herbivorous dinosaurs might have been a factor in stimulating flowering plants to evolve.

Yet another look at the possible causes of the angiosperm radiation suggests that fires paved the way for their diversification. The Cretaceous atmosphere was probably something like 25% oxygen, compared to today’s 21%, so wildfires would be more likely. Fires would have cleared the gymnosperm forests, allowing for invasion by angiosperms. To my mind, one problem with this is why didn’t it happen earlier, or why did it happen so suddenly, relatively? Of course there might have been some threshold that was crossed, but if angiosperms were present at all, it seems that they might have been able to take advantage of wildfire situations before the mid-Cretaceous. But maybe not. The modern savannas, grasslands on many continents, appear to have evolved over a short period, say 10 million years or so, and wildfires may be a factor in their evolution, too. Stay tuned.

By the end of the Cretaceous something like 80% of modern groups, including oaks and maples, had been established. The forests were much more modern in appearance than they were at the start of the period.

I have links below to reports on three of the research topics I’ve mentioned today.

* * *

Today’s birthday is Aleksandr Fersman, a Russian mineralogist born November 8, 1883, in St. Petersburg. His work in economic geology helped provide for the needs of the Soviet Union during its post-World War I industrialization. It’s perhaps a minor footnote to his career, but he was responsible for evacuating 80,000 mineral specimens from the Moscow mineral museum during World War II. As a mineral collector myself, I can relate to that.

—Richard I. Gibson

LINKS:

Origin of angiosperms
Cretaceous angiosperm radiation
Mammals declined with plant expansion
Fires and plants

Magnolia photo by WibblyWibbly (public domain)

October 29. Cycads and Gingkoes

We’ve touched on the prolific plant life of the Jurassic several times. It resulted at least to some extent from the greenhouse conditions that prevailed over much of the globe for much of the period. And I mentioned yesterday that there were pretty much no flowering plants. The Jurassic forests were still dominated by conifers, ferns, rushes, and cycads. Not too different from the Carboniferous.

Cycads have cones with exposed seeds, and they first appear in the fossil record during the Permian, with some possible Carboniferous examples. They flourished during the Jurassic, so that they were characteristic of many Jurassic forests, to the point that the Jurassic is sometimes called the Age of Cycads. They are the primary constituents of many Jurassic coal beds. 

Jurassic gingkoes from Oregon.
USGS Monograph XLVIII, by Lester Ward, 1905.

Cycads are gymnosperms, with cones containing exposed seeds. They range from tiny plants only a few centimeters tall to large trees, and while many of them looked like palms, they are not closely related. Cycads today live mostly in tropical environments, but some varieties are found in desert conditions. Modern cycads have been called “living fossils” because they seem to be pretty much unchanged from their Jurassic ancestors, but a recent study has suggested that the diversity of modern cycads only dates to the past 10 million years or so – a second wave of cycad diversification. According to that research based on DNA analysis, cycads declined at and after the end of the Cretaceous but then radiated again during the Miocene, about 10 million years ago. (Note: text above modified thanks to the comment, below)

The giant herbivorous dinosaurs, Sauropods and others, undoubtedly munched on cycads as part of their diet. 

Gingkoes were also common in Jurassic forests. They are fairly closely related to cycads and conifers, although their precise relationship is not certain. The earliest fossils of the Gingko genus come from the early Jurassic, and while their abundance and diversity peaked during the Jurassic, gingkoes declined during the Cretaceous and later, so that today, there is only one living species, Gingko biloba, native to China.

Jurassic gingkoes lived across the northern continents, what are now North America, Europe, Siberia, and China. Today’s gingko trees can be well over 100 feet tall, and Jurassic varieties were probably of similar size. Gingko leaves dominate some fossil assemblages.

* * *

It’s appropriate that today’s birthday is Othniel Charles Marsh, one of the most prominent students of Jurassic vertebrate fossils in the United States. He was born October 29, 1831, near Lockport, New York, and he spent most of his career with Yale University and the U.S. Geological Survey. O.C. Marsh’s competition with Edward Cope in the rush to find and identify dinosaur fossils is known as the “bone wars,” but we’d probably say Marsh won the wars – he named at least 43 orders, families, and genera of dinosaur, and described 80 new species to Cope’s 56. The legacy of both Marsh and Cope is dinosaur fossils in museums around the United States.

—Richard I. Gibson

LINK
Modern cycads not so old?  
Cycads 

September 22. Triassic plants

Plants during the Triassic were much like those of the preceding periods, although much diminished in extent by the Permian extinction. I mentioned the coal gap a few days ago, the early Triassic time from which no coal beds are known, with the implication that large coal-making forests were decimated by the extinction event, but compare that to the forests known in Antarctica, that we talked about yesterday. The earth is wildly varied, and was in the Triassic, too. 

The ferns, conifers, and other plants of the Triassic were largely gymnosperms, plants that make seeds without fruiting bodies to enclose them. The name gymnosperm means “naked seed.” Angiosperms’ seeds are enclosed in fruits and other protective matter, and they also have flowers. The name means “seeds enclosed in a protective vessel.” Angiosperms are today’s flowering plants. But there were no flowers during the Triassic – the oldest likely possible angiosperms are from the Jurassic, about 160 million years ago, and they really aren’t abundant and confidently identifiable until the Cretaceous, 125 to 130 million years ago. But the ancestors of the angiosperms were probably beginning to diverge from the gymnosperms by Late Triassic time. 

To an extent, flowering plants burst on the scene in the fossil record quite suddenly. Charles Darwin saw this as a problem for his theory of evolution, and called it an “abominable mystery.” But more recent discoveries have extended the story so that we can see much of the step-wise evolution of flowering plants, though much of the story remains obscure.

The genus Sanmiguelia, ferns found in late Triassic rocks of northwest Texas, has sometimes been suggested as the first angiosperm. The fossils, from about 230 million years ago, preserve the plants in growth position, allowing for more accurate interpretations of their lives. It was probably a herbaceous plant that grew in swampy wetlands that had angiosperm-like flowers. If it was not a primitive angiosperm, it seems likely that it was a close ancestor to them. There are also some spores that seem to be angiosperm-type pollen grains, dated to the middle Triassic, about 245 million years ago in Switzerland.

The question remains, if they existed, why did angiosperms not diversify and expand for almost 150 million years more, until the mid-Cretaceous, when they underwent a huge radiation. That question still stumps researchers, and we can’t be entirely certain when flowering plants, angiosperms, began. If they are as old as Triassic, or even Jurassic, they certainly were not common then.

—Richard I. Gibson

Links:
Sanmiguelia 
Purported Triassic angiosperms

Image from R.W. Brown, USGS Prof. Paper 274-H (public domain)

September 18. The Petrified Forest: Chinle Formation

As we get later into the Triassic, environments were changing from the common hot arid desert settings to more complex systems. In what is now Arizona, the variegated Chinle Formation was laid down. It did include extensive wind-blown desert sands, but it also was a time for lakes, swamps, and river systems. The Chinle and its equivalent, the Dockum Group, extend from western Kansas west across the Colorado Plateau into Nevada, and south from Colorado and Utah into Arizona and New Mexico. Stratigraphically, the Chinle typically lies on top of the Moenkopi Formation that we talked about September 7, but there is usually an unconformity between the two packages of rock, indicating either a period of non-deposition, or deposition and erosion, or both. 

Fossil animals in the Chinle formation include various reptiles, including semi-aquatic crocodile-like phytosaurs, small dinosaurs including Coelophysis which we mentioned yesterday, amphibians, lungfish, and sharks. But probably the most famous life forms in the Chinle are trees.

Petrified wood, Arizona. Photo by Kumar Appaiah used under Creative Commons license

The Petrified Forest of Arizona is in the Chinle Formation. Fossil logs represent trees that grew up to 200 feet high and two feet in diameter. They were mostly conifers, like modern pines. The trees grew along river channels, and when they died and fell into the rivers, logjams sometimes developed. The region was also one where occasional volcanic ash falls occurred. Groundwater dissolved silica from the volcanic ash and carried it into buried tree trunks, where the wood was replaced by silica in the form of multicolored agate. The diverse colors reflect trace elements such as iron. In some cases, the replacement was so delicate, practically on a molecular scale, that bark and tree rings are preserved. Since silica is quartz, a resistant mineral, the petrified logs typically weather out of the soft shale that constitutes much of the Chinle formation.

The Petrified Forest contains at least 200 different plant species, making it the richest Triassic plant fossil locality in the world. The most common tree fossils are conifers.

The nearby Painted Desert is also in the Chinle Formation. The alternating shales, mudstones, sands, lakebeds, and volcanic ash are colored mostly by iron and manganese in various amounts.

—Richard I. Gibson

Photo by Kumar Appaiah used under Creative Commons license. 

August 8. Glossopteris

Glossopteris leaves

One of the first lines of evidence leading to the acceptance of continental drift was a shrub called glossopteris. It was a seed fern that lived during Permian time (with some questionable successors in the Triassic). They were among the dominant plants of the Permian, and contributed significantly to the Permian coal beds that are important reserves today in South America, South Africa, India, Australia, and even Antarctica, where they were collected by Robert Scott’s expedition. That’s right, despite the episode titled “The End of Coal” last month, plants did continue to make coal under the right circumstances.  

Glossopteris seeds were large – too large to be distributed by the wind. That observation and the presence of glossopteris across the southern continents that are now widely separated led Austrian geologist Eduard Suess to suggest that the southern continents had once been attached, either via land bridges or in a single supercontinent, Gondwana. It wasn’t much of a leap to connect Gondwana with the other continents in Pangaea, although it was tectonic and stratigraphic evidence that led to that, rather than glossopteris. Some glossopteris-like fossils have been found in the northern continents, but I think the general consensus is that it was restricted to the southern continent of Gondwana.

Glossopteris distribution (dark green) across Gondwana

The plant was a shrub or tree, with some as tall as 100 feet. The name means “tongue-leaf” for the simple lobate shape of the leaves.

* * *

Benjamin Silliman was born August 8, 1779, in New Stratford, Connecticut. He was a professor of chemistry and geology at Yale, where he built up the mineralogy collection that formed the core of the Yale Peabody Museum. He was one of America’s first real science professors, and he established the American Journal of Science, the oldest scientific journal in the U.S.

—Richard I. Gibson

Photo by Daderot (public domain) 

Glossopteris distribution map by Petter Bøckman (public domain, via Wikipedia) 

July 25. Fossil spores

With all the ferns and other spore-bearing plants around during the Pennsylvanian, you’d expect that there would be a lot of fossil spores, too. You’d be right. The science of palynology is the study of fossil spores and pollen. Because they’re microscopic, assemblages of spores and pollen make really good markers for particular time periods – often pretty short time periods, so they’re useful in biostratigraphy. They help explorers for oil and natural gas pin down pretty exactly where you are in the stratigraphic section, even when the information comes from tiny cuttings in wells. 

The word palynology comes from a Greek word meaning “strewn,” or “sprinkle,” so it’s the study of things that are scattered or strewn.

Pennsylvanian spore Reinschospora magnifica, about 70 microns across.
From Kosanke, 1950, Illinois State Geol. Survey Bulletin 74.

The smallest pollen grains are around 6 micrometers across – it would take more than 100 to span a millimeter. Spores are similar in size, but they can range up to at least 90 micrometers across. Fossil spores come in a wild array of shapes, just as modern pollen and spores do. They’ve been studied scientifically since the 1800s but modern palynology really took off in the 1950s and 1960s. The American Association of Stratigraphic Palynologists was founded in 1967, and today under the name The Palynological Society the organization has around 500 members worldwide.

—Richard I. Gibson

Pennsylvanian spore Reinschospora magnifica, about 70 microns across. From Kosanke, 1950, Illinois State Geol. Survey Bulletin 74.

July 11. Mazon Creek

Pennsylvanian shrimp Acanthotelson stimpsoni,
from Dugger Formation, Indiana.

Another remarkable Pennsylvanian fossil assemblage is found at Mazon Creek, Illinois. It’s a lagerstatte, one of those natural collections that’s remarkable in its state of preservation. The Pennsylvanian rock unit that these fossils are found in is a shale – but the fossils aren’t in shale but rather are in concretions within the shale. The concretions are called ironstone, oval to circular concentrations of iron in the form of siderite, iron carbonate that seem to have preferentially focused on the fossils, encasing and entombing them. Possibly the bacteria that began to decompose plant and animal parts generated carbon dioxide which combined with iron – which must have been anomalously high in concentration in the water to make iron carbonate. The concretion is still mostly shale or siltstone, but the iron dissolved from the rock makes the concretion enclosing a fossil harder than the general rock, and they weather out nicely. Collectors split the concretions to reveal plant and animal fossils in wild diversity. 

The overall setting for the Mazon Creek rocks was probably a large river delta, with sediment and iron eroding off the uplifting Allegheny and Appalachian Mountains to the east. It was a tropical environment.

Over 400 species of plants and more than 300 species of animals have been found in Mazon Creek concretions. Leaves such as parts of fern fronds are common, as well as seeds and cones from plants. The animals include jellyfish, worms, shrimp, snails, and fish, plus centipedes, insects, spiders, and beetles – the oldest known beetle is from Mazon Creek, and it was described in 2009.

Tully Monster, collection of Mike Hamilton, photo by Steve Henderson

One interesting animal, found only at Mazon Creek, is called the Tully Monster. It was an invertebrate with no hard parts that ranged in size from about 8 to 35 centimeters, or three to 14 inches. It had a long proboscis with teeth at the end, with which it presumably fed on small animals or debris in muddy water, and a linear bar with possible eyes on each end. We flat-out do not know what the Tully Monster is. We don’t know its phylum or its affinities, beyond a suggestion that it might be some variation on some of the worm-like themes seen in the Burgess Shale of Cambrian age.

—Richard I. Gibson

Oldest beetle
Tully Monster 

Pennsylvanian shrimp Acanthotelson stimpsoni, from Dugger Formation, Indiana. Collected by Richard Gibson. Photo by Steve Henderson. One half of this fossil is in Montana, the other half is in Georgia. Both photos used by permission from Steve Henderson.

May 19. Devonian plants: the first forests

The Devonian saw the most fundamental change in the appearance of the land in several hundred million years. Plants, which had gotten started on land maybe by very late Ordovician time and for sure by the Silurian, began to spread across the landscape. For the first time, soils with organic matter began to form in abundance. You can visualize the development of soil as a sort of symbiotic relationship with plants – chemicals from plants, plus the mechanical action of their root systems, broke up rocks and changed them to the stuff we’d call soil. Soil in turn served as a reservoir of nutrients for future plants, as well as a substrate that was softer than hard rock, a place for plants to grow. The cycle of plant growth, death, and soils had begun.

Devonian forest

Early Devonian plants were still pretty primitive, but by the end of the period many diverse plants with true leaves and root systems were covering large areas. Many were relatives of modern ferns and horsetail rushes, but early varieties of other plants, such as pro-gymnosperms, spore-bearing plants that eventually gave rise to conifers, were also around. And the first true seed-bearing plants had evolved by the end of the Devonian.

You remember Cooksonia, from April 25, back in the Silurian? In contrast to those spindly stalks a couple inches tall, the Devonian saw the development of the first woody plants – trees – and the first real forests. The oldest known tree is called Wattieza, a fern-like tree from New York dating to about 385 million years ago, the Middle to Late Devonian. Some of these early trees were more than 30 feet tall. The oldest known forest, at Gilboa, New York, has upright stumps, roots and trunks that are interpreted as part of an extensive ecosystem that can only reasonably be called a forest. The stumps and trunks had been known since 1870, but it wasn’t until 2007 that the crowns of the trees were found and connected to show the geometry of the entire tree.

Apart from bragging rights for being the oldest forest, this discovery has huge consequences for the history of the earth. If large plants were widespread on the earth’s surface, it would have had a significant impact on the atmosphere – carbon dioxide in, oxygen out, and dying plants would be returning their elements – largely carbon, into that new product on the land, soil. All of this is part of the carbon cycle, the shifting of carbon around in the atmosphere, hydrosphere, and soils and rocks of the solid earth. That, in turn, has a great impact on the nature of climate and the kinds of life that can inhabit various ecological zones.

* * *

May 19, 1871, was the birthdate of Reginald Aldworth Daly, at Napanee, Ontario. Daly worked as a geologist surveying the Canada-U.S. boundary for many years, leading to a massive report entitled North America Cordillera: Forty-Ninth Parallel. His work also resulted in a definitive book called Igneous Rocks and their Origins. Daly served as the head of the geology department at Harvard for 30 years. He worked on impact theory, and there are craters on the moon and Mars named for him.

Carl Beck, my graduate school major professor of mineralogy, was also born on this day in 1916. He put me on the convoluted path that took me from kidney stone mineralogy to geophysics in oil exploration.

—Richard I. Gibson

Devonian landscape painted by Eduard Riou, 1872 (public domain)

Link: Gilboa fossil forest

April 25. Cooksonia

We talked about early plants on April 11, and last month I mentioned that there are some fossil spores from the Ordovician that indicate there were some primitive plants on land that long ago. But really, even though there are some early to middle Silurian land plants known, there still wasn’t much, even towards the end of the Silurian, coming up at about 416 million years ago. 

Most of the land surface was bare rock or the kind of soil that forms from the physical weathering of rock. Plants seem to have mostly lived mostly along sea coasts, in wetlands, and it’s reasonable to suppose they might have grown along some rivers. But even with that limited range, they were still pretty much worldwide – from what is now Greenland to Siberia to Australia – all areas that were within the temperate or tropical zones during the Silurian.

Cooksonia

Cooksonia was the first plant on land that had an upright stalk, rather than mossy ground cover. It was a vascular plant, meaning it had a system for delivering nutrients and water around its body. It wasn’t very tall, though – just a few inches at most. It seems that the stalk was mostly there to help disperse the plant’s spores, which were in clusters on the tips of the branched stalks. Cooksonia was discovered in 1937 in very late Silurian rocks in England and Wales, but it’s since been found all over the world.

Cooksonia didn’t have leaves, or flowers, or a root system. William Lang, who described the first specimens, named it for Isabel Cookson, an Australian botanist and paleobotanist who worked with him in Britain. Her work on Silurian and Devonian plants was important in establishing theories of the evolution of land plants.

So, we’re getting there – the land isn’t just bare rock any more. Short stalked plants, mosses, and a millipede here and there to eat the dead plant detritus, and that’s about it. But it was a start.

—Richard I. Gibson

Drawing by Smith609 via Wikipedia, under Creative Commons license

March 9. Life invades the Land

Up until now, all the life we’ve been talking about has been in the sea. OK, maybe some trilobites ran up onto a wet beach above the water’s edge, and maybe some of the shelly critters lived in the intertidal zone, just as such animals do today, but they were fundamentally dependent on ocean water for their existence. And algae and bacteria can live almost anywhere today, so maybe they were exploiting some of the niches on land at a pretty early date.

Liverworts

It probably won’t surprise you to hear that the first big life on land was plants – moss-like plants, to be specific. The earliest evidence for them is spores in the fossil record of the Middle Ordovician, about 470 million years ago or maybe a few million years older. The spores are similar to the spores of modern liverworts, which are mossy plants. Spores from more complex vascular plants are found in Upper Ordovician rocks. Vascular plants have tissues for shipping fluids and nutrients around their bodies, and include modern trees and flowering plants.

Once plants were on land, they began the chemical weathering of rocks, which until then had largely been subjected mainly to physical weathering, breaking apart by freezing and thawing and such. Certainly there were some chemical reactions among rocks, water, and the atmosphere, but plants must have accelerated that chemical alteration considerably.

Chemical weathering of rocks removes things like calcium and other nutrients that plants can use – and combined with photosynthesis, which converts carbon dioxide to carbon and oxygen – might have removed enough CO₂ from the earth’s Ordovician atmosphere to lead to a decrease in average temperature. So much so, according to some speculations, that it might have contributed to the onset of the Late Ordovician ice age. We’ll talk about that glacial period towards the end of the Ordovician, later in March.  But never underestimate the power of plants acting over a long period of time!

We don’t find actual body fossils of plants in the geologic record until the Silurian, but the fossil spores in Ordovician rocks are pretty conclusive evidence that there were plants on land by Middle Ordovician time.

—Richard I. Gibson

Liverworts image from Ernst Haeckel's Kunstformen der Natur, 1904 (public domain)


Moss froze the planet?