Plants, although mainly made of soft tissue, left traces of their evolution in the fossil record just like animals. In particular, the great humid forests of the Carboniferous Period, known as the Age of Coal, provide us with many excellent examples of fossilized plants worldwide.

Before becoming widespread on land, plants first made their appearance in water. More than 3.5 billion years ago (3.5 Ga), small single-celled organisms acquired the capacity to photosynthesize, meaning they could use the energy of the sun to produce glucose from carbon dioxide with free oxygen (O2) as a waste product. Free oxygen was rare when the Earth was young, but gradually accumulated, thanks to photosynthesis, until reaching 20% of the air we breathe today.

These first photosynthesizing organisms, cyanobacteria, later gave rise to algae, the first aquatic plants. Plants were restricted to the aquatic world until three billion years later, at the end of the Ordovician period, when we find signs of their tentative presence on land. After many more millions of years, they resembled patches of moss a few centimeters high. But things changed in the Devonian Period, and these primi Read More
Plants, although mainly made of soft tissue, left traces of their evolution in the fossil record just like animals. In particular, the great humid forests of the Carboniferous Period, known as the Age of Coal, provide us with many excellent examples of fossilized plants worldwide.

Before becoming widespread on land, plants first made their appearance in water. More than 3.5 billion years ago (3.5 Ga), small single-celled organisms acquired the capacity to photosynthesize, meaning they could use the energy of the sun to produce glucose from carbon dioxide with free oxygen (O2) as a waste product. Free oxygen was rare when the Earth was young, but gradually accumulated, thanks to photosynthesis, until reaching 20% of the air we breathe today.

These first photosynthesizing organisms, cyanobacteria, later gave rise to algae, the first aquatic plants. Plants were restricted to the aquatic world until three billion years later, at the end of the Ordovician period, when we find signs of their tentative presence on land. After many more millions of years, they resembled patches of moss a few centimeters high. But things changed in the Devonian Period, and these primitive plants finally developed many of the more familiar characteristics of modern day land plants.

© Miguasha National Park 2007

A Seed Fern of <i>Mariopteris nervosa</i>

Mariopteris nervosa, a seed fern of the Carboniferous of New Brunswick, a plant characteristic of the Coal Age.

Miguasha National Park
2007
© Miguasha National Park


When they left the aquatic world to invade land at the end of the Silurian Period, plants had to overcome a number of problems related to desiccation, weight, and the availability of water and nutrients. To counteract the drying effects of the air and sun, plants developed a protective layer, the cuticle, covered by a thin coating of lipids and wax. Small openings in this protective layer called stomata facilitated gas exchange.

Plants also developed a vascular system, which consists of a network of internal tubes and vessels for conveying water through the tissues to keep them hydrated. This is necessary since not all parts of the plant will be in contact with water. In a fossil plant, the presence of stomata and small internal conduits for sap leave no doubt as to the terrestrial habitat of the original plant.

The first vascular plants in the Upper Silurian, such as Cooksonia and Rhynia, had small erect shoot systems several centimetres tall. The branches bore sporangia at their tips, which released spores that dispersed and colonized the surrounding ground. Being highly dependent on water, these plants were restricted to narrow green strips of land along r Read More
When they left the aquatic world to invade land at the end of the Silurian Period, plants had to overcome a number of problems related to desiccation, weight, and the availability of water and nutrients. To counteract the drying effects of the air and sun, plants developed a protective layer, the cuticle, covered by a thin coating of lipids and wax. Small openings in this protective layer called stomata facilitated gas exchange.

Plants also developed a vascular system, which consists of a network of internal tubes and vessels for conveying water through the tissues to keep them hydrated. This is necessary since not all parts of the plant will be in contact with water. In a fossil plant, the presence of stomata and small internal conduits for sap leave no doubt as to the terrestrial habitat of the original plant.

The first vascular plants in the Upper Silurian, such as Cooksonia and Rhynia, had small erect shoot systems several centimetres tall. The branches bore sporangia at their tips, which released spores that dispersed and colonized the surrounding ground. Being highly dependent on water, these plants were restricted to narrow green strips of land along rivers and streams.

Plants rapidly improved during the Devonian Period as they became better suited to life on land. Photosynthesis, initially limited to the vertical surface of the stems, began to take place in small horizontal structures called enations. According to a current hypothesis, these features eventually became vascular and developed into leaves. Because leaves increased the surface area available for evaporation, roots had to develop to obtain more water for the plant.

At the beginning of the Devonian, vascular plants diversified into increasingly complex communities along the margins of water bodies. The delta and estuary environments represented by rocks of the Lower Devonian Gaspé Sandstone Group contain excellent examples of the advances of these communities over time. In particular, the coastal rock sequences of the Baie de Gaspé and northern New Brunswick include beds with abundant fossils of remarkably preserved plants trapped in the very spot where they once lived.

The work of American paleobotanist Patricia G. Gensel on the Gaspé Sandstone Group focuses on the evolution of flora during the Lower Devonian. Her work paints a picture of banks that became progressively greener over time. The flora included bryophytes, rhyniopsids, zosterophylls, lycopsids, trimerophytes, the arborescent fungus Prototaxites, and a crust-forming lichen, all of which were fairly representative of worldwide flora at that time. These plants were part of lineages that eventually led to modern-day plant groups.

The appearance of root systems in many plant groups promoted the development of soils, and fossil soil layers from the Lower Devonian are evidence that vascular plants were already contributing to the reduction of carbon dioxide in the atmosphere while increasing the amount of oxygen.

© Miguasha National Park 2007

<i>Psilophyton forbesii</i>

Psilophyton forbesii from the Cap-Aux-Os Member of the Battery Point Formation, Lower Devonian, Baie de Gaspé.

Miguasha National Park

© Miguasha National Park 2007


<i>Sawdonia ornata</i>

Sawdonia ornata from the Cap-Aux-Os Member of the Battery Point Formation, Lower Devonian, Baie de Gaspé.

Miguasha National Park
2007
© Miguasha National Park 2007


The various types of plants that developed during Middle Devonian time had to overcome, one by one, the problems imposed by life out of water. Drying out and insufficient gas exchange were the main risks, followed by the difficulty in maintaining structural support. Whereas aquatic algae were surrounded by a liquid medium and species could be soft and virtually support-free, land-dwelling plants had to struggle with the reality of gravity.

The thick epidermis of the small, tubular stems of the first plants was rigid enough to allow them to grow about a dozen centimetres high. Faced with competition from many other plant groups colonizing the land, taller species could disperse their spores farther, and were therefore the first to colonize distant virgin territory. Not to mention that a new biological innovation – the leaf – was creating shade, and taller plants had a distinct advantage in the race toward sunlight.

A stem that simply grows tall will eventually break under its own weight or will be blown over by strong winds. To grow higher, a plant’s stem must also become thicker and thus stronger. This ability first appeared among a few plan Read More
The various types of plants that developed during Middle Devonian time had to overcome, one by one, the problems imposed by life out of water. Drying out and insufficient gas exchange were the main risks, followed by the difficulty in maintaining structural support. Whereas aquatic algae were surrounded by a liquid medium and species could be soft and virtually support-free, land-dwelling plants had to struggle with the reality of gravity.

The thick epidermis of the small, tubular stems of the first plants was rigid enough to allow them to grow about a dozen centimetres high. Faced with competition from many other plant groups colonizing the land, taller species could disperse their spores farther, and were therefore the first to colonize distant virgin territory. Not to mention that a new biological innovation – the leaf – was creating shade, and taller plants had a distinct advantage in the race toward sunlight.

A stem that simply grows tall will eventually break under its own weight or will be blown over by strong winds. To grow higher, a plant’s stem must also become thicker and thus stronger. This ability first appeared among a few plant groups in Upper Devonian time. In addition to primary (upward) growth, secondary growth (in circumference) allowed lignified material to be added and accumulated in a continuous manner. This was the beginning of wood, a substance that permitted early trees to attain substantial height and form the first forests.

The Middle Devonian flora of Gilboa, in the State of New York, existed just before the first forests formed and includes a few plants that grew several metres tall. But these were restricted to thin bands that bordered the unstable channels of an alluvial plain, and it was not until the arrival of Archaeopteris in Upper Devonian time that the first true forests were established. These great trees possessed an extensive root system that greatly accelerated soil formation, producing a new terrestrial habitat that encouraged the diversification of invertebrate animals venturing onto the land. Archaeopteris forests, which lined the shores of the ancient Miguasha estuary, spread around the world during the Upper Devonian.

At long last, the success of terrestrial plants was assured. Their abundance notably increased photosynthetic activity across the planet, and over millions of years, more than 80% of atmospheric carbon dioxide (CO2) was gradually consumed. Scientists attribute this decrease in carbon dioxide – a notorious greenhouse gas – to the cooling period that occurred on Earth at the end of the Devonian Period.

© Miguasha National Park 2007

Fossil Roots

The roots of fossil plants in the Middle Devonian Pirate Cove Formation in the Anse aux Pirates cliff at Escuminac. The development of roots in Devonian plants led to the formation of soils and new habitats for land-dwelling creatures. The appearance of the first forests in Upper Devonian time greatly accelerated the process of soil formation.

Miguasha National Park
2007
© Miguasha National Park 2007


Learning Objectives

The learner will:
  • identify and classify different types of fossils;
  • explain the stages of fossilization and the best conditions to create and preserve fossils;
  • make assumptions about the evolution of living beings;
  • make assumptions as to the explanation of the disappearance of some species.

Teachers' Centre Home Page | Find Learning Resources & Lesson Plans