In Carboniferous forests dragonflies grew as big as ravens. Trees and other vegetation likewise attained outsized proportions...
Bill Bryson, A Short History of Nearly Everything (2003)
These are the lasting images of the Carboniferous - great forests of strange fern-like trees, and huge insects flying between their trunks. The other image of course is of the legacy of those lush forests, the vast coal mines where tons of coal are stripped from the surface or hewn from coal seams at depth. The Carboniferous was a crucial time in the evolution of life on land. This was the time when plants and animals really cemented their land-living adaptations, and took on all habitats and all continents. The rapid rise of insects, tetrapods, and plants marked the future structure of terrestrial ecosystems.
Life in the sea was no less rich. Tropical reefs abounded, some of them a kilometre or more in length, and composed of dozens of species of corals. Brachiopods, molluscs, and echinoderms lived among those reef organisms, and conical and coiled molluscs swam above side by side with sharks and other fishes, some of them like modern forms, others much more weird and wonderful. Some Carboniferous sharks were long and thin, others were deep-bodied, some had long pointed snouts, others had great coils
| of teeth at the front of their mouths, and some even had great J! bony spines covered in teeth that extended like sunshades from their foreheads.
But it is life on land we will explore here. The key biological inventions were forests and flight (Fig. 15). Before the Carboniferous, plants were sparse, and focused around water bodies, and animals crept around on the ground. Land life exploded in the Carboniferous, forests clothed much of the landscape, and numerous insects buzzed and whizzed in the sky. Was it pure chance that these changes happened at this time, some 320 million years ago, or was there something special about the Carboniferous world?
The Carboniferous Period, from 360 to 300 million years ago, was a time of continental fusion. In the Devonian there had been several continents, a major northern continent that consisted of most of North America and Europe, as well as several southern continents. In the Carboniferous, these northern and southern landmasses began to fuse together, and indeed, in a line roughly along the modern Mediterranean, there was a great collision as Africa drove north into North America and Europe, causing earthquakes and volcanoes, and raising a chain of mountains from the Appalachians, across Ireland and Germany, to Poland. Much of Europe and North America lay around the Carboniferous equator, and tropical conditions prevailed throughout these regions.
Climates through the Early and Mid Carboniferous were warm, but conditions changed towards the end of the period. A huge glaciation began to develop at the South Pole. Our modern world is unusual in that we have an ice cap at both North and South o
Poles, and it is important to realize that there was no ice cap at | either pole for much of the Earth's history. The absence of ice caps, a as in much of the Carboniferous, means that there was much less fl temperature differentiation from the equator to the poles than t there is today.
But why do we have ice caps today if this is not the norm? The general assumption is that ice can grow at the poles only if there is land at the poles. This is the so-called albedo effect, that cold begets cold. An ice cap of some size is necessary to set things going. Sunlight is reflected from white surfaces (such as ice) and absorbed by dark surfaces. So an ice cap is somewhat self-sustaining by being white; albedo is the extent to which an object reflects light, hence the term. If the ice cap sits on land, at one of the poles, or in the high mountains, the ice tends to remain and not melt, even under modest sunlight. At the poles, it is never really warm because of the angle of the Earth to the Sun, so the winter ice can survive all through summer. Antarctica sits squarely over the South Pole today, and Greenland is close enough to the North Pole to have the appropriate effect.
In the Carboniferous, and much of the history of the Earth, the poles of course were cold, but there was no land there. If oceans lie at the poles, the winter sea ice disappears each summer, and an ice cap cannot develop. The circulation and mixing of sea water also helps break up ice caps - deep cold waters move in stately fashion from the poles towards the equator, and rise, thereby pushing currents of warm water, ever so slowly, back towards the poles.
In the Mid Carboniferous, the southern supercontinent of Gondwana (including what is now South America, Africa, Antarctica, and Australia) was moving south, and it approached the South Pole. As this happened, an ice cap began to build up, and this survived for 30 or 40 million years, into the subsequent Permian Period, when the ice finally disappeared as Gondwana drifted away from the pole. There is extensive evidence for this southern glaciation, both geological and palaeontological. For a
5 time, there was no life around the South Pole - the coals o t disappeared - and the rocks show clear evidence of glaciation:
| glacial tills and rock scratching, as seen earlier in the
¡= Neoproterozoic (see pp. 47-9), as well as sands compressed and contorted by the weight of ice, and scattered erratic ('wandering') blocks, rocks that had been torn up by glaciers and dropped elsewhere randomly.
Early observations of the evidence for Carboniferous glaciation were key elements in proving the concept of continental drift. About 1900, geologists pooled their evidence for glaciation in Australia, South Africa, South America, and India. They noted that the glacial features all pointed back to a source of ice in South Africa. Glaciers had evidently radiated from there, eastwards across Australia, westwards across South America, and northwards across India. On the modern map, India of course lies in the northern hemisphere, so how on earth, these early geologists asked, could ice have flowed all the way from the South Pole and across the equator?
The Carboniferous is famous for its coal. Indeed the name Carboniferous comes from the French carbonifère, meaning 'coal-bearing'. Huge coal deposits occur in Mid and Upper Carboniferous rocks of Europe and North America. Coal is almost pure organic carbon, composed of the remains of tree trunks, leaves, branches, and other plant debris that have been buried and compressed. Sometimes these plant remains may still be seen in the coal; in high-grade coals such as anthracite, the plant remains have been compressed and heated at depth and very little trace of their structure remains.
It is a puzzle why there is so much Carboniferous coal, and not much from other times. There are indeed some coal deposits from younger rocks, and some of these are commercially exploited, but these deposits pale into insignificance beside those from the Carboniferous. The clue may be partly in the worldwide models for the accumulation of coal: because of the particular Earth movements at the time, and perhaps because of the southern polar ice cap, land was subsiding rapidly in many areas, and huge thicknesses of terrestrial sediments, including coal beds, accumulated. But why were lush forests so extensive then?
The clue might come from studies of ancient atmospheres. It might seem incredible that geologists can reconstruct ancient atmospheres, or indeed that ancient atmospheres might have been different from today's. We have seen that the earliest Earth had an atmosphere that was devoid of oxygen (see p. 39), and that oxygen levels built up to near-modern levels by the end of the Precambrian. However, there is extensive evidence that levels of oxygen and carbon dioxide have varied considerably through the past 500 million years, and the Carboniferous was a time of extraordinarily high oxygen levels.
Geochemists focus in particular on isotopes of carbon and oxygen in the rocks. The secret is that the waters of seas and lakes contain similar proportions of gases to the atmosphere of the time, and these proportions may be locked into the skeletons of shellfish and planktonic organisms, as well as into limestones deposited on the seabed or certain kinds of ancient soils. It's essential of course to choose samples that have not been altered by any subsequent geological activity, so that the chemical measurements actually reflect conditions all those millions of years ago.
Many elements may exist as several isotopes, forms with different atomic weights. Isotopes are important in radiometric dating (see pp. 17, 20), and also in reconstructing ancient environments. Carbon, for example, exists generally as carbon-12 in living organisms, as carbon-13 in inorganic reservoirs, and even as the radioactive form carbon-14 in some settings. Nearly 99 per
5 cent of carbon on Earth is in the form of carbon-12, and the other o t two isotopes make up much smaller quantities. Oxygen exists in
| three isotopic forms, oxygen-16, which is commonest, as well as
¡1 oxygen-17 and oxygen-18. By measuring the ratios of these isotopes in ancient rock and fossil samples, geochemists can reconstruct ancient temperatures (from the ratio of oxygen-18 to oxygen-16), and detect perturbations to the carbon cycle (for example major extinctions, volcanic eruptions, releases of carbon from deep stores) from the ratio of carbon-13 to carbon-12.
All the measurements indicate that the Carboniferous atmosphere carried something like 35 per cent of oxygen, compared to 21 per cent today: this was the highest level that oxygen ever reached. The reasons are debated. It could simply be that the huge rise in plant diversity and abundance meant that global levels of photosynthesis increased, and so vast amounts of additional oxygen were pumped into the atmosphere. The problem with this idea is that oxygen levels fell to modern levels soon after the
Carboniferous, and yet plant richness remained as high as ever. Another reason might be that large quantities of wood were buried during this period because nothing could eat the lignin: today there are specialist bacteria that can reduce lignin rapidly, and wood-eating animals such as termites and beavers have such bacteria in their guts. If huge quantities of wood were buried, as they were, carbon was removed from the atmosphere through photosynthesis of carbon dioxide, and locked up in the buried, but not decomposed, plant cells, so releasing more gaseous oxygen and building up overall atmospheric oxygen levels.
Many palaeontologists have speculated that the extraordinarily high levels of oxygen in the Carboniferous might have permitted huge insects to evolve. Indeed, there were dragonflies like birds, cockroaches as large as your hand, 2-metre-long millipedes, and so on. Gravity was just the same in the Carboniferous, so why would o insects be larger? There may be two reasons. Some physiologists | have suggested that the oxygen-rich Carboniferous atmosphere a would have been denser than today's, and so would have provided fl more lift and thus made it easier for them to fly. This sounds a bit t weak, and in fact it probably wasn't the whole story.
The key may be in respiration. Insects 'breathe' by diffusion, and do not pump air, as we do, in and out of lungs. Insects have pores in their cuticle that extend deep into the body. Oxygen diffuses passively into their tissues through branching tubes, and this limits the size of an insect, and indeed the size of most arthropods. Because oxygen diffuses into the body, the cross-section of an insect is limited, and insects generally cannot be much larger than the largest dragonfly today, with a 15-centimetre wingspan, and a body as thin as a pencil. Meganeura, the Carboniferous dragonfly, had a 75-centimetre wingspan, and a body more like a frankfurter than a pencil. An atmosphere rich in oxygen may have been sufficient to allow arthropods to achieve gigantic size even with their passive diffusive system of respiration.
Damp forests of vast trees and lush undergrowth became widespread in the Middle and Late Carboniferous. The plants included giant clubmosses, horsetails up to 15 metres tall, ferns, and seed-ferns (Fig. 15). There were no flowering plants - these came much later, in the Cretaceous (see pp. 15,143) - and conifers were rare. These Carboniferous plants though had some of the first proper leaves - necessary to promote photosynthesis at a time when carbon dioxide levels were low.
Clubmosses were generally low shrubs, but some of them became huge in the Carboniferous. The best known is Lepidodendron, a clubmoss that reached 35 metres or more in height. Fossils of Lepidodendron have been recognized for 200 years because they are commonly found in association with
5 commercial coalfields in North America and Europe. At first, o t the separate parts - roots, trunk, bark, branches, leaves, cones,
| and spores - were given different names, but over the years they
¡= have been assembled to produce a clear picture of the whole plant. The massive roots of Lepidodendron specimens may be seen in situ in a famous Victorian museum in Glasgow. When the specimens were found in an old quarry in 1887, they were carefully excavated, and covered over by a marvellous greenhouse constructed from cast iron and glass, and this can still be seen in the Victoria Park.
Horsetails are familiar to gardeners today as small pernicious weeds. Their upright green shoots, with a characteristic jointed structure, are linked by underground rhizome systems. Horsetails today may be rather obscure, and generally small, but this was a significant group in the Carboniferous that grew in incredibly dense, bamboo-like thickets. One form, Calamites reached nearly 20 metres in height, but shows the jointed stems and whorls of leaves at the nodes that are typical of modern smaller horsetails. The trunk of Calamites generally arose from a massive underground rhizome. The leaves formed radiating bunches at nodes along the side branch, and there were usually two types of cones.
The clubmosses and horsetails occupied the low-lying floodplains. Seed-ferns, conifers, and ferns were adapted to drier conditions, and they occupied elevated locations such as levees, the banks of sand thrown up along the sides of rivers. Ferns today are generally low-growing herbaceous plants, common in many environments. Some of the Carboniferous ferns were tree-like, with their fronds borne on a vertical trunk, while others were smaller, like typical modern ferns.
Seed-ferns were a diverse group of shrubs and trees. Conifers today include pine, spruce, and monkey puzzle. The Carboniferous cone-bearers, as today, were adapted to dry conditions. The earliest conifers had cones and long needle-like leaves that were adapted to save water. More familiar conifers only evolved rather later, in the times of the dinosaurs.
Some of the best evidence about Carboniferous plants comes, ironically perhaps, from burnt charcoal that was buried after wildfires. In the tropics today there are often vast wildfires that burn up hundreds of acres of forest. Fires may be started by a carelessly thrown cigarette or a bottle that focuses the rays of the sun, but usually the causes are natural; fallen branches and leaves may just be so dry that a chance lightning strike may spark off a huge conflagration that burns for days or weeks. The charcoal that is left after the burning still preserves all the fine cellular detail of the ancient wood, and ancient examples can reveal a huge amount of information.
Wildfires are not always destructive; indeed, many plants rely on occasional fires to clear old timber and to allow new shoots to grow. And the ash from the fire provides phosphorus and other nutrients. Wildfires were common in the past, and particularly in the tropical belt during the Carboniferous, and the high atmospheric oxygen levels surely stimulated more burning than happens today. Investigations show that fires were commonest in the higher areas, away from the banks of rivers, where plant debris could become very dry. Some of the Carboniferous wildfires might have been set off by nearby volcanic eruptions, or they might have been set off regularly during particularly hot or dry seasons. Wildfires were probably a regular part of the growth and regrowth of forests, and also destabilized hill slopes, triggering occasional landslides.
The new forest habitats of the Carboniferous opened up great possibilities for the early tetrapods, and they diversified extensively. As we have seen (p. 79), Late Devonian tetrapods are 5 quite rare, and they still remained largely in the water. The groups o t diversified into some forty families in the Carboniferous, some of | which continued to exploit freshwater fishes by becoming ¡= secondarily aquatic, while others became adapted to feed on the insects and millipedes.
There used to be a major gap in our knowledge of tetrapod evolution in the Early Carboniferous, but new work on localities in Scotland has revealed some extraordinary animals from this time. One of the strangest is Crassigyrinus. It has a large skull with heavily sculptured bones and massive jaws, which show it was clearly a fish-eater. In fact, Crassigyrinus was really a massive head driven by a bulky body and minute limbs, and it could barely have struggled about on land.
The whatcheeriids, known from Scotland and the United States, were metre-long animals, also with massive heads, but perhaps mixing their diet of fish with occasional tetrapods too. The baphetids, known from Europe, had very low skulls. Their broad curved jaws were lined with small teeth, and they may have hunted rather small fishes. This head shape, roughly semicircular, with broad grinning jaws, and a skull that is about the same depth as the lower jaws, was typical also of a major group of Carboniferous tetrapods, the temnospondyls. There were many different lines of temnospondyls, and the group diversified and was important through the Permian and Triassic, and survived right into the Cretaceous, some 200 million years later. Most temnospondyls were up to a metre in length, and they were generally fish-eaters, but some were much larger; a few much smaller.
The most unusual Carboniferous tetrapods were the lepospondyls. There were three lepospondyl groups, the small insect-eating and quite terrestrially adapted microsaurs, the newt-like nectrideans, some with extraordinary broad heads shaped like boomerangs, o and the enigmatic limbless ai'stopods. The microsaurs dashed |
about on dry land, the nectrideans were strongly adapted to life in a the water and may have snapped at insects above the water, while fl the ai'stopods perhaps hunted slugs and worms in the damp leaf t litter of the forest floor.
All the tetrapod groups mentioned so far were on the amphibian side of the fence, and it is probable that modern amphibians evolved from this group. The oldest frogs are Triassic in age, and probably evolved from among the temnospondyls. Salamanders might have arisen at the same time, but their oldest representatives are known first from the Jurassic.
The other major tetrapod branch were the reptiliomorphs ('reptile-like'). Basal reptiliomorphs included the Carboniferous anthracosaurs, a group of medium-sized rather aquatic animals that chased fishes in the rivers and ponds. But, midway through the Carboniferous, the reptiliomorphs spawned something surprising: the first reptile.
The great Scottish geologist Sir Charles Lyell (1797-1875) had the surprise of his life when he visited the wind-lashed shores of Nova Scotia in 1852. He had been there before, in 1842, a hazardous and brave journey for a geologist more used to the quarries and coasts of Europe and the debating rooms of the Geological Society in London. Lyell had written his epochal Principles of geology in the early 1830s, and these had set the new science of geology on a firm road for its future development. Lyell was committed to understanding how the Earth worked, and he undertook travels to remote continents to augment his understanding and to provide materials for his hugely popular textbooks.
In 1852 Lyell was exploring the cliffs at a locality called Joggins, on the north shore of Nova Scotia, in company with the
5 Canadian geologist William Dawson (1820-99) who had earlier o t found some remarkable specimens of tetrapods preserved in an
| ancient tree trunk. Lyell was amazed by what he saw. A tree trunk
¡= stood there in the cliff, upright, in the position of growth. Modern erosion by the sea had worn away the rock and the two geologists could see inside the ancient tree stump. There, within the sand inside the trunk, were the tiny bones of a tetrapod. Lyell later reported that the Joggins locality was 'the finest example in the world'.
What Dawson and Lyell had found was a specimen of the oldest fossil reptile, later named Hylonomus. This animal was about 30 centimetres in length, roughly lizard-shaped, with a long tail and long limbs. Its small sharp teeth show that this was an insect-eater. The specimens of Hylonomus are superbly preserved because they have sat undisturbed within the ancient tree stumps; after 1852, many more examples of this stump style of fossil preservation have been found at the Joggins Cliff. It was only in the 1960s, over 100 years after its first discovery, that palaeontologists realized that Hylonomus was not a microsaur, but actually a reptile. Its high skull sets it apart from most of the Carboniferous tetrapods, which had low skulls. But more importantly, Hylonomus has a major ankle bone called the astragalus, a bone not seen in amphibians, but typical of reptiles, birds, and mammals.
How did these animals die? It seems that the trees were felled by a sudden flood in the Mid Carboniferous, and the trunks and branches were swept away. The tree stumps were presumably firmly enough rooted that they did not budge. The flood waters washed sand and mud around the tree trunks, partly burying them. As the core of the trunks rotted away, the woody tissues may have been attacked by insects, and the reptiles may have followed them in, looking for a snack. Why in the end the reptiles became trapped is not certain. Surely they could have climbed out of the hollow trunk? Or maybe a further flood swamped them before they could escape.
Why is Hylonomus so significant in evolution? The point is that a it was the first tetrapod to lay eggs, and so to escape from the fl hold of the water. As we saw (p. 83), the first tetrapods, the t amphibians, still relied on their proximity to the water to avoid desiccation and to lay their spawn. All other tetrapods, the reptiles, birds, and mammals, have made the break away from the water, and Hylonomus was the first. And yet there are no fossil eggs known from a Carboniferous reptile, so how do we know that Hylonomus and its descendants laid eggs?
The answer comes from phylogeny and the idea of homology (see p. 12). Modern reptiles, birds, and mammals lay very similar eggs, called amniotic eggs. The amniotic eggshell is usually hard and made from calcite, but some lizards and snakes have leathery eggshells. The shell retains water, preventing evaporation, but allows the passage of gases, oxygen in and carbon dioxide out. The developing embryo is protected from the outside world, and there is no need to lay the eggs in water, nor is there a larval stage in development. Inside the eggshell is a set of membranes that enclose the embryo, that collect waste, and that line the eggshell. The developing animal is sustained by the highly proteinaceous yolk.
Reptiles, birds, and mammals are called collectively the amniotes, or the amniota, because they all share the same ultimate ancestor, an animal close to Hylonomus. Birds evolved from dinosaurs in the Jurassic (see p. 138) and mammals evolved from another reptilian group in the Triassic (see p. 130). But do mammals lay eggs, as I have just claimed? Well, the most primitive living mammals, the platypus and echidna from Australia, do in fact lay eggs with a hard calcareous shell. The details of the anatomy of reptilian, avian, and mammalian eggs are all the same, and so these all evolved from a single ancestor. Track back down the evolutionary tree, and you arrive at Hylonomus, so Hylonomus must have laid the same kind of egg.
o o t The Carboniferous then seems to have been some kind of acme, or o
| high point, in the evolution of life both in the sea, and especially
1= on land. Nothing could ever equal the high oxygen levels, the vast acreages of lush tropical forests, and the giant insects. Indeed, only 50 million years later, at the end of the subsequent Permian Period, the most devastating mass extinction of all time would kill off nearly all of life.
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