Geology and terrestrial life of the Carboniferous

The Carboniferous Period is a fascinating time in earth history. It spanned 60myrs (359.2 to 299.0mya), towards the end of the Palaeozoic era, falling between the Devonian and Permian. During the Carboniferous, the supercontinent Pangaea was assembling and the oceans were home to invertebrates such as corals, bryozoa, ammonoids, echinoderms, trilobites and crustaceans. Fish were also well represented (especially sharks), which were rapidly diversifying at the time. The continents were no barren wasteland either – they were host to some of the first widespread terrestrial forest and swamp ecosystems. In these lived both invertebrates, which had crawled onto land by the Silurian period (at least 423mya) and vertebrates, which were relative newcomers to this realm.

This article provides us with an excuse to write about the Carboniferous. We will first introduce the geology and palaeogeography of the Carboniferous, including an overview of the most common mode of preservation we see in terrestrial fossils. Then, we will provide an overview of terrestrial life during the period, as land-based ecosystems of this age are among the best known from the Palaeozoic and an exciting time in the history of life.

Global paleogeographic reconstruction of the Earth in the late Carboniferous period 300mya. (C)opyright Dr Ron Blakey (Wikipedia Creative Commons).

Carboniferous geology

The Carboniferous is split into two epochs, the Mississippian (or Lower Carboniferous; 359.2 to 318.1mya) and the Pennsylvanian (or Upper Carboniferous; 318.1 to 299.0mya). As we shall see, the two are associated with very different rocks. The palaeogeography – that is, arrangement of the continents at the time – was shaped by a mountain building event known as the Variscan Orogeny (or Hercynian in some regions, and Alleghenian in America), which was at its peak. This was the result of the collision between a southern landmass called Gondwana and, in the north, the continent Laurussia. The latter had itself been assembled in the early Palaeozoic during the earlier Caledonian Orogeny. It is a little complicated in detail but, by the end of the Carboniferous, Pangaea – the supercontinent on which the dinosaurs lived millions of years later – had almost finished assembling. In it, there was an equatorial mountain chain similar in scale to the present day Himalayas, the remnants of which are found today in Russia, Western Europe and North America. There were two major oceans at the time – Panthalassa, a super-ocean covering much of the globe, and the smaller Paleo-Tethys, which was between the two halves of the assembling Pangaea and surrounded by a ring of continental crust.

soudley-valleyCoal seam from a road cutting along the Soudley Valley.

There was a slight rise in sea level at the start of the Carboniferous, which created large areas of epicontinental seas, and the rocks dating from the Mississippian epoch are generally limestones from such environments (usually carbonate ramps). They rarely record terrestrial life, despite occasional terrestrial deposits. This all changed in the late Carboniferous. The Pennsylvanian was a time with a large ice mass situated across the southern continent of Gondwana, which straddled the South Pole. Despite the chilly conditions in the south, a wide zone – spanning the tropics and even some higher latitudes – maintained coal forests. These were low lying, flat and swampy areas – wet environments, which allowed organic matter to accumulate. Flooding and wildfires (evidence of occasional drier periods) were widespread. Scientists still debate the true extent of these forests – whether they were actually widespread or whether they result from a taphonomic (preservational) bias within the rock record.

Either way, many Pennsylvanian rock deposits come from lush, tropical forests, in which a large amount of organic plant material was deposited in waterlogged, boggy sediments. These are found as part of cycles alternating between marine and terrestrial deposition known as the Coal Measures in the UK, because the plant and, therefore, carbon-rich layers are now found as coal seams. These coals are frequently interbedded with mudstones and siltstones into packets of terrestrial sediments, often found between layers of sandstones and occasionally limestones. Such deposits are known as cyclothems and are very important – the Coal Measures were responsible for fuelling the industrial revolution and remain economically important today. Without Carboniferous coal, it is almost certain that Great Britain – and the world – would be a very different place today.

The coal forests generally got dryer as the Carboniferous progressed and, while a small enclave survived in China until the end of the Permian (299 to 251mya), the majority died out at the end of the Carboniferous. The reasons for this remain unclear – it could be the result of climatic change or the newly formed Variscan mountains breaking up the habitats.

Carboniferous fossils

The only reason we know a lot about life during the Carboniferous is because we have fossils to work with and where the time period really comes into its own is its records of early terrestrial life. The advent of plant colonisation of land reaches back to the Upper Ordovician (about 450mya) with the preservation of micro-fossils (sporomorphs) and cuticles. Recognisable vascular plants appear in the Late Silurian to Early Devonian with classic examples such as Cooksonia. Our first evidence that animal life had come on land is in the form of fossils of millipedes, about 423myrs old. These, along with insects and arachnids, made up the earliest animal ecosystems. Vertebrates like us only came onto land relatively late in the game by the Devonian (about 375mya). However, by the Carboniferous, the varied plants of the coal forests played host to a relatively diverse collection of creepy crawlies and early vertebrates. The time period is also the first that has widespread preservation of terrestrial fossils. This widespread and excellent preservation of terrestrial sites is unexpected and can be explained by a fortuitous combination of factors during the Carboniferous. In the cyclothems, typical deposits reflect broad coastal deltas, formed when the mouth of a river flows into a larger body of water and drops much of its sediment. The resulting deposits are close to land and so terrestrial fossils can be washed in. Many of the fossils from this period were buried in shallow marine basins, estuaries, or the bays and lagoons between the fingers of a delta. The low-lying coal forests were intimately associated with such environments and – as we learned previously – sporadic inundations left a large number of organic, carbon-rich deposits that eventually became coal. The prevalence of both deltas and coal swamps stems from the palaeogeography of the era, with a large swathe of low-lying land in the tropics and lots of shallow seas.

Carboniferous brachiopod (Antiquatonia hindi) from Hall Dale Quarry, Matlock.

These deposits, with high levels of organic carbon, would have led to local anoxia, layering of the water body and varied salinity. Such conditions favour exceptional preservation of fossils, as they slow the decay of soft tissues. However, rapid burial and high carbon concentrations, in the absence of sulphate (preventing the formation of pyrite) and anoxia (preventing oxidation) can result in the formation of siderite concretions (FeCO3, otherwise known as ironstone). The formation of this mineral is further aided by low magnesium and calcium concentrations, which would otherwise lock up the carbonate in limestone or dolomite. All this makes siderite formation most common in fresh and brackish waters – found close to or on land. These concretions form very rapidly and early in the history of the host sediment, and usually before any compaction of the rock occurs – some even show evidence of spring and neap tides, suggesting formation within days. In such conditions, the remains of organic matter tend to decay by microbial methanogenesis, which causes organic material to become a locus for precipitation of carbonate nodules. Therefore, it is common for siderite nodules to form around dead organisms and protect the resulting fossils from further deformation. These fossils are commonly found as moulds (that is, voids) within siderite nodules, but a surprising proportion of siderite nodules do host surviving mineralised organic material.

Fish coprolite in siderite nodule from Scotland.

So, to recap – Carboniferous palaeogeography lent itself to a prevalence of shallow marine and deltaic environments, close to land. These created the ideal conditions for siderite nodule formation, which in turn makes exceptional and three-dimensional preservation a common feature, providing the earliest extensive record of terrestrial ecosystems. These are found in sites of exceptional preservation (Lagerstätten) such as Mazon Creek (Illinois, USA), Coseley (near Dudley, UK) and Montceau-les-Mines (France). This is all helped out by our Victorians ancestors, assiderite formation is concentrated immediately above or below coal seams, and many of these deposits are associated with extensive mining operations. Therefore, a lot of the earliest work on these fossils dates from the Victorian era (1837 to 1901), when coal mining was prevalent in the UK and the rest of Europe, with some deposits even being commercially mined at the time for their high iron content.

Terrestrial Carboniferous life

The majority of Carboniferous, siderite-hosted deposits record life from the shallow seas they were deposited in and also the adjacent coal forests. This article will focus on the life on land during this period. Scientists still debate whether the fossils we find were typical of the Carboniferous Period or if it just so happens that fossils sample coal forests preferentially. Either way, these remains show that land was host to complex ecosystems, which we will introduce here. Our account will be split into three separate sections – the first will introduce the plants that constituted coal forests and played host to the other two groups. The second will introduce terrestrial vertebrates of the time – relative newcomers to the terrestrial realm, but ones already reflecting some of the major splits between different terrestrial vertebrate groups around today. However, then as now, the vast majority of the diversity of life was in a group called the arthropods and, therefore, we will finish by outlining the different arthropods groups forging a life in the Carboniferous coal forests. This is clearly a selective approach, as we are omitting marine life and also terrestrial bacteria and other single-celled organisms.

The largest plants both in physical size and biomass contributing to the carbon system were the arborescent lycopsids. These plants, which were related to the living genus Lycopodiella (left) and Selaginella (right), formed the main body of lowland swampy forests. © Wikimedia Commons, Selaginella – Luis Fernández García, Lycopodiella – Franz Xaver.

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