Geology and terrestrial life of the Carboniferous
Russell Garwood and Alan Spencer (UK)
The Carboniferous Period is a fascinating time in earth history. It spanned 60Ma (359.2 to 299.0Ma), 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.
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.
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 423Ma 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.
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.
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.
Plants
The wide-spanning tropical region that, like today’s rainforests, exhibited a warm and humid environment, provided perfect growing conditions for a diverse array of plant groups. There was a global average temperature of 20°C during the Mississippian cooling to 12°C during the Pennsylvanian, with global atmospheric carbon dioxide (CO2) levels that started at about 1,500ppm dropping to about 350ppm (comparable to our modern day level) by the Permian. This warmth helped some species to evolve gigantism and arborescence (that is, having a tree-like structure).1
The largest plants of the era, both in physical size and biomass, contributing to the carbon system (70% during the Westphalian dropping to just 5% during the Stephanian) were the arborescent Lycopsids. These plants, which were related to the living genus Lycopodiella (bog clubmosses) and Selaginella (spikemosses), formed the main body of lowland swampy forests.
Growing to a height in excess of 50m, these giants of the forest consisted of a single trunk supported by shallow roots, which bore leaves and reproductive organs. All of this conjures up an image of modern day trees, yet they possessed a body-form and growth pattern that is unlike any plant now living. One of the commonest found fossils is that of Lepidodendron, a compression fossil of the outer trunk surface, which shows the asymmetric rhomboidal shaped scars caused by the detachment of leaf ‘cushions’.
These pads acted as the photosynthesis centres for the plants and covered the whole trunk. Supporting the trunk is a network of roots and rootlets (Stigmaria), which spread at a shallow depth within the palaeosol (fossil soil). The trunks grew rapidly over a 10 to 15-year-period and only towards the end of this time, with the plant reaching maturity, did a crown of branches sprout from its top. The branches supported a tree-like canopy and a reproductive system of strombili (reminiscent of cones), which bore both megaspores (female) and microspores (male) parts. Therefore, unlike the rainforests we see today, the forests of the lowland Carboniferous were fairly well lit, with a high proportion of ground that lower growing plants could colonise.
One of the groups that inhabited this well-lit ground, as well as colonising the swampy levees, were the Sphenophytes (commonly referred to as horsetails) of which only one genus still lives today – Equisetum (with around 20 named species). Horsetails are defined by their distinctive appearance of an erect, soft-centred stem bearing branches and leaves in whorls at nodes. Evidence within the fossil record also points to underground rhizomes that, like the modern day Equisetum, allowed sideward spreading of the plant.
This process should not be confused with sexual reproduction which, as in the living species, was only achieved through spore transfer borne within cones at the end of the stems. During the Mississippian, the order Sphenophyllates took off and began to flourish in the Pennsylvanian. These plants had horsetail-like features, exhibiting whorled leaves, ribbed stems and cones with whorls of sporangiophores (branches bearing sporangia) and bracts. However, the stem does not conform to the same anatomical structure, having a triangular cross-section and a solid centre. Their growth forms varied for species to species, for instance, some had hooks and spines for climbing, as well as occupying disturbed and open ground. Within the swamp ecosystem, the Sphenophyllates became part of the lower ground cover.
Simultaneously during the Mississippian, the order Archaeocalmitaceae become abundant, forming a large percentage of the land’s vegetational component. For a long time, this order was a defining feature of Mississippian-aged rocks, not having being found within the Pennsylvanian. However, recent discoveries are revising this view and some Permian fossils now appear to show identical features to Archaeocalmites (a fossilised stem). This could indicate a displacement during the Pennsylvanian, from the wetter coal swamps to the drier uplands where fossil preservation is less evident.
So far, we have discussed some of the low growing plants of this order, yet during the Pennsylvanian, the sphenophytes reached their maximum physical size and diversity. Calamostachyaceae seemed to have preferred wetter areas of the palaeo-tropical belt where they flourished around the edges of lakes and rivers. They grew up to 10m in height, spreading like most sphenophytes through underground rhizomes that produce vertical stems, with branching at nodes. These branches were able to bare several orders of branches, supporting leafy shoots with whorls of leaves (for example, Annularia) and reproductive organs in the form of cones (for example, Palaeostachya sp). They have been referred to as the “giant bottle brushes” of the Carboniferous.
Ferns were a common sight in the coal forests, exhibiting a variety of morphologies – from small plants to larger arborescent forms. During the Carboniferous, there was a change in the ferns seen – three major groups of early ferns became extinct (Cladoxylates, Zygopteridales and Rhacophytales), one continued and flourished (Stauropteridales), and three groups of modern ferns came into existence (Asterotheceae, Umatopteridales and Marattiaceae). Many were small and herbaceous, living throughout the coal swamps and forests.
Of these, one of the most important groups is the Marattiaceae. These still survive today (for example, Christensenia) and it is this group that we will focus on. During the late Carboniferous, they were common (but not abundant), growing in mainly drier localities (that is, raised river levees). Yet, at the end of the Carboniferous, they had replaced the arborescent lycophytes as the dominant tree flora. Marattialeans are described as “tree ferns”, possessing a wide trunk up to 1m in girth and reaching a maximum height of 10m.
Within the fossil record, the stems are referred to as Psaronius (where anatomical preservation is seen), which were anchored to the ground by a mass of adventitious roots (that is, roots with the ability to grow in unusual places, for example, on aerial stems), which also gave the trunk a conical shape. The top of the trunk bore the leaves, which expanded on fronds from the crosiers at the apex. The frond may well have been up to 3m in length, with highly divided leaves. On death, the frond was shed, leaving a residual scar on the trunk. This dead plant material became the basis of widespread peats which, especially towards the end of the Pennsylvanian in Europe and North America, formed many of the coals seen today.
So far, we have seen a wide variety of plants that inhabited the Carboniferous, which share one defining feature – sexual reproduction that depended on moisture. These plants were not able to successfully dissociate themselves from the wetter environments they first colonised. It was not until the Late Devonian that nature provided a solution – the seed. Seed-bearing plants are generally separated into two groups: the angiosperms (flowering plants with proactive ovules, which won’t figure in this article as they had not yet evolved) and the gymnosperms (seeds with no protective ovules). Within the Palaeozoic gymnosperms, there are three informal groups: pteridosperms, cordaites and glossopterids; the last of which we also won’t cover as they only arose at the beginning of the Permian (299.0mya) becoming extinct by the end of the Triassic (199.6mya).
Pteridosperms (seed-ferns) grew as both small trees and scrambling creepers, probably colonising the levees around rivers. Their leaves are fern-like, which explains the name, with fossils being placed in a large number of form-taxa. The earliest know pteridosperms belong to the order Lyginopteridales, in which there are two well-documented families. The Elkinsiaceae were widespread during the late Devonian and early Mississippian along the palaeo-tropical belt, but being replaced during the Pennsylvanian by Lyginopteridaceae. Many were relatively small plants (for example, Elkinsia, which was less than one meter high), yet some like Pitys had trunks up to 2m in girth – it is thought that some maximised their photosynthesis opportunity by climbing up trunks of tree as vines. But, however much they had evolved during the Carboniferous, by the late Pennsylvanian, they had become extinct.
The late Mississippian brought about another change within the seed-ferns with the advent of the Medullosaleans. These were one of the most successful gymnosperms throughout the Pennsylvanian, fossilised remain having been found in Europe, North America, China and Central Asia. The middle Pennsylvanian medullosales were small to medium-sized trees, which diversified later to include scrambling modes of life. The arborescent form bore fronds up to 7m in size comprising a central stem, which could branch in to sub-rachises, supporting leaflets. The seed they produced tended to be large and had a characteristic three-fold symmetry (for example, Stephanospermum). However, with a few exceptions, we do not yet know much about how they were attached to the parent plant.
Cordites are the last group of gymnosperms we will discuss in relation to the Pennsylvanian. They were scrambling shrubs and trees, which inhabited a varied number of ecosystems, including the drier areas in more marginal parts of the wetlands and areas similar to modern-day mangroves. Unlike all other gymnosperms of the time, they did not produce fronds. Instead, they grew elongate tongue-shaped leaves with straight veins along their surface (not too dissimilar to modern-day Dracaena). Cordites are generally seen as early relatives of our modern-day conifers, although there is continuing debate as to the exact relationship.
Vertebrates
The earliest terrestrial vertebrates date from the Devonian, about 375mya. Sadly, their story in the Carboniferous is messy and there is a lot of complex vocabulary involved, which unfortunately cannot really be avoided. Several pivotal phases of early tetrapod evolution of occurred in the Carboniferous, largely in taxa from the moist coal forests, where these creatures usually lived as predators, consuming freshwater fish or terrestrial arthropods (a group we will introduce below). The split between true amphibians (Lissamphibia, including frogs, caecilians and salamanders) and amniotes (mammals and sauropsids, the latter being reptiles and birds) probably occurred in the late Devonian, so a little earlier than the timeframe discussed in this article.
However, the split between the ancestors of mammals (the class Synapsida) and the sauropsids occurred in the mid-Carboniferous. Much of the lack of clarity surrounding these tetrapods stems from the fact that the new forest habitats created a great many opportunities, resulting in a diversification of the early land vertebrate fauna to over 40 families by the end of the period. This situation is aggravated by the fossils’ importance – the ancestors of all extant vertebrates have their origins in the Devonian and Carboniferous.
Accordingly, the relatively small number of fossils has been researched extensively and this complex history of study has resulted in competing theories, frequent uncertainty and regular changes. For example, most of the fossils introduced below have traditionally been placed in a now obsolete subclass called Labyrinthodontia, which was based on a complex tooth structure and large body size. However, we shall do our best to give a balanced account of the situation.
Fossils from the Lower Carboniferous are relatively sparse, with few fossils from the first 15 years of the Mississippian – a phenomenon known as ‘Romer’s Gap’. However, Mississippian vertebrates have been discovered in recent years, so we will introduce a couple here. These include a group called the colosteids – basal tetrapods, superficially similar to a later group, the temnospondyls, which are also introduced below. One example is Greererpeton (found in West Virginia, USA), which possessed an elongate body, broad tail, short limbs and a low, flat skull, indicative of an aquatic lifestyle. A more recent discovery in Scotland is the fossil Crassigyrinus, which possessed a robust skull and sharp fangs, suggesting it was carnivorous. It also had a flattened tail bearing a broad fin, again suggesting an aquatic creature, so it probably preyed on fish.
A recent discovery could bridge the gap between Devonian forms and those found in the Carboniferous. This family, Whatcheeriidae, is known from a single specimen, about a metre in length, with a deep lower jaw and sharp, recurved teeth suggestive of a carnivorous diet. It also had some fish-like features (for example, palate bones), but other features are indicative of later tetrapods, such as a lightly sculpted skull and five digits. This mixture of both primitive and derived characters suggests it could lie between Devonian forms and the more derived Carboniferous ones, and it has been placed here using computer analysis in recent years.
The assemblage of tetrapods known from the Upper Carboniferous is more diverse. While often referred to as amphibians, these are not true members of the group Lissamphibia – although some could be early relatives that predate the split between frogs, caecilians and salamanders. A great deal of uncertainty surrounds the contentious issue of lissamphibian origins and their position in the tetrapods varies between different workers.
However, the most widely accepted hypothesis is that the true amphibians lie close to a group called the Temnospondyli. This is the most abundant and diverse group of early tetrapods, which was widespread until the Triassic (251 to 200mya) and became extinct in the Lower Cretaceous (Aptian, 125 to 112mya). Members of the group are defined by broad skulls, with a rounded front margin and an open palate at least half as wide as the skull. Their stout limbs, with strong shoulder and hip girdles, suggest terrestriality, although aquatic examples are known. There was a great deal of variety in the group, from salamander-like examples with short legs and squat bodies, to more robust and crocodile-like forms.
Another major group around in the Carboniferous was the Lepospondyli. While some studies in the last two decades have suggested that the Lissamphibia could be more closely related to this group, most recent work suggests the group probably lies closer to the amniotes. Generally smaller than the Temnospondyli, these creatures also show a large variety.
The largest group of lepospondyls in the Carboniferous were the salamander-like microsaurs. Some, like Tuditanus, had lizard-like proportions, powerful limbs, a strong skull and short teeth capable of penetrating arthropod cuticle. They were most likely fully terrestrial, but a handful of species appear to have been secondarily aquatic (for example, Microbrachis). Others show evidence of burrowing/leaf litter dwelling in the form of reduced skull bones, long bodies and short limbs (for example, members of the family Brachystelechidae), while others were newt-like in appearance (for example, Sauropleura), with long flattened tails.
The Nectridea are another lepospondyl group found in Pennsylvanian deposits. They often had extremely long tails and many were newt-like in appearance (for example, Sauropleura). These ones had very long flattened tails, and vertebrae with ornamented symmetrical spines above and below. The resulting deep, flat-sided tail could have been used for propulsion and is indicative of an aquatic group.
Very few cranial features of nectrideans are uniquely shared by all members of the group. Some stranger examples include the taxa Diplocaulus and Diploceraspis, which bore horn-like outgrowths on the skull that are ambiguous in function, but possibly an aid to capture prey in weak currents. There are a number of other lepospondyl groups, including Aïstopods that were similar in appearance to modern snakes – they lacked limbs, but possessed up to 230 vertebrae. Some even may have possessed extra joints in the skull, allowing them to open their jaws unusually wide as some snakes do.
There is also a messy collection of groups that probably lie fairly close to the origin of modern amniotes, which are sometimes called the Reptiliomorpha. This name applies to the most reptile-like early amphibians (and their descendants, including Amniota). They include anthracosaurs – creatures around from the early Carboniferous to the early Triassic, of which both terrestrial and secondarily aquatic examples are known.
One group – the Diadectomorpha – is also known from the late Carboniferous and possessed both amphibian and reptilian characteristics, so probably also falls close to the origin of the amniotes. One notable example is Diadectes, fossils of which have been found in the USA and Germany. This creature was heavily built, with strong limb girdles, short limbs and heavy ribs and vertebrae. However, most excitingly, it ranks among the first terrestrial vertebrates to exhibit specialisations towards a herbivorous diet in the form of short peg-like teeth at the front of the jaw and, behind these, rows of broad, blunt cheek teeth for grinding.
We’ve also been keeping a secret until this point – the oldest true amniotes are, in fact, Carboniferous in age. Two amniotes of this age – Hylonomus and Paleothyris – were found in Nova Scotia. They possessed a slender, approximately 20cm-long body, with a diminutive head and small sharp teeth. This suggests the earliest amniotes preyed-upon invertebrates – a diet facilitated by an increase in the strength of the jaws and associated musculature, compared to the basal tetrapods. Both were lightly built and have been found within fossilised tree-trunks in a typical coal forest environment. All Carboniferous amniotes were similar in form, being small to medium-sized insect-eaters. It was during the Permian that there was an explosion in amniote diversity.
Arthropods
This brings us to the arthropods. These are a very important group of animals, given that more than half of all described living species are arthropods and they have been the dominant contributor to animal species diversity for all of the past 520Ma. Arthropods share a suite of features, including a segmented body with specialised regions (called tagma), an open circulatory system featuring a dorsal heart and an exoskeleton composed of articulated plates, which is moulted as the animal grows.
There are five constituent groups in the phylum Arthropoda – the extinct trilobites, the insects (and a couple of smaller groups, making up the haxapods), the crustaceans (for example, crabs, lobsters, shrimp and many more, including woodlice), the myriapods (millipedes, centipedes and relatives) and the chelicerates (arachnids and horseshoe crabs). Three major arthropod groups, introduced below, had a terrestrial presence during the Carboniferous and all are common in Upper Carboniferous Lagerstätten. There are very few Lower Carboniferous arthropod-bearing deposits and, therefore, it appears that major diversification events (in, for example, the insects) remain entirely unrecorded in the fossil record.
As in today’s terrestrial ecosystems, a vital part of those in the Carboniferous were the hexapods. A large variety of insects lived in the coal forests. They included members of a group called the Archaeognatha, or jumping bristletails, which are primitive, flightless insects that have been found in France and Mazon Creek, Illinois. These are rare in comparison to fossils of the winged (or pterygote) insects. One of the most common of these found as fossils are the roachoids – cockroach-like insects, which probably pre-date the split between cockroaches (and termites which are, in fact, highly derived eusocial roaches) and the praying mantises.
These creatures had long antennae, were flattened in form and were probably very fast runners with long legs held at a low angle to the body – modern true cockroaches are among the fastest organisms for each unit of weight in the animal kingdom. The roachoids exhibit a number of the specialisations that help them move so very fast. The mayflies (Ephemeroptera), or ancestors of these, were also present, but it is not clear if these early forms had aquatic young, as modern mayflies do.
Also commonly preserved are examples of an entirely extinct group – the Palaeodictyopteroidea – which made up around half of all known Palaeozoic insect species. The group’s diversity of form suggests these insects filled a wide range of ecological niches until their extinction in the Permian. However, some common themes can be found in their morphology. They appear to have been largely herbivorous and numerous species had a third pair of wings at the front of their thorax – something that is not found in any modern insect groups. These have veins and look superficially like a third pair of reduced flight wings, but their function remains ambiguous. A lack of articulation makes a role in flight unlikely, but they are tantalising glimpses at what early insect wings might have looked like.
Most possessed sucking mouthparts, which were used to puncture plant tissues, as attested to by a wide range of plant damage in exceptionally preserved plant fossils from this period. It is thought they could be closely related to the ancestors of dragonflies and damselflies (the Odonatoptera) and are the only significant insect lineage to have become entirely extinct. The Odonatoptera were also present and included a group called the Protodonata or griffenflies. These were similar in appearance to dragonflies, but – in part due to the high oxygen levels in the Carboniferous atmosphere – could reach large sizes, with one species (Meganeuropsis permiana) possessing a wingspan of about 71cm. It is likely the Odonatoptera were aerial predators, their prey including other insects and even small vertebrates.
Another important component of Carboniferous terrestrial ecosystems were the myriapods, in no small part because they were probably responsible for churning up the leaf litter and creating soil on the floors of coal forests. They also appear to have been widespread and fossils of both millipedes and centipedes are known from the Carboniferous. Both lived then, as now, in the leaf litter on the forest floor – an environment that lends itself towards the rapid decay of organic matter, with abundant bacteria and fungi. This makes their fossils somewhat rarer than they might be otherwise, as decay is much more likely than fossilisation. This is especially true of the softer-bodied centipedes. Two members of extant centipede groups are known from Carboniferous fossils, but their preservation leaves something to be desired.
Millipedes actually have a higher presentation potential because they have a more robust exoskeleton – in most groups it is reinforced with calcium carbonate. However, their fossil record remains poor, possibly due to their behaviour. The cuticle is usually consumed after moulting to recycle the calcium and therefore, moulted exoskeletons rarely become fossilised. The Carboniferous provides the best early fossil record of the millipedes, much of the work being carried out in late Victorian times and the following two decades.
The most numerous Carboniferous millipedes are members of a group called the Archipolypoda, especially the heavily spined group, Euphoberiida, which, in addition to having an impressive battery of (defensive) spines, had ozopores. These were openings from which nasty substances could presumably be released, making them a still less attractive meal to tetrapod predators. Despite these long, branched spines and chemical defences, euphoberiid remains have been found in coprolites, so such measures were clearly not 100% effective.
Some also had modified legs midway down their length, probably to grasp the opposite sex while mating. However, the kings of this woodland realm – at least in terms of myriapods – were a group called the Arthropleuridea. These were common in the Carboniferous, with one genus (the Arthropleura) growing to over two metres in length. Their tri-lobed body possessed around 29 segments and they had a narrow head at the front. It is thought they were herbivorous, but a number of questions regarding their biology remain (to date) unanswered.
The final major terrestrial arthropod group in Carboniferous ecosystems – almost always the predators of our story – are the arachnids. There were a wide range of these around at the time, a few of which we’ll introduce here.
Commonly found as fossils in Carboniferous deposits are members of the Trigonotarbida. The species in this extinct arachnid group are superficially similar in appearance to spiders, but they lacked silk-producing spinnerets and the back half of their body was segmented (these segments are fused in most spiders). They were among the first predators in nascent terrestrial ecosystems and peaked in diversity during the Upper Carboniferous, by which time numerous families had developed heavy ornamentation in the form of spines. This was probably an adaptation to make them a less pleasant meal for tetrapod predators.
They preyed on other arachnids and appear to have had varied strategies for doing so, including running them down and ambushing them. More familiar creepy-crawlies making life dangerous in the coal forests were the scorpions. In the Carboniferous, there is an apparent peak in the diversity of fossil scorpion lineages, but it is unclear if this is a true signal or just a result of better preservation. They had tails like modern scorpions, with stings on the end, and further adaptations for catching prey. For example, some Carboniferous scorpions appear to have had trichobothria – hair-like structures that detect minor changes in air currents and aid prey location.
True spiders had also evolved by the Carboniferous. In fact, a series of fossils from late Carboniferous deposits in the UK are currently the earliest described examples. The spiders themselves are split into two groups, the Mesothelae and the more derived, Opisthothelae. It is the former that we find in the Carboniferous. Some exceptionally preserved specimens from France even preserve spinnerets – specialised silk-spinning organs. This demonstrates that they could spin silk by this time, but what exactly they used the silk for is open to debate and there is no evidence to suggest it was used for webs. These (French) Carboniferous fossils date the split between the Mesothelae and Opisthothelae, but examples of the latter are not found until later in the Triassic.
We’ll finish with some of the meanest looking arachnids out there – the Amblypygi or whip spiders. These get their name from their highly elongate first pair of legs. They often walk sideways, and use these long legs like antennae to feel ahead of them and locate prey. Two examples are known from the Carboniferous, one from the UK and the other from Mazon Creek, US. They appear to be related to a primitive but extant family called the Paracharontidae.
The Carboniferous
With that we come to the end of our description of the Carboniferous, a time when the ancient supercontinent Pangaea was assembling. This helped create unique conditions, which made the widespread preservation of terrestrial fossils far more common than we would otherwise expect. As a result, these are among the best known early terrestrial ecosystems. In particular, lush coal forests, with a wide range of plants, played host to a surprisingly diverse range of creatures, from giant insects and venomous scorpions – and to the ancestors of all major groups of terrestrial vertebrates including us.