Unusual Titans

At about 336 million years ago, this is the Early Carboniferous Subperiod. What in the future will be called Scotland is now a lush and swampy rainforest. The continental disposition has not changed dramatically since our last trip, with the large Gondwana still being found to the south and the continents Euramerica and Siberia to the north, the latter positioned the northernmost. Dwelling east, one will encounter the North China and South China landmasses. The already-stated forest is located reasonably close to the Equator, not too far from the coast of Euramerica and, consequently, not very distant from the coast of Gondwana either. This proximity will intensify over the next million years and lead to the collision of both, originating the famous supercontinent Pangaea. Anyhow, the Carboniferous is a very peculiar time and this is a direct consequence of two phenomena that had been previously discussed in our voyage to the Devonian: the spread of vegetation and the start of an ice age, which, by lowering sea levels during glacial periods, made available new areas for land plant colonization.

In this context, great amounts of carbon were being sequestered from the atmosphere by plant life and then trapped underground. But how exactly did such process take place? Formerly, it was thought that the bark and wood from the trees of this period were rich in lignin, a polymer not easily decomposed, requiring special enzymes in order to be broken down. It was also believed that fungi sporting these enzymes had simply not yet evolved, which basically meant that a huge amount of undegraded lignin accumulated. However, decomposers capable of breaking down this substance, which actually was not even that abundant in widespread plants of the time, were likely already present. As such, the real reason for the collection of plant material was a conjunction of widespread swampy habitats, where vast swathes of organic matter were produced and struggled to decay due to anoxic conditions of stagnant water, together with the tectonics fueling the already mentioned origin of Pangaea, which formed basins where the vegetal material was deposited, allowing it to, over several millions of years, become a significant portion of the coal that, when burned today, releases all the carbon that was once captured by those ancient photosynthesizers. 

Consequently, the mere number of plants and the aforementioned fact that some of their parts did not decompose resulted in a higher concentration of oxygen gas in the atmosphere, provoking increased and more severe wildfires. Ironically, this turn of events would somewhat aid in ending itself. As Pangaea formed, oceanic currents changed, aiding the glacial pulses to become more pronounced, which further promoted the spread of forests to lowland habitats exposed by the receding oceans. Apparently, the colder conditions were also wetter, unlike in our current ice age where colder usually means drier, the reasons for this opposition probably being various, such as the distinct locations and extent of ice coverage, the vastly contrasting continental layout, among others. 

Therefore, the number of plants climbed up yet more, with these sucking up even more carbon dioxide from the atmosphere, eventually generating cooling to the point that aridification of the climate and, as a result, the fragmentation of the once typical rainforests, which thrived in humid, water-logged habitats, ensued. It has even been suggested that the sequestration of carbon dioxide was so intense that Earth only narrowly avoided a global glaciation (such as the ones that occurred in the Great Oxidation Event and in the Cryogenian Period, both from before the Phanerozoic) due to the plant dieoff just described. Either way, this gloomy twist is reserved only for the end of the Late Carboniferous though, and, at least for now, the masses of greenery, not even in their maximum extent, rule unabated. However, it must be noted that the successions of glacial and interglacial episodes dynamically affect the extent of wetter and drier areas, either increasing or decreasing them, making the landscapes of the Carboniferous Period far from static. 

With these explanations out of the way, let us pay close attention to the environment we are in. Many different sounds can be heard if one is attentive enough, from the misty breeze that rustles against the leaves, to the croak-like noises emitted by amphibians, to the scuttling of invertebrates among the undergrowth. Medullosa seed ferns are seen with ease, being distinguished by their large wingless pollen, indicating that they may depend on pollinators in order to reproduce sexually, and also large seeds with lots of flesh, perhaps also dependent on animals for dispersion or maybe capable of being carried over to suitable grounds by the waterways. While these Medullosa are self-supporting ones, many others are incapable of bearing their own weight due to the morphology of their stems: they are climbing forms, using other plants as foundations. Regardless, the terminology "seed fern" is informal and simply refers to a wide collection of spermatophytes (seed-bearing plants, including gymnosperms and angiosperms) that encompasses representatives sometimes more related to non-members of the group than to other members proper. Medullosa in particular are related to cycads, extant gymnosperms. Cordaites trees, situated here and there, are gymnosperms too, producing sizeable seeds, like the aforestated Medullosa. They are part of the group Cordaitales, which includes various representatives, some very different from each other and occupying equally differing habitats: from very wet, like the one we find ourselves in right now, to ones seasonally dry, but still humid, where fires run even more rampant, with loads of plants charred and burned.  

Some trees, though, are especially common, these being of the genus Lepidodendron. With a few individuals reaching over 30 meters in height, they are true titans. Curiously, though, these lifeforms are not closely related to the tall trees of modern times and belong, instead, to the lycophytes, a group of vascular plants that reproduce by spores and which only have rather small surviving members. Also lycophytes are the unusual Sigillaria, being capable of living, like some Cordaitales, in areas not as humid or as wet as this one, which is quite atypical for members of Lycophyta. The two are quite peculiar in the sense they lack a wooden core as seen in most other trees, with their photosynthetic trunks actually being composed mainly of primary tissue, with only minimal amounts of wood and bark, the latter being important to resist the common wildfires. Additional adaptations of the sort include their rapid growth, enabling them to attain high statures quickly, and the great distance between their branches and the ground, serving to impede fire from destroying the leaves by climbing up.

All these plants are not merely taking advantage of this wet environment, but actively contributing to its formation. The damp and foggy atmosphere that surrounds us is, for instance, a result of their evapotranspiration. Being all vascular, they, despite differences in this system (the lycophytes have a more rudimentary one when compared to the one of Medullosa for example), share the same principal mechanism for sap conduction and it functions the following way: in the ground, roots absorb surrounding water and the fluid goes up the plant through the xylem (composed of dead, vessel-forming cells), with it only going upwards because, on the leaves, water is being simultaneously lost, evaporating into the surrounding air. What happens is that water molecules up on the leaves essentially pull the other water molecules, generating a chain reaction that extends all the way down, culminating in a tube of ascending water (this property of water, also shared with other liquids, is called capillarity, being the result of the attraction between the molecules forming the liquid in question). 

Water escapes from leaves through microscopic openings called stomata, constituted by two cells known as guard cells, which, in drier conditions, lose water and close, preserving the plant's water content by inhibiting the cited evapotranspiration, being also essential in inhibiting embolism, in which the xylem gets stuffed with air, breaking the water column, thus compromising its transport. Either way, the lycophytes are especially important in maintaining this steamy air, not utilizing water very efficiently and thus evapotranspirating lots, much more than the seed ferns like Medullosa, more efficient water users. Once the water reaches the leaves (or any other photosynthetic part), it plays a part in many essential processes to plant survival, including the iconic and very mentioned photosynthesis. The products of such metabolic pathway need also to be transported, but, in this case, they follow the reverse direction: starting up and going down. They form a sugary sap and go through phloem, formed by living vessels, with the nutrients being delivered to cells either through openings on their cell walls (connecting their cytoplasms) or through the cell walls themselves.  

Clumsily climbing down the scale-like bark of one Lepidodendron is an enigmatic little critter of burrowing habits known as Westlothiana, normally not found on trees, but going up this one when, hungry, it hurriedly followed a small opilionid arthropod (arachnids with long, thin legs fairly reminiscent of spiders) into unfamiliar terrain. Despite resembling a lizard, it is not a reptile. As a matter of fact, its classification is the subject of much uncertainty. Currently, it is placed as a basal member of Lepospondyli, a taxon that also is fairly uncertain in its phylogenetic placement. In accordance with the most accepted idea, modern amphibians (lissamphibians) are descended from temnospondyl ancestors and lepospondyls (of which Westlothiana is part) are more closely related to amniotes, which developed in this period. Take into account that amniotes, represented today by reptiles and mammals, are the tetrapods characterized by a membrane (amnion) that protects the embryo, allowing it to avoid dehydration. In a nutshell, Westlothiana, from what is understood nowadays, shares a more recent common ancestor with us than it does with a frog.

All of this discussion is of no importance to the small, around 20-centimeter-long animal, which, initially going after arachnid food, is now trying to avoid becoming a meal for another arachnid by fleeing from a Pulmonoscorpius, a scorpion able to grow to 70 centimeters in length. The pursuit continues and, close to the bottom of the tree, the hunter catches the tetrapod by its tail using one of its pincers. The little vertebrate wiggles desperately, trying to free itself and even opening its mouth in a futile attempt to intimidate or even nibble its attacker. This is the prey’s lucky day, as it, after a few seconds, manages to break free by autotomizing its tail just before the stinger moves down to hit its long, fragile body. The scorpion stays motionless for an instant, observing the would-be meal quickly run away from it over the forest floor, and then proceeds to eat the still squiggling tail, one that the lepospondyl will regenerate completely. Though not getting the whole meal, it will be more than sufficient for the Pulmonoscorpius, which, in contrast with modern scorpions, sporting reduced eyes and usually nocturnal lifestyles, is a visual predator, locating potential victims during hours of daylight.

Inside some Westlothiana live several animals, much more than their unimpressive size would seem to indicate. Moving around and reproducing in its digestive tract are several nematodes, commonly known as roundworms due to their cylindrical shape. These fairly simple creatures undertake a great variety of lifestyles, being incredibly widespread and abundant, a disposition that remains to the present day. Despite their unsegmented bodies and sleek aspect, they are part of the superphylum Ecdysozoa, which, unifying organisms that, besides other traits, grow through molting a cuticle, includes arthropods, priapulids (the stem-priapulid Ottoia was seen during the Cambrian), and more. The nematodes occupying this unfortunate tetrapod's bowels are more specifically part of the Ascaridoidea superfamily, one which has recently evolved, taking on terrestrial vertebrates such as these as their hosts. Fairly large among their phylum Nematoda, the ascaridoids are characterized by a mouth surrounded by three lips (bulbous-like projections of varying shapes) and by the following sexually dimorphic traits: the male has a tail that is curved and that contains two spicules. These are hardened structures used during mating, helping the male hold on to the female and also opening up the latter's genital pore (eggs also come out through this opening), allowing for the passage of sperm. 

Either way, they feed on whatever their harborer eats, possessing a fluid-filled body cavity called a pseudocoelom, which not only helps transport nutrients (these may be prevenient from digestion in the roundworm's gut or may be directly absorbed from the surroundings via the cuticle) but also functions as a hydrostatic skeleton, helping sustain them. The Ascaridoidea are only beginning their still-to-be-long and successful history: not only will they continue parasitizing tetrapods (Ascaris lumbricoides, a very large nematode several centimeters in length that parasitizes quite a significant number of humans, is one example of the superfamily), but they will expand to other hosts and habitats, becoming common in the aquatic non-tetrapod fishes. 

On a fallen Lepidodendron log not far away from the just observed scene, a Balanerpeton stands idly. It is a constituent of the previously mentioned Temnospondyli and breathes through buccal pumping (the aforementioned amniotes breathe with the help of muscles in the chest, a process called aspiration pumping), a method also utilized by lissamphibians and non-tetrapod air-breathing fishes which basically consists of gulping air through the mouth (buccal pumping is also practiced by other non-tetrapod, non-air-breathing fishes that use it to draw in water to ventilate their gills). This, together with a less kinetic skull allowing for tightly-held tympanic membranes and for the dissociation of the stapes (very small bones) from respiration (thus solidifying their function as carriers of vibrations from the just stated tympana to the inner ear), makes Balanerpeton and fellow temnospondyl relatives pioneers in better perceiving airborne sound (keep in mind tympanic hearing probably evolved in several tetrapod groups independently, with this temnospondyl hearing likely being homologous to that of the lissamphibians which still display such ability, with some having lost tympanic hearing altogether).

Another large arthropod, the gigantic Arthropleura, up to 2.6 meters in length, is also on the structure, passing on top of it, its many legs scuttling patiently in a wave-like motion. Unlike the Pulmonoscorpius, this myriapod is detritivorous, consuming decaying plant matter of varied origins, including wood from medullosans, horsetails, and lycophytes. Protected by a hard exoskeleton, it has virtually no predators and lives a calm existence after acquiring its full size. This individual, in specific, is just passing by these grounds, since it, just like other adults and semi-adults of its kind, normally roams more open spaces dominated by woody trees, quite different from this swampy environment in which the mostly wood-lacking lycophytes are predominant. Though a stem-millipede (herbivorous/detritivorous myriapods with two pairs of legs per body segment), Arthropleura possesses traits of their relatives: centipedes (carnivorous myriapods with one pair of legs per body segment), evident, for instance, in their mouthparts. Apart from this, it also has some peculiar characteristics of its own, such as its stalked eyes. 

How and why do such big arthropods, here represented by Pulmonoscorpius and Arthropleura (though some species of the latter are quite small), even exist? The lack of large terrestrial tetrapod predators could be the primary reason, with the spread and subsequent diversification of amniotes following the aridification of the Late Carboniferous playing a role, for instance, in the extinction of Arthropleura at the start of the next period (the Permian), with the gentle giant already on a vulnerable spot due to habitat loss as a result of the increasing dryness. Additionally, it is possible that the high oxygen levels also contributed by facilitating the breathing of some terrestrial arthropods. In general, various of these animals (such as myriapods and insects) have what is known as the tracheal system, which is a differentiated respiratory system (likely evolved independently in many arthropod lineages) in which air enters the organism’s body via several holes (spiracles), which then direct it to tiny tubes (trachea), which then allow, via several blind-ended ramifications (tracheoles), each tissue to be oxygenated. Notice that this process is not associated with a circulatory system, like in our respiration, and, therefore, there is no way for gases to be distributed around the organism other than going through the trachea, the longer size of which compromises how far oxygen can travel in adequate concentrations. Consequently, when there is more oxygen in the air, more oxygen gets diffused deeper into the body, allowing for an increase in size. Scorpions rely on book lungs, rather than trachea, and have a circulatory system associated with respiration (instead of hemoglobin, hemocyanin is used as a respiratory pigment), but it is an open circulatory system, where the blood (hemolymph in this case) bathes organs directly, mixing with the extracellular fluid. Since this system is not as "efficient" as a closed circulatory system in some aspects, it is conceivable the greater amount of oxygen favored more expressive proportions as well.

In regards to insects, they have already evolved (it is estimated that they first appeared during the Ordovician), but many groups alive in the present have not yet developed and the true burst in this group’s diversity, with a highlight on flying forms, will occur only later in the Carboniferous, with these flyers being more capable of regulating gas exchange by opening or closing their spiracles, allowing greater fluxes of oxygen to fuel their more energetically demanding activities. Undoubtedly some of the most impressive of these flyers will be the members of Meganeuridae, a family related to modern dragon and damselflies and too sporting predatory habits, with huge eyes, powerful and sharp mouthparts as well spiny limbs, ideal for caging up any captured prey item. Meganeura, their namesake, will reach up to 71 centimeters in wingspan, and will constantly fly over more open habitats on the lookout for smaller, but still quite large, relatives, such as Meganeurites, with a wingspan of 32 centimeters. Those smaller relatives will, for their part, go after even smaller flying invertebrates (such as palaeodictyopterans, mentioned in more detail here), constituting a true food chain of titans. Beetles, for instance, probably originate further down in this period, but will only diversify more significantly during the Mesozoic Era. The important order Hymenoptera will undergo diversification in the Permian, even though some its most famous members (bees and ants) will also only arise in the Mesozoic.

Focusing, now, on a body of water, one can spot many Sphenophyllum horsetails close to its edge. These are small vascular plants and some of their horsetail relatives are still found on Earth today (only represented by the genus Equisetum), with both reproducing by spores, like the abundant Lepidodendron. They form extensive rhizomes underground, which allow them to spread rapidly over an area by means of asexual reproduction, forming various clones. This makes them apt to quickly recover after fires, to the point they actually may benefit from these destructive phenomena. Fairly similar, but very much larger (up to incredible 30 meters tall), are the Calamites dotted along the landscape, forming thick aggregations close to rivers, which aid them to persist even in seasonally dry locations.

In the water proper, a Crassigyrinus lies motionless, with the top of its head and part of the back above the waterline. It has an eel-like body and four stubby limbs, which denounce its purely aquatic lifestyle whilst aiding it in maneuvering through tangled underwater vegetation. Considered a stem tetrapod, this creature has large jaws and is able to open them relatively wide, enabling it to consume a variety of different prey items, mostly other fish (some quite sizeable), besides having the capability to deliver quick snaps to ensure no targets escape. The top jaw contains not one, but two rows of teeth, with the teeth of the second row being quite big. Its relatively large eyes are possibly an adaptation for the murky waters it inhabits. Capable of getting to almost 2 meters in length, it is an active swimmer, a trait made feasible by its paddle-like, laterally compressed tail. 

Despite such an active lifestyle, it shares a trait with a few modern-day ambush hunters, such as the Suriname toad (Pipa pipa) and the mata mata turtle (Chelus), two South American creatures that resemble dead leaves lying on the bottom of bodies of water. Though we are unable to see it, Crassigyrinus counts with similar camouflage, having various flaps of skin and wart-like protuberances poking out from its sides, which allow it to cross the jammed waterways unnoticed until it is too late for its unfortunate targets. Apart from serving such a function, these extensions increase the vertebrate's surface area and, being filled with blood vessels, further aid in one of the ways it gets oxygen: through cutaneous respiration. For an animal its size that is dwelling in habitats often with little oxygen, every extra amount of the reactive gas is appreciated and, apart from the respiratory role, the skin also has ion transporters, essential for it to keep the right amount of water and its solutes in its body as it moves across waterways differing in their chemistry, ensuring it does not dehydrate if the water is too ion-rich and does not swell up should it be too ion-poor.

Swimming away from where it is located is a Tristychius, exposed by its two dorsal fins, the two of which are accompanied by characteristic spines that adorn this 60-centimeter shark-like chondrichthyan. These spines can serve a defensive purpose, with many individuals sporting bite marks on them, evidence of the larger predators, such as Crassigyrinus, lurking in swamps like this. Using its expandable mouth, the Tristychius captures smaller creatures by suction feeding, acting as the bane of the bottom-dwellers, pressuring them into harder-to-access hideouts, such as deeper burrows. Not a true shark, it rather is a member of Hybodontiformes, a clade that is closely related to both sharks and rays (the two constitute the grouping Neoselachii), likely diverging from their common ancestor in the Late Devonian. 

On the opposite shore where all previous action took place, a Hibbertopterus hauls itself onto land: it is an eurypterid, member of a successful group of aquatic arthropods related to arachnids. Displaying a carapace 65 centimeters wide and a total length not arriving quite at 2 meters, this, like the Arthropleura, is another titan. When submerged, juveniles rake through the sediment, consuming any small animals unfortunate enough to be caught in their path, while adults adopt a strategy known as sweep feeding, capturing food particles from the water column. With an amphibious lifestyle, Hibbertopterus has a few adaptations allowing it to temporarily abandon aquatic habitats, including (but not limited to) a thickened cuticle and muscle attachments on its abdomen responsible for helping the respiratory structures function appropriately.

Later the same day, we can find the same Arthropleura somewhere else, scuttling on a sandy shore, next to a river surrounded by Cordaites and Medullosa, with abundant Sphenophyllum growing close on the banks. The area is quite a bit more open than the dense forest from earlier, and the blue sky is more visible, not so concealed by the dense fog. Soon, another myriapod of the same genus makes an appearance, emerging from behind a few rocks. It quickly makes its way to the one we have been observing and tries mounting on top of it. This is courtship: the new Arthropleura is a male, and he is now trying his luck with the massive female. She, however, does not seem impressed and moves a little faster, making the male lose his grip and fall right behind her. He, for her initial displeasure, is a persistent bugger and will not give up so easily. Night falls and he is still trying, but he has had more success, being much closer to her head than earlier, having steadily made advances on the top of her back. As he goes, his many legs tap her rhythmically and gently: it is an attempt at seduction, and it seems to be working. 

Progressively, the female slows her pace and allows the male to get closer and closer. Like most modern millipedes, the genital openings of the females are located close to their heads, while the genital appendages of the males, specialized legs with which they introduce their sperm, are on their seventh to eighth body segments. To ensure both couple up, the male needs to move beyond the female. Soon, he does just that, as the female rears up and he envelops her in a tight embrace that may last many hours. After the deed is done, both go their separate ways. She stores the male's sperm in an organ called a spermatheca. It will stay there until she is ready to lay her eggs, the time in which fertilization will occur. Their progeny will be buried somewhere with much organic matter, perhaps in the forest floor, and then abandoned. They will be born pale and much softer than their parents, with fewer segments. These will increase in number after each molt and, when they become sexually mature, only size will increase each time they shed their exoskeleton. When that time arrives, they will be the new generation of Carboniferous titans.

Finally, the third tale ends. The lushness we have just experienced, as said before, is going to end and the world will enter a new age: the Permian. By then, tetrapods, mainly amniotes, will have solidified their dominion over land and diversified into many new forms (this amniote expansion will greatly favour the aforementioned ascaridoid nematodes and aid in their diversification further down the line). However, all of this would be put to a rigorous test, the most destructive trial Phanerozoic life would ever have to endure, for now at least.

***

1-Arthropleura 

2-Balanerpeton 

3-Pulmonoscorpius 

4-Westlothiana 

5-Sphenophyllum

6-Crassigyrinus 

7-Tristychius 

8-Cordaites

9-Sigillaria

10-Lepidodendron

11-Hibbertopterus

12-Calamites

13-Medullosa