Terror of Patagonia

At about 15 million years ago, this is the Miocene Epoch of the Neogene Period. What in the future will become the Argentine province of Chubut is now a location of humid subtropical climate, with distinct and well-demarcated seasons, influenced by nearby volcanic activity. Since last time, continents have not moved dramatically, though still continuing previous patterns: Africa and Australasia have stayed moving upwards (the former’s movement has even promoted the collision of the Arabian Peninsula with the Asian mainland) and the two Americas are still getting progressively closer. In terms of global climate, Earth has, following our stop at the Eocene, finally, come out of the prolonged greenhouse state that started all the way back at the end of the Paleozoic. Though ice has fully established itself in Antarctica, taking the shape of a large icy sheet, it is just now returning to the North Pole, since volcanic eruptions two million years back, arising from the Yellowstone hotspot (other hotspots are present, for example, in Hawaii and Iceland, as mentioned here) in Western North America, pumped sufficient amounts of carbon dioxide into the atmosphere to generate a brief warm period inside the longer ice age.

Here in Patagonia, where our tale takes place, local factors are also influencing the climate. Going west, towards the Pacific Ocean, the uplifting of the Andes has provoked dryness in various areas. The mountains effectively block air currents carrying humidity into the continent: due to their high elevation, the ascending water vapor condenses and pours down along one mountain side, without properly reaching the other, leaving it much drier. This area suffers only a bit from this phenomenon, called a rain shadow effect, but others, like the Atacama Desert, a region that has recently become hyperarid due to increased Andean heights (in the modern day, it still is an extreme environment, being the world’s driest place), are much more affected. Due to these general drying trends, the vegetation has undergone a significant shift.

Although grasses (family Poaceae) are still not as dominant as they are today (currently, grasslands cover 40% of the Earth’s land surface), these monocots have been increasing in their range. Originating back in the Late Cretaceous, grasses possess several traits that render them incredibly well-adapted to colonize and disperse through different environments, forming continuous and widespread mats of vegetal cover never before seen in our planet’s long history. First of all, their fruits count with various and effective modes of transport: some get stuck to animals passing by and may end up carried great distances, others are moved on by the wind (as is most of their pollen, liberated bountifully into the air), and others are ingested and then released somewhere else. A few species can even be dispersed by more than one method. These dispersal events are not rarely dependent on awn, bristles that can, for instance, attach the fruit to the hair of a mammal or help plant the seed (the bristles can twist with humidity and help push the seed into the substrate). Regarding the seed, the embryo inside it is quite remarkable on its own: differently from other plants, it is already well developed when reaching the soil and thus germinates much faster, ensuring a quick and literal settling down of roots.

Additionally, even if the number of seeds that manage to establish themselves in a previously uncolonized portion of land is small, the fact that most grasses are polyploid (meaning they contain more than two sets of chromosomes) confers them greater genetic variability despite inbreeding and guarantees that even grasses that reproduce without meiosis (generating seeds that are clones of the mother plant) will have a more varied genetic repertoire than would otherwise be possible. Besides, Poaceae rapidly propagate clonally through rhizomes (also observed in some previously seen plants) or through stolons, which, unlike the former, occur at the soil surface, not underground (some grasses, however, do not spread in such a manner, forming, instead, clumps). Also close to the soil surface or in the soil proper are the buds of many grasses, from where they can regenerate if damaged. Due to this positioning, the photosynthesizer’s upper portion, containing its leaves and reproductive structures, can be harmed with no consequence to its regrowing abilities, allowing it to be grazed or burned and still survive. The expendable superior part of grasses reflects in their root morphology: since there is no need to maintain aboveground structures during harsh conditions, they do not go deep into the soil and, staying and spreading superficially, are better able to absorb any incoming water. Even the stomata (first mentioned during our Carboniferous visit) of these hardy angiosperms are different: they are composed of two extra cells, opening wider but also responding faster to changes in moisture, generating higher photosynthetic rates and less water loss.

Not only do grasses achieve success for themselves, but also bring about ruin to many of their embryophyte relatives. Due to their rapid growth, they outcompete, without much effort, other seedlings and monopolize resources. Apart from this, the members of Poaceae are readily flammable, with fires helping them further secure their grip: due to their protected buds, lack of woody tissues (except in bamboo), and nutrient storage underground, they can rapidly grow back after fires that may significantly damage fellow plants. This is not the only family, however, that has expanded in the wake of the Cenozoic’s climatic trends. Another one is the eudicot Asteraceae, a family that includes sunflowers, daisies, lettuce, and many others, growing preferentially in open habitats, which have become more and more widespread. The largest of the angiosperm families, the representatives of this grouping come in different forms, from the more herbaceous varieties just mentioned, to vines, shrubs, and even trees. They are characterized by forming inflorescences (clusters of flowers) that may resemble a single large flower from a distance (what is typically considered a sunflower’s “flower”, for instance, is actually just a collection of many tiny flowers, with peripheral flowers being the ones from which the large petals that surround the whole inflorescence emerge).

The also eudicot family Amaranthaceae (including plants such as spinach, the ornamental plumed cockscomb, and its namesake amaranth, a pseudograin) has been another one to expand. Its members are commonly found in more arid spaces, but can also be encountered in other environment types across the globe to the present day. Such members may take the shape of herbs, shrubs, vines, and rarely trees, some possessing succulent stems and leaves, though these can come in other formats, despite simple leaves overall. Nevertheless, they are adapted to these growingly dry conditions and curiously are plants exhibiting an anomalous vascular arrangement, with their xylem and phloem forming alternating rings (as also seen in the Early Carboniferous, xylem is normally at the core of the plant, while phloem surrounds it at the periphery). 

One of the most remarkable representatives of this family is the famous tumbleweed, corresponding primarily to the Salsola genus. These dry-adapted plants are native to semi-arid environments of Eurasia and North Africa, but they became better known after being accidentally introduced to North America by European settlers, where they proceeded to spread massively, at home in the many dry environments they encountered there. While this genus begins quite green and succulent, it becomes progressively drier and stiffer as it ages until it eventually detaches from its roots, forming a ball-shaped tangle of twigs and crispy leaves that goes rolling and tumbling along with the wind, dispersing its seeds wherever it goes. Incredibly flammable (similar to grasses), it can be quite dangerous, as many tumbleweeds encounter each other and get stuck, forming masses of dried material ready to be lit up.

Returning to the Miocene, other plant families, however, have not fared so well. One of them is the conifer family Podocarpaceae (members of which were seen in our journey to the Jurassic). A typical Gondwanan family (in reference to the Gondwana continent, which aggregated all the southern continents), it is adapted to humid conditions and, as such, is suffering quite a bit. Even so, these gymnosperms will manage to eke out into our present-age Patagonia, though restricted to some specific forests. Despite this restriction in Patagonia, the podocarps nowadays remain spread across the world as a whole, occupying other areas in South America and also in Central America, Africa, Australasia, and even Asia, a distribution evidencing their Gondwanan past just cited. Another is too typical of the long gone Gondwana: the eudicot Nothofagaceae, containing a single and extant genus, Nothofagus. These angiosperms actually expanded and diversified considerably in the Eocene, taking advantage of a more temperate climate to form expansive forests, outcompeting other flora. This was not meant to last though and the augmented dryness led to the end of such temporary dominance.

Further north in South America, the monocot family Bromeliaceae, together with the Poaceae in the order Poales, is only now expanding its reaches. Likely originating back in the Early Cretaceous, they have been so far restricted to highlands, but the sea levels have fallen just enough to allow them to finally disperse and, in due time, they will expand across the Americas and even get into Africa. Bromeliads are quite curious plants, taking on either the more usual terrestrial habits or being epiphytic, growing on other plants. Those which subscribe to the second lifestyle can absorb neither their water nor their nutrients from the ground and, as a consequence, employ different strategies to secure an adequate supply of essential substances.

In these forms, roots may serve as mere anchors and, in a few cases, may be completely absent as a matter of fact. Leaves are the main ones responsible for absorption. In some, they form rosettes that act as tanks, collecting not only water but also organic material and even serving as microhabitats for other organisms. Others absorb water directly from the atmosphere. In either case, trichomes, tiny protuberances arising from the leaf, play a central part. They are not at all exclusive to bromeliads and can be found in all manner of plants with all manner of functions, but, in this case, they are essential for water uptake. When dry, the trichomes, which usually resemble incredibly small flowers, have their “petals” erect, allowing water to get into the plant via their permeable base. As water gets in, the cells composing the trichome swell and the formerly erect “petals” descend and become parallel to the leaf’s surface. While in this new conformation water cannot get in so easily, it also has more difficulty evaporating out (because the permeable base in which the trichome is anchored is covered by the “petals”). Thanks to these dynamic structures, some bromeliads have no need at all for collecting water in tanks, having a fuzzy and pronounced covering of trichomes that suffices all their needs, absorbing even the very diluted solutes that come amidst the water molecules.

Finally turning our attention to the landscape itself, it can be noticed that, due to this area’s humid climate, the Podocarpaceae and Nothofagus still have a worthy presence. Between the homogenous forests formed by the latter, more open patches of grassland are found, also containing plants such as Asteraceae and Amaranthaceae. At the edge of one of these Nothofagus forests, lie the first animals of our visit: Theosodon, a genus of South American Native Ungulate (mentioned in the previous chapter). As also said then, these ungulates (though perhaps not all of them) are prevenient from North American ancestors that migrated during the very end of the Cretaceous, around the time of the fifth mass extinction. It is more specifically part of the order Litopterna, which, together with the Notoungulata order (a very diverse clade, containing gracile to rhino-like forms), another example of South American Native Ungulate, possibly forms a monophyletic group closely related to Perissodactyla, the odd-toed ungulates (though the positioning of Notoungulata is disputed, as will be mentioned further along). In this regard, litopterns are quite similar to some “conventional” ungulates, bearing a resemblance to camelids in particular (which are artiodactyls, even-toed ungulates), having cursorial habits and three-toed feet, with the middle digit forming the axis of the foot. They are thus odd-toed ungulates in a literal sense, but not in a phylogenetic one!

With around 2 meters in length, Theosodon is a fairly large litoptern (though not as large as later forms, like the vastly more well-known Macrauchenia). It has a gracile masticatory apparatus and a preference for softer plants, like eudicots such as the Nothofagus various individuals are currently eating from. Thus, and like others of Litopterna, this genus inhabits primarily closed and forested environments. They are not very adapted to consuming the ever more abundant grasses, which incorporate significant amounts of silica into their tissues, rendering them an abrasive and not-so-easy-to-process food item (the ash from volcanoes that deposits on plants also lends them abrasive qualities and is a possible reason for the higher crowned teeth which have been increasingly observed in this region’s mammalian herbivores). Either way, the vegetal matter that is indeed consumed undergoes degradation primarily in the anaerobic habitat of the hindgut, where an exuberant microbiota is essential for the breakdown of carbohydrates, which are fermented to fatty acids, absorbed by the herbivore’s circulation and then converted into glucose in their liver. In the case of foregut fermenters (the case of many artiodactyls), plant carbohydrate breakdown takes place in an also anaerobic compartment before the stomach and the gastric digestion of the microorganisms found in this chamber is the main source of proteins for these creatures.

Bacteria (primarily of the phyla Bacteroidetes, previously seen, and Bacillota, a diverse phylum containing parasitic and non-parasitic bacteria with all sorts of lifestyles, usually counting with a cell wall most similar to the ones of Actinobacteria and of which the species Listeria monocytogenes, cited in the previous tale, is a representative), non-eukaryotic Archaea, and eukaryotic Archaea, in the form of protozoans and fungi, all take part in this process, interacting with each other and with host factors. The mucus lining of the intestinal epithelium, for instance, not only protects the gut tissue against possible microbial invasion but also offers a substrate for colonization and biofilm formation. Even secreted antibodies, by interacting with bacterial surface molecules and aggregating the microorganisms, can facilitate the development of biofilms while compromising the pathogenic potential of these beings.

Viruses, of course, are also present. Bacteriophages, which parasitize bacteria, are important for controlling their numbers, also aiding the mammalian host in avoiding infection. Other viruses associate with bacteria in different ways: some adhere to their surfaces and get transported by them to the interior of the host tissues and a few get stabilized by bacterial products, as is the case of the poliovirus, a non-enveloped RNA virus with a polygonal capsid benefited by lipopolyssacharides (for more detail, return to the first tale), which, in humans, can rarely invade the nervous system, severely damaging motor neurons and leading to paralysis. Additionally, immune stimulation by the microbiota can decrease the chances of certain viral infections due to more robust immune responses.

Returning to the macroscopic world, one of the most peculiar characteristics of Theosodon, along with the aforementioned Macrauchenia and others, must certainly be their snouts. Not proboscises in a strict sense (unlike, for example, the ones of elephants and tapirs, which are quite mobile and utilized in food manipulation), they are quite alike the ones of saiga antelopes (Saiga tatarica), extant artiodactyls that utilize their noses to filter dust emanating from the semi-arid soils of their dwellings and to vocalize, and of moose (Alces alces), also extant artiodactyls that use their nasal structures to trap air and thus become more buoyant, facilitating water crossings. In the case of the observed litopterns, many of the functions remain enigmatic, but vocalization seems to be one of them. Like in the saiga antelopes, male Theosodon close their mouths and inflate their “trunks” to produce nasal roars, a way to intimidate rivals and signal their fitness to females. Thus, physical confrontations are especially rare and most disputes are resolved by way of the nose. Vocalizations also have other uses, as can be expected and is to be seen.

The Theosodon herd suddenly stops their browsing as one of their members emits a loud, ear-piercing whistle. While some swivel their heads on their long necks to look at the cause of the signal, most just run for the forest, skillfully avoiding trees and taking sharp turns. Amidst the confusion, a fleeing mother notices the absence of her calf. Dazed by the loss of her progeny and by the quick shadows passing by her, she struggles to locate any distressed calls that might telltale the location of the lost youngling. She desperately emits soft noises that get muffled by the surrounding commotion. At last, she hears a response and rapidly moves to the edge of the forest. There, illuminated by the afternoon’s Sun, she finally sees her calf. It stands under the talons of a large flightless bird, 2.5 meters tall and 3 meters in length, with an enlarged second digit, reminiscent of the sickle claws of deinonychosaurians of old. Its beak is already tinted with blood. Rapidly bobbing its head to the side, it opens its mouth wide, sending a loud shriek at an approaching peer. Distracted by the competition, it lifts its foot from the calf, which feebly moves. The mother Theosodon gallops into the open terrain and, from the forest, her companions anxiously observe, moving their heads up and down, stomping their feet, and inflating their noses.

As she approaches, the two birds stop exchanging pecks and fix their intimidating gaze upon her. The litoptern, however, is not intimidated and roars aggressively using its nose, making the two dinosaurs recoil slightly. She lowers her head down to the calf, which weakly responds by touching its “trunk” with its mother’s. Breathing heavily, it soon breaks contact and falls to the ground once more. Simultaneously, the two birds finally dart in towards the mother, which turns fast enough to run back into the forest, taking one last look at her baby. It was a victim of the terror of Patagonia, for here, in South America, theropods have remained apex predators. These are Kelenken, carnivorous birds of the Phorusrhacidae family, commonly called the terror birds. They are part of the more inclusive order Cariamiformes, which, in the modern day, is represented by seriemas, long-legged, primarily ground birds of omnivorous diet that are small when compared to these older forms, but still quite sizeable. It is curious because the seriema body plan likely is the ancestral condition of phorusrhacids, with some members of the family being smaller and retaining such traits, while others, like Kelenken, have grown extremely large, with boxy and rigid skulls that are adapted to taking down big game.

To kill, they stab their prey, landing precise attacks with their beaks. With the Theosodon calf, it was no different. Using its speed, the Kelenken easily caught up to the placental and proceeded to land a single blow that was already enough to bring the poor synapsid to the ground. Fortunately for the rest of the herd, the terror birds, despite being quite speedy, comparable to and, in some cases, even surpassing the current ostrich, are not apt at making turns, very much unlike the targeted litopterns. Consequently, the ungulates are very safe in the forest, from where they will not be leaving so soon. Another mammal that saw it all safely and now is currently watching the two saurischians battle for the corpse with kicks and pecks is a mother Neotamandua with her baby, tightly gripping her furry back. Fortunately for them, the Theosodon had earlier scared her into climbing a Nothofagus and now they will not be going down for some time.

A little away from scientific nomenclature, these are anteaters and, as the name suggests, they consume ants, but also other insects, like termites. For that, they possess a very strange, cylindrical snout containing a very long and sticky tongue (longer even than the skull), adapted for penetrating inside the nests of such arthropods, taking away any of the unfortunate critters caught in the way. Besides, they also have powerful claws, which can easily dig into the just cited nests. Neotamandua is most closely related to the extant giant anteater (Myrmecophaga tridactyla), which is fully terrestrial, unlike other anteaters, which display arboreality and are markedly smaller. However, this species in particular, N. australis, displays a greater resemblance to arboreal anteaters than the more northern species, N. conspicua, being also smaller. Either way, even the giant anteater of today has the capability of climbing trees when in need, thanks to its large claws and strong limbs, something shared with the Neotamandua genus, which, like it, also walks on its knuckles, a behavior that preserves the claws by limiting their contact with the ground.

Anteaters, together with sloths, comprise the order Pilosa that, together with armadillos (which comprise the order Cingulata), form a very unique superorder of placental mammals: Xenarthra. Xenarthrans, originating right here in the isolated South American continent, possess several traits that render them distinct from other members of Placentalia, including different skeletal arrangements and joints, apart from lower metabolic rates, lack of incisors, etc. Besides all of these xenarthran peculiarities, anteaters are additionally bizarre because they do not produce stomach acid, counting on the formic acid (produced by ants of the subfamily Formicinae, such as the Titanomyrma from the last chapter) of some of their prey to aid in digestion.

A few of the ants they avoid, due to their large jaws contained in big, bulbous heads, are those of the subtribe Attina, remarkable insects that have developed, like humans, agriculture. Unlike ourselves, their agricultural endeavor has taken millions of years, starting close to the Mesozoic-Cenozoic boundary, and still is to reach the scale it possesses in the current times. Their association is with fungi and both eukaryotes have been forever changed by this symbiosis, as has happened between us and our domesticated plants (though we have also domesticated some mushrooms), but arguably to a lesser degree. It all started with some fungi that, in an “accidental” manner, grew inside these ants’ nests, living off the plant material that decayed within the hymenopterans’ chambers. It is possible that, due to the devastation left in the wake of the impact, the lack of alternate food sources and the proliferation of fungi, due to abundant matter to be decomposed, facilitated such interaction. Either way, this event took place in the South American rainforests, where these fungi freely lived, while at the same time infiltrating inside the arthropods’ dwellings. As this repeatedly occurred and the contact between the organisms increased, the ants gradually became more and more dependent on the fungi, becoming incapable of producing the amino acid arginine (already supplied by the fellow eukaryote) and better able of degrading chitin.

While they became fully dependent, the fungi were still able to reproduce with fellow fungi that grew outside their nests. 15 million years before this tale though, things changed. As some attine ants dispersed to drier environments that were inadequate for free-living fungi or to locations outside of South America, where these free-living fungi were absent, the ones inside the nests became reproductively isolated and finally turned dependent on the ants, initiating a new level of interspecies cooperation that saw the fungi develop specialized ant-feeding structures and grow to much larger sizes, as the ants simultaneously started feeding them with fresh leaves (this justifies their large jaws, used to cut up plants), fueling their unprecedented growth.

The ants also experienced changes, with larger colonies and more castes (instead of only workers, for example, some may have major workers, which are larger and play a more active role in defense, medium workers, which act mostly in leaf-cutting, minor workers, which process the leaves inside the nest, and even caretakers, which tend to the fungus and control the growth of potentially harmful microbes). In a bit more than 5 million years, the fungal cultivars will become even grander and fuel even greater ant numbers, at last reaching the modern-day level of the more specialized attines (many, to the present day, still associate with fungi that are not dependent on them).

Other ants that have a curious symbiosis are the turtle ants (Cephalotes), which have existed at least since 35 million years back, being characterized by their very thick and usually dark exoskeletons, justifying the use of turtle in their names. Consequently, the main defense of these arboreal hymenopterans usually is their tough armor and, to guard their nests, soldier castes block nest entrances with their huge heads, which may come in square, dome, disc, or dish shapes, varying not only in shape but also in size among the many species (these head shapes are also extremely plastic evolutionarily and have come, and still are to come, in various backs and fourths). These nests are located in tree trunks and are curiously not dug up by them, but reutilized from wood-boring beetles. Due to this defense strategy, for a hole to be ideal for a turtle ant colony, it must be of a specific size, either small enough so that only one soldier can block the entrance or big enough to guarantee sufficient space for various soldiers to form the living barricade.

They are herbivorous, feeding on sap, honeydew (produced as an excretion product of sap-sucking insects, such as aphids, more explored here), and nectar. Such diet is poor in nitrogen and, apart from consuming more nitrogen-rich foods such as feces and urine, they count on the help of bacterial symbionts residing in their gut, which help recycle the element (essential in the synthesis of amino acids and chitin, which is a nitrogenated sugar) and also directly synthesize some amino acids. Their gut is specialized for housing these many bacteria, with enlarged and occasionally invaginated compartments with such function. The transmission of these microorganisms occurs both vertically (from queen to offspring) and horizontally: ants ingest the anal secretions of their peers, populating their own guts with this rich biota.

Other much more conspicuous insects of the forest are cicadas, members of the order Hemiptera. They are large for insect standards, with robust, barrel-shaped bodies. Modern forms arose at the beginning of the Cenozoic, but cicadas as a whole extend back into the Mesozoic. Moving primarily through flying, these invertebrates are mainly characterized by the usually very loud, buzzing noises emitted by males in order to attract females (though these sounds vary significantly, both in character and in intensity). They are generally produced through the tymbal organ, a structure located at the start of the cicada’s abdomen which consists of a ridged and stiff membrane that, when deformed by the activity of the tymbal muscle, makes sound. To hear these sounds, cicadas have well-developed auditory organs also located on their abdomen, lined with hundreds of mechanoreceptors.

With a proboscis, like other hemipterans, these animals are herbivorous, feeding on xylem sap. This is quite a nutritionally poor diet and, like turtle ants, cicadas associate with several bacteria to complement their nutritional needs, with these being endosymbiotic (unlike the ones of Cephalotes, which are extracellular), present inside their gut tissue and forming amino acids as well as vitamins. The sap-based diet also manifests in another way in the physiology of these creatures: they are the only ones, besides larger animals, that urinate in jets, a consequence of the high-water content of their food, allowing them to more efficiently process the ingested material by quickly replacing it. These jets can even serve to deter predators.

After mating, females lay their eggs on plants and eventually emerge nymphs, wingless and possessing strong front legs apt for digging. They drop to the ground and burrow next to the roots, also feeding on xylem sap and doing so for a few years, generally from 2 to 6 (though some can stay underground considerably longer), after which they finally emerge, anchoring themselves to tree trunks and metamorphizing into their adult stage, leaving behind golden brown husks of their old exoskeletons, which stay there, clung on, memories of transformations formerly undertaken. During the underground phase, cicada urine continues to be important, helping agglutinate soil to build tunnels and even cleaning their bodies from debris.

Opposite to the forest and in a shallow lake is a medium-sized grouping of Astrapotherium, three-meter-long hefty South American Native Ungulates, though perhaps not true ungulates after all. These fairly rotund mammals are part of the order Astrapotheria, one which has a very contentious and poorly resolved classification. They are perhaps part of a clade known as Afrotheria, which, separate from other placental groups, includes representatives such as elephants, sea cows, and hyraxes, generally small and plump mammals to which they would be most closely related, along with possibly Notoungulata and other South American Native Ungulate orders (if such relation is indeed true, it is possible their ancestors came from Africa during the Paleogene, crossing a hypothetical island chain extending along the south of the Atlantic Ocean). And with elephants, these tetrapods have quite a few resemblances it must be said, though these are intriguingly a result of convergent evolution. Not only do they, unlike the Theosodon, possess true trunks, aiding them in acquiring vegetation, but they also have large and very noticeable tusks (the tusks of elephants though are modified incisors, while the ones of astrapotheres are modified canines). These teeth are not only for show and are intensely employed by males (which have larger tusks) to defend their harems from rivals, generating violent disputes that can turn quite bloody. However, most of the time, the Astrapotherium are gentle and water-loving herbivores, spending their days at or near lakes and rivers, selectively picking plants with their proboscises and yanking out with their tusks those better anchored to the substrate, while also using these same strategies to gather leaves located higher in branches and stems. Such feeding habits are in contrast with those of other astrapotheres, which are not picky eaters, being indiscriminate grazers.

One of the Astrapotherium is just comfortably lying down on the watery ground, occasionally moving and rubbing itself against the lake’s muddy substrate. This is a peculiar, but widespread behavior among various animals known as wallowing, especially well-known among swine. During this time of day, when the Sun is high in the sky, rolling on mud can be quite refreshing, in particular for mammals as large as it and which have few sweat glands. There are other functions for this behavior also: it can protect the animal from pests, acting as a plaster that inhibits, for instance, fly feeding. The rubbing on its own too serves to eliminate external parasites, like ticks, and helps in shedding hair and skin as well. And though all these functions likely played a role in selecting this behavior across several generations, it is perhaps even more important to mention the creature’s main motivation for doing so: it simply is an act of pleasure or one that alleviates discomfort.

Either way, mosquitoes are the flies (order Diptera) most affected by a layer of mud, for these are blood-suckers, at least in the case of females. Though both males and females feed on nectar and fruit fluids, females of the great majority of species require bloodmeals in order to support egg development. For this, they locate victims using various possible means: visualizing infrared radiation (invisible to human eyes and only perceived as warmth), sensing carbon dioxide concentrations, and taking on olfactory cues, these latter two also being essential for honing in on flowers. Before blood is acquired through the female mosquito’s proboscis, which pierces the skin and sucks in the liquid, she mates. The antennae of both the male and the female are indispensable for this event (they are also the sensory organs responsible for picking up odors), as are their wings, for the two engage in an elaborate courtship process in which the pair synchronize the sounds produced by their wing beats. The synchronization is only possible due to the sound-detecting properties of the antenna base (which is even more complex than the well-developed auditory organs of cicadas) and serves to attract the two of them, culminating in aerial mating.

After this, the female stores the male’s sperm and goes on looking for blood. Eventually, the eggs are laid either directly in water or close to it (even the water collected in tank bromeliads may serve as a nursing spot for these young flies). The larvae are aquatic and resemble segmented worms, containing a caudal tip that serves to take in air. Most filter-feed, consuming much smaller organisms (like algae and bacteria) and detritus until they are ready to pupate, becoming a non-feeding, comma-shaped form that, even so, is quite skittish, quickly responding to any perturbations in the medium. From this form finally emerges the adult mosquito, ready to start the cycle anew.

Other insects that also depend on vertebrates for their sustenance are dung beetles, but they do so with no negative effects on their distant deuterostome relatives, indirectly helping them actually. Originating in the Cretaceous in Gondwana, dung beetles initially evolved consuming dinosaur droppings, diversifying due to the spread of angiosperms and their more palatable foliage. Some, now gone, consumed preferentially the ones originating from the very large herbivorous dinosaurs, such as sauropods and hadrosaurs. However, those that, more generalist, consumed the poop from smaller dinosaurs and those from mammals managed to survive the extinction and go on to colonize and spread through the Cenozoic world. In this environment, they come in three main lifestyles.

The ones most visible are the rollers. These, such as the ones gathering the materials recently defecated by the wallowing Astrapotherium, form characteristic balls of dung and roll them along some distance, until they bury such “treasure” to either feed on it or lay eggs, with their larvae, vermiform in character, developing inside it. Some, called dwellers, nest within the dung or at the interface between dung and soil, being like the rollers in the other cited traits. Finally, tunellers dig into the soil beneath the feces, forming tunnels, as the name suggests, in which they store such materials, with their larvae possibly developing deep within the soil.

The way they help their nourishers is two-fold: by messing around with and in the poop, they help distribute nutrients that will aid in plant growth and, consequently, in the nutrition of the larger animals, besides also increasing earthworm activity, which as previously mentioned, are quite important for soil health. Additionally, they can help control the number of some parasitic nematodes. Sometimes, they bury the balls of dung so deep that the nematode larvae which hatch from the eggs released in the feces are unable to reach the upper parts of the soil and, consequently, cannot get accidentally ingested by their hosts. However, the action of some dung beetles, by aerating pieces of dung, can positively impact nematode eggs, decreasing the formation of anaerobic environments and promoting their hatching.

Nearby, a Patagosmilus continues to harass a Neosteiromys, an affair which has extended for quite some time, even before the Kelenken attack on the Theosodon herd. Despite its great canines, which continuously grow throughout its life, this mammal has not managed to do much against the prickly hide of its desired prey. Unlike other members of Mammalia seen thus far in this chapter, it is a metatherian, part of a group of carnivorous forms called Sparassodonta. Like the also isolated Australia, South America has maintained an extensive fauna of non-placentals and, though not a marsupial proper, Patagosmilus and its sparassodont relatives possess the characteristic pouches found in many female marsupials. Inside these pouches, very precocious young lactate and mature into more well-developed babies, while, in placentals, babies already emerge from the mother in a more advanced stage of development. The Neosteiromys, in contrast, is a representative of Placentalia and part of the order Rodentia, which coincidentally shares with the Patagosmilus and some of its relatives the same pattern of tooth growth. In their case, however, the incisors are the ones that grow continually, grinding against each other and thus leaving them constantly sharpened.

Like what possibly happened with astrapotheres and notoungulates (among others), rodents likely arrived in South America from Africa, eventually getting stranded on the continent and diversifying into a myriad of different forms, a path also followed by the New World Monkeys, African immigrants too. These South American rodents are known as the caviomorphs and include a very great variety, from capybaras to chinchillas to porcupines such as Neosteiromys. This genus in particular is better adapted to consume abrasive foodstuffs, more prevalent in the open environments it preferentially inhabits, with its large body size, along with spines, serving as sufficient deterrent against smaller predators such as Patagosmilus, though it remains a mystery if they would be as efficient against the much larger Kelenken. Unlike the modern New World Porcupines, which are overall arboreal, Neosteiromys is completely terrestrial, but it also shares with them many traits, including spines with barbs (these spines being modified hairs), a primarily herbivorous diet, and nocturnal habits: this individual was rather unfortunately caught in the open by the persistent sparassodont.

Farther away, the first Patagosmilus’ mate is trying its luck with another possible prey item, this time a xenarthran of the Cingulata order or, in other terms, an armadillo, one of the genus Peltephilus, around the size of the extant greater long-nosed armadillo (Dasypus kappleri), capable of reaching lengths up to 1 meter. A very fossorial animal, it, like other armadillos, has a body armor composed of osteoderms, giving it quite a formidable protection against predators, with the two horns over its snout giving it a most distinct appearance. In most circumstances, such protection is not needed, since most nutrition, originating from roots and tubers, can be found inside their burrows, but occasionally it wanders outside attracted by the smell of carrion, which serves as an important complement to its diet.

Earlier today, this Peltephilus was brought out of its burrow to deal with the nuisance resulting from a pair of Protypotherium, small notoungulates that are well-adapted to feeding on the abrasive grasses, constituting a speciose genus with several members, these being P. colloncurensis, a larger-sized species. They were on the look for a new hideout and believed that a Peltephilus burrow was just the right match. For some time, they annoyed the xenarthran, darting back and forth while grunting. In response, only aggressive hiss or purrs and several nipping attempts at its far nimbler adversaries when they got too close, trying to land their paws on the armored and horned face of their opponent.

All three were too occupied to notice a much larger menace creeping from behind. It was only when the two Protypotherium suddenly turned their backs and went on the run that the Peltephilus noticed something was not right and, in a second, it felt something heavy hit the osteoderms on its back. As it tried to wiggle, it realized it was pinned to the ground and soon a hot and strong breath had reached its nostrils. Then came a decisive wiggle, for its pinner shook about and missed its target, its large canines only hitting the bony scales of its victim. Then another decisive wiggle and another miss and then one more.

With that, the Patagosmilus had lost its grip and the Peltephilus managed to return to its burrow, going deep into it, far from the clawed paws of its attacker, growling and sticking its arms the maximum it could inside the hole. As far as one can see, it will be two failed hunts for this fierce couple. The Peltephilus can surely count itself lucky, for the Patagosmilus’ sabers are an efficient killing weapon: though without a strong bite, the Patagosmilus displays strong neck muscles that allow it to deliver fatal stabbing motions with its canines. The cingulate was not only protected by its armor but in an advantageous position, as these stabbing attacks are usually done at the throat, quickly incapacitating prey. After the quarry is killed, only the softest organs are eaten, for these predatory metatherians, like their saber-toothed relatives, do not process well tougher items, leaving them for carnivores such as the phorusrhacids, which have kicks strong enough to rip bones open, exposing the nutritious marrow inside.

As a result of all the commotion around the Peltephilus’ burrow, a horde of darkly-colored hemipterans with orange markings emerged from the darkness. They are triatomines, a subfamily also called the kissing bugs, being of the genus Panstrogylus. Not in accordance with the cicadas from before, their proboscises are used to suck blood, with this genus usually correlated to habitats such as this one, though it may also occur at tree cavities. Wherever it may live, the most important for it is hosts, for kissing bugs are obligate blood-feeders, be they male or female. Other members of Triatominae occupy other locations and, in this sense, they present quite a striking niche partitioning, with each one being surprisingly well adapted to its dwellings: some have a pinkish coloration that renders them good camouflage on the bromeliads they reside, others live between the leaves of nests, being dorsoventrally flattened to adequately accommodate themselves in such a restricted environment, a few are true generalists while others still stick to very specific hosts.

Deriving from predatory ancestors, triatomines may have become bloodsuckers following a stepwise process, first allocating to vertebrate nests, feeding on other arthropods found in such nests, and then, eventually, moving on to feed on the nesters themselves, a process consummated more or less 25 million of years before this tale. From that transition onwards, these hemipterans have become increasingly adapted to this food source, with the development of a mostly painless bite and modifications of their salivary glands, which started, for instance, producing anticoagulants to ensure a continuous flow of blood into their proboscises. The kissing bugs are intimately associated with an organism much smaller than themselves: the Trypanosoma cruzi protozoa, a parasitic euglenozoan. Likely brought to South America by infected bats, this unicellular eukaryote quickly associated with triatomines and later with other mammals. Xenarthrans, to the present day, constitute an important reservoir of these parasites, which are transmitted through two main ways: orally, via the consumption of infected triatomines, and by the feces of such triatomines, which, deposited close to the bite site, can end up allowing trypanosome infiltration into the tissues by means of the opening.

Like other euglenozoans (a grouping of which the already mentioned euglenids are a part of), T. cruzi possess a flagellum (despite the fact most euglenozoans have two) and a paraflagellar rod, a structure that serves to support the former. More specifically, T. cruzi is a kinetoplastid, a more restricted grouping that is, even so, incredibly diverse, containing both free-living and parasitic forms. Even more restricted is the Trypanosomatidae family, formed by T. cruzi and its closer relatives, counting with only one flagellum and only one mitochondrion, which is large and branched, extending all over the cell. The mitochondrial DNA forms a grid-like pattern and is connected, through proteic filaments, to the flagellum, which emerges from an invagination of the cellular membrane. Such flagellum is normally profusely associated with the cell body, forming an undulating membrane, such as the one also seen in the tyrannosaurid protozoan parasites. Thanks to this long flagellum and the equivalently long membrane it forms, the trypanosome can swim even in a medium as viscous and crowded as blood, generally moving in the same direction of the tip of its undulating flagellum, undergoing a rotational motion. However, if it encounters an unsuitable environment that may trap it, it can use its flagellum to propel it backward and, once danger clears, it resumes its normal movement. The blood forms of T. cruzi are non-feeding and count on reservosomes (organelles that store several nutrients) until they find a host cell, because a part of their life cycle is intracellular.

In regards to finding a host cell, T. cruzi targets especially cardiac muscle cells and glial cells (varied cells that participate in the maintenance of neurons), though it can infect virtually any cell type. After locating a cell and getting internalized by it, the euglenozoan suffers a radical transformation: its formerly elongated and slim body becomes much more circular and its long flagellum becomes a truncated stub, while the undulating membrane disappears completely. Apart from this, they develop one more invagination of the plasma membrane that, just like the one from which the flagellum emerges, functions to take up nutrients from the outside environment. With a nourishment source secured, these rounded forms proceed to multiply asexually, eventually reaching numbers so high they metamorphize back into blood forms and rupture the cell, going on to infect more. To this day, T. cruzi remains active. In humans, for instance, the effects of this single-celled eukaryote can be quite severe. In a minority of those infected, symptoms will only appear many years after contact with the protozoan and result from chronic aggressions to tissue that, over a long time, generate great consequences.

In the case of the heart, characteristic signals of infection and cellular damage promote the infiltration of leukocytes, initially bringers of an innate response. While this, on its own, already promotes damage, the latter appearance of lymphocytes, spousing varied antibodies (including some against the organism itself), aggravates the situation even more. Though at the end of this process the local source of infection is cleared, the tissue has been irreversibly damaged. The accumulation of such events leads to hypertrophied cardiac muscle, as progressively decreasing cardiac muscle cells cause the remaining ones to develop more in order to maintain normal heart function. Such cardiac function is not regained however and the heart becomes frayed and enlarged, unable to preserve its proper shape and to adequately pump blood.

In the case of the glial cells, the immune response is so unbalanced that even neurons, which are not infected by T. cruzi, are also targeted. As a result, intestinal nerve plexuses are gradually degenerated, which may result in two conditions: megaesophagus and megacolon. The two originate from inadequate nervous stimulation of the gut musculature due to neuronal destruction: in the first, the esophagus is unable to empty its contents into the stomach and there they gradually accumulate, making the organ larger and larger, while in the second, the colon is unable to propel the feces forward, leaving them stuck and accumulating, also making the organ larger and larger. One potential mechanism behind some of these pathogenic effects may actually even be horizontal gene transfer involving the trypanosome’s mitochondrial DNA, which can integrate into the hosts’ genomes, negatively affecting their normal functioning.

Returning to the triatomine, it gets infected with T. cruzi when taking a blood meal from an infected vertebrate. It mainly ingests blood forms, but the rounded, intracellular ones can also end up in its stomach. There, they suffer other transformations, but eventually end up in the intestine where they assume an appearance that is quite similar to that of blood forms, though with a few differences. These forms, for example, possess an extra invagination, which is also utilized to gather nutrients and fuel their asexual multiplication. Besides, they lack a carbohydrate covering as thick as that of the forms encountered in the host, since in the kissing bug’s gut they are not subject to the pressures exerted by the immune system of vertebrates. Here, though, they have to contend with competition from fellow microbes, such as bacteria from the phyla Proteobacteria, Actinobacteria, Bacteroidetes, and Bacillota, with which they form filamentous biofilms. It is important to mention that, although T. cruzi mainly undergoes asexual reproduction, there are instances of sexual reproduction, which help explain the great variety existent in this species. Be that as it may, as the trypanosomes approach the rectum, nutrients become scarcer and they once again start to transform. There, they attach to the organ’s cuticle using their flagellum (which develops adhesion proteins, with its tip enlarging) and shift into blood forms, which will end up in the feces and, if all goes well for the parasites, find a new vertebrate host to colonize.

Armadillos do not serve as host only for this protozoan, however: their burrowing habits make them come into frequent contact with soil-dwelling fungi, as is the case with Paracoccidioides. Similarly to the T. cruzi vector Panstrogylus, such fungus currently does not occur in Patagonia, but the warmer and more humid climate of the region, as is in this tale, allows it a wider distribution than nowadays. Nevertheless, Paracoccidioides does not have such an expressive presence in the environment either way, as its thin and long hyphae grow all too slowly and do not seem to fare all too well with the fellow microbial competition. However, when producing conidiospores (spores produced asexually and that can go airborne), they may establish themselves on a new frontier in which there is far more success to be had: the insides of an animal like the Peltephilus. As the conidiospores germinate inside the xenarthran's lung, they do not adopt the form of hyphae, but instead become yeasts, which are quite varied in shape and size, ranging from oval to spherical to quite elongate. From these yeasts, many others bud out, sometimes forming a quite peculiar arrangement that resembles a captain's wheel. In this form, the parasitic eukaryote can, in many circumstances, resist the host's immune attack, especially in the case of these placentals of choice, which, as mentioned earlier, have a lower body temperature (but that still is high enough to promote the metamorphosis into yeast).

Disseminating around the animal's body, the fungi can occasionally promote lesions in various organs, including the lymph nodes and the liver, apart from many other locations. These lesions are characterized by being granulomatous, this being a type of immune response in which lymphocytes stimulate macrophages to differentiate and essentially form a cellular ball around the infectious agent, serving as a means to stop its spread and hopefully kill it off by cutting its oxygen supply and exposing it to antimicrobial compounds, like reactive oxygen/nitrogen species and various enzymes. Unfortunately for the mammalian defenses, Paracoccidioides is exceptionally adapted to survive within these structures and resists for many, many years. In the case of the armadillos, this does not affect them much: the majority do not show signs of disease. In humans, though, which are to become new hosts once they arrive in South America, many millions of years in the future, more serious disease tends to be more common, though still not the rule overall. Other fungi of the order Onygenales exhibit a lifestyle that is remarkably similar to the one described and share traits that make them generally prone to associate with animals, like their ability to degrade keratin. Despite this, transmission between infected animals does not occur, except perhaps in cases in which an uninfected individual consumes an infected one. So, one can wonder how these organisms became so finely tuned to survive inside the bodies of their eukaryotic relatives.

A potential evolutionary pressure in that regard was withstanding predation from ameobae (as was the case of Listeria monocytogenes, talked about in the preceding chapter) and from nematodes, the former being quite similar to macrophages and the latter being an animal itself. Resistance to these agressors likely helped these fungi develop the "tools" that, one day, would make them able to colonize metazoan hosts. Apart from this, adoption of animals as hosts can facilitate sexual reproduction by promoting the encounter of opposite mating types, which can often infect the same host. In the case of Paracoccidioides specifically, its lackluster growth in the environment is in stark contrast to the plenty of opportunities it encounters once inside a mammalian body, which offers it a viable alternative that can ensure much more reproductive success. Meanwhile, it does not stay trapped in the body of its host forever, as, once its supplier dies, it can easily resume its purely decomposing role and start degrading the carcass, eventually getting reintroduced in the environment.

Later in the day, around sunset, one of the Kelenken directs itself to the Astrapotherium lake, with the hefty astrapotheres wisely giving the bird quite a wide aperture. For its part, the theropod does not display any form of aggression. It is fed and, even if it was not, it would not be a good decision to attack such large creatures, especially when they are in such numbers. Calmly, the terror bird sits on its legs and lowers its head to drink. After a few minutes of such activity, another Kelenken appears. The two look at each other and the drinking one stands up and moves into the water, increasing the distance between it and the newcomer. A few more instants of silence pass, only broken up by occasional trumpets from the Astrapotherium. Then, one of them emits a short vocalization. It is repeated by the other and, soon, both are emitting a series of cries that resemble a deep, booming laugh. In the modern day, seriemas still engage in quite a similar behavior, forming duets of “laughing” pairs that move their necks up and down in repetition.

After some months, the results of the twilight encounter can be seen in the form of two downy chicks. They reside in a nest made of twigs and branches, with each parent taking turns to care for the youngsters, which are quite voracious little ones, hungrily consuming the pieces of meat the adults carefully tear apart into small chunks. Even though their parents are this place's undisputed apex predators, they are, for many other creatures, easy prey or apparently so at least. One animal that considers them as such is a boa. It is dawn and the fairly large snake slithers silently through the grasses, attentive to any possible meal. Like pythons and pit vipers, boas are capable of sensing infrared radiation, integrating this sense with the one of the more classical visual capabilities, both signals being processed in the same area of the central nervous system, thus giving them a unique, broader vision that we, lacking that ability, can barely imagine. 

In the semi-darkened environment provided by the Sun's first rays, the detection of infrared light is especially useful and, with it, the lizard localizes three foci from which emanate a considerable amount of heat: two of them are small and one of them is quite large. It moves silently and gets closer, closer until the sleeping chicks are at eating distance and, then, it starts opening its flexible mouth. In an instant, the worst pain it ever felt floods its senses, a pain both sharp and dull but absolute in its intensity, though also fast, certainly the fastest pain it ever felt: a pain that, in almost every regard, is unprecedented. 

Illuminated by the orangish light, one of the Kelenken, carrying the limp body of a Patagosmilus between its beaked jaws, looks down at the would-be predator. The kick of the bird was so powerful it completely shattered the fellow reptile's cranium, its talons so strong and sharp the head of the intruder was severed from the rest of its body, laying there motionless while blood squirts and pools from it and around it. Due to the action, the chicks wake up and proceed to chirp loudly and incessantly while the killer's mate quickly stands up, surprised by the sudden appearance of a mangled snake right in their nest. In a few days, the new generation of grassland predators is already taking their its first large strides, being pursued by its parents across the vast expanses. The youglings excitedly run from their progenitors and try to hide in any bushes they can find, only to be flushed out and pursued on and on once again. It is a game of sorts, but one which stimulates the survival tactics of the tiny birds, on their path to assuming the mantles of their parents as terrors of Patagonia.

In one of the adults, though, other animals are also producing their next generation. Located inside some blood vessels of one of the Kelenken are schistosomes, parasitic worms of the phylum Platyhelminthes, commonly known as flatworms, due to their slender complexion. This slender complexion comes accompanied by the fact that they have no circulatory system to speak of and their nervous system, for this reason, is incredibly important, since it is through it that signals can be more efficiently transmitted from cell to cell. They, in contrast to ourselves and to the also sometimes parasitic roundworms, have no medium in which hormones or other signalers can circulate and affect cellular behavior: consequently, the nervous system is truly of paramount and unparalleled significance.

Too unlike the sometimes parasitic roundworms, which are part of the superphylum Ecdysozoa and thus share a fairly close relation with arthropods, flatworms group with the other protostomes, being nearer, consequently, to invertebrates like mollusks, annelids, and others. Also unlike the nematodes, they do not possess a cuticle, but rather a covering made of the fused cytoplasm of various cells, which, located further down, send projections to form this layer. It is partly through this integument through which schistosomes absorb nutrients, taken up from the surrounding blood. Apart from this, they also feed using an oral sucker (they have another sucker, which, located on their bellies and a bit further down from their oral sucker, is used to attach to the blood vessel wall), located in one of their extremities and connected to a blind-ended digestive tube.

Orienting this orifice against blood flow, the worms ingest the liquid and process it, eventually regurgitating the undigested material, since it can only leave from where it originally came. Blood is curiously a fairly dangerous meal, since the respiratory pigment hemoglobin can release potentially harmful, pro-oxidant products when it is broken down and, as such, it is converted into a more benign, insoluble form known as hemozoin, which is dark and so gives the otherwise pale flatworms a more brownish color. It is female worms that are usually darker, since they consume more blood. Unlike males, which are quite wider, they possess a more typical vermiform aspect and consequently have less surface area to absorb nutrients through their surface, helping explain their greater appetite for bloody meals. The males are not only wider but count with an excavation running along their bellies, an excavation into which females nicely fit, with these worms thus constantly embraced, the male tightly involving his longer but less muscular companion.

The association between these roughly 1-centimeter-long worms is actually quite long-lasting, with some couples being completely monogamous, only breeding between themselves for their whole lives, though a few “divorces” can also occur. The reasons for this monogamous behavior are perhaps many and, first off, it is essential to mention that the schistosome ancestors, as most flatworms, were hermaphroditic, but their specialization into becoming parasites of birds and mammals (as will be more elucidated later), which constitute a habitat with many hazards, brought forth the separation of sexes: not only did this guarantee more genetic variability (since in hermaphrodite individuals there would be the possibility of self-fertilization) but also allowed for a division of tasks (the male is responsible for movement and the female is responsible for egg-production) that ensured better outcomes for the parasite.

Regarding monogamy, females can only attain sexual maturity when paired with males and, as such, there is significant male investment in order to promote reproductive success. Consequently, maybe engaging with more than one female would not be beneficial for the male and would actually result in fewer of his offspring, thus explaining the selection of monogamy (though it is important to mention that some schistosome species are indeed not monogamous, with some having males that associate with multiple females, all laid inside his ventral excavation). Additionally, the eggs released by females are the main provokers of immune responses and, consequently, if many eggs end up being laid in one same vessel, the ensuing immune response could destroy the suitability of that environment and compromise the reproductive success of the flatworms.

Such eggs count on their immunogenic potential to help them migrate through host tissues, eventually reaching either the bird’s feces or its nasal mucosa (depending on which vessels the adults were parasitizing). If those eggs get into water, they hatch and release a roughly leaf-shaped larva that is non-feeding, rapidly moving by the means of various cilia attached to epidermal plates. Unless it encounters certain freshwater snails, the larva dies, but it has mechanisms to increase its chances. Not only is it attracted to light (to which the snails sometimes also are), but it can detect chemical components emanating from the snails, helping it localize its targets. Once they are localized, it moves to the snail’s soft parts and, while moving violently, releases enzymes from a forward protuberance, penetrating into the mollusk’s body if successful.

This penetration does not guarantee the worm will succeed in establishing infection, as the snail, despite not counting with the adaptive immune system typical of vertebrates, nevertheless has effective defenses of its own. Hemocytes (previously mentioned at this trip) can adhere to the invading platyhelminth and form a capsule around it, releasing toxic components, such as reactive oxygen species. Apart from cellular responses, the mollusk possesses molecules with immune properties, some of which are expressed generally, but others which are only produced in response to specific antigens.

Either way, the larva, once inside, loses its cilia and many of its internal organs, and germinal cells inside it start to proliferate until it eventually just becomes a sac containing these cells. As nutrients are absorbed from the snail through the worm’s cytoplasmatic integument, the cells differentiate and form structures that leave the sac and move to other snail tissues, away from the site of penetration. Inside these structures, there are more germinal cells that can form additional generations of their own, but that also generate the vertebrate infective form of the schistosome: a less than half a millimeter long larva that counts with two body portions, one that is more barrel-shaped, housing its essential organs and the two suckers first mentioned in adults, and another that is a bifurcated, fish-like tail, helping it swim in the water and track its host. Like the other water-dwelling larva, it is non-feeding and has a limited time to find its host before its energy reserves run out.

Emerging from the snail at specific times of day, it uses several factors to locate a suitable vertebrate host, such as shadows, water turbulence, and chemical cues from the host’s skin. Once the target is located, the larva proceeds with a new penetration event, adhering using its oral sucker and losing its tail in the process. Other modifications take place, such as the loss of its carbohydrate covering, one of the many schistosomal adaptations for immune evasion. As it reaches the host’s bloodstream and migrates around its body, the worm enlarges and eventually pairs up with another member of the opposite sex. Since the snail-infecting larva already emerges from the egg with a determined sex, if a bird or mammal is only infected with males, for example, they may actually enter the excavations of each other, filling in the literal hole females would normally fill.

At the beginning of this era, schistosomes transitioned from their until then only avian hosts to mammals, probably rodents initially. While this occurred in Asia, the eventual spread of these worms to ungulates in the last few million years has yielded a great wave of diversification, one which has carried them all the way to Africa. In the African continent, the schistosomes will eventually adopt primates as hosts and, even later, make their way into humans. Humans, for their part, would be the ones responsible for finally introducing the mammalian schistosomes into the Americas. This would occur as a result of the Atlantic slave trade, when millions of African slaves would be brought into the American continent by Europeans, Africans who would bring with them the species Schistosoma mansoni, which would here find, in South America, a space ripe for colonization.

Though we end this tenth tale with a demonstration of life’s hallmark trait of self-propagation, a few organisms will be disappearing in the following million years. The gradual cooling and drying will very soon change the environment we have witnessed in the paragraphs above and animals like the astrapotheres will go extinct by the end of this epoch, while others like Protypotherium will move to increasingly lower latitudes until they too finally meet their end. That being said, many will persist until the next chapter, when we will once again return to this continent, and beyond. Even if it has now turned commonplace to end each one of our expeditions with these grim reminders, it is maybe comforting to remember that most organisms, even those that inevitably fell into extinction, still lived normal lives: the eventual disappearance of their kind did not annihilate the significance of all their individual experiences and certainly did not mean anything for them, for we are constantly living in the present and have no true certainty of what the next instant has in store for us.

***

1-Kelenken

2-Astrapotherium

3-Neosteroimys

4-Dung beetles

5-Protypotherium

6-Peltephilus

7-Panstrogylus

8-Patagosmilus

9-Neotamandua

10-Theosodon

11-Nothofagus

12-Podocarpaceae

13-Amaranthaceae

14-Asteraceae

15-Poaceae