First-Time Mother

At about 103 million years ago, this is the Early Cretaceous Epoch. A region that one day will belong to the Australian state of Queensland is now a coastal setting marked by the presence of an inland sea. Despite the high latitude position, the temperatures here are quite warm, as is generally the case for the rest of the planet, which still finds itself inserted in the greenhouse phase extending all the way to the start of the Mesozoic. Some millions of years prior, the cooler period that began in the previous chapter ended, a scenario too contributing to the higher temperatures. In regards to our last journey, the continents have moved quite a bit since then. In the Northern Hemisphere, Eurasia has taken a more recognizable shape and North America has been essentially cut in half by an inland sea of its own, dividing it into a western and an eastern landmass. Down south, Gondwana has split: South America and Africa are continuously growing apart, forming the South Atlantic in the process, and, concomitantly, India along with Madagascar are drifting northward, while Antarctica and Australasia remain together in the south pole.

On one of Australia’s most boreal coasts is where this tale takes place. Not far from a beach, a female Kronosaurus slowly swims about. She does so in a “flying” manner, twisting her flippers to propel herself through the liquid medium. At more than 10 meters in length, she is the largest animal of these expanses and can only be put in danger by others of her kind. Thus, for most of the year, Kronosaurus intentionally avoid each other, setting up loose perimeters in which they hunt without competition. However, currently, some are actively seeking out others. Right now, for instance, she is crossing a male’s territory and, after noticing several low-frequency calls, the female realizes he is approaching with no offensive purposes. The male finally reaches her, swimming by her side at a reasonable distance while attentively scanning his possible mate’s reactions: he would not like getting bit by a disgruntled female. As minutes go by, he gets progressively closer and, soon enough, they are nuzzling their snouts, very innervated and containing sensitive mechanoreceptors, which, normally used for detecting prey, here serve a different purpose. Eventually, the male delivers his gametes and wanders away. For him, this was a brief affair, but, for her, it is only the beginning of a long and intense process, especially since she is a first-time mother.

A few weeks later, her baby is growing more and more developed while other animals are still engaged in the mating season, brought forth by seasonal blooms in phytoplankton related to changes in day length characteristic of high-latitude locations (as mentioned before). Eromangasaurus are a few of such creatures, sharing with the Kronosaurus many notable features, such as four flippers, short tails, and quite toothy jaws. This is no coincidence since both are plesiosaurians, members of the larger Sauropterygia, a group of possibly archosauromorph reptiles that took to the water during the Triassic. From then on, they have diversified into numerous forms and the contrasting differences between the Eromangasaurus and Kronosaurus, despite their similarities, show this also, the first being an elasmosaurid and having a long neck (not very flexible, making it useful for rapid swimming while not compromising its downward motion, used to capture targets below) accompanied by a small head, and the latter being a brachauchenine pliosaurid, spousing a short neck but a very large head, a pattern that comes in many variations among plesiosaurians as a whole. Both additionally have in common less visible traits (a viviparous mode of reproduction and high metabolic rates, making them endothermic).

Though far from small at 7 meters from snout to tail, the Eromangasaurus are dwarfed by their brachauchenine relatives and, as a consequence, sometimes find themselves on their menu, meaning they try to avoid the Kronosaurus even more than the Kronosaurus try avoiding themselves, content on feeding upon small fish and the abundant cephalopods (pliosaurids, in general, will also consume these foodstuffs in substantial amounts), using gastroliths to help process the ingested material. Even so, guaranteeing the next generation is now the elasmosaurids’ main worry. Courtship is reasonably elaborate, with males vying for the attention of the females through acrobatic movements, each one undertaking a choreography of his own, some further incrementing their displays by blowing bubbles. Females are picky and will snap at pretenders who insist after rejection. The ones to have their “dances” approved will close in on the females and, following a moment of synchronized swimming of variable duration, finally copulate.

An Eromangasaurus couple is currently partaking in the courtship process, the male and female staying side by side, calmly cruising just below the surface. The nearby commotion caused by a shoal of Pachyrhizodus, ray-finned fish with a tuna-like body shape, does not distract them. There, though, many other predators have coalesced to hunt the 45-centimeter-long osteichthyans, which, under other circumstances, are quick, active carnivores. They, in particular, belong to the P. grawi species, which contrasts with P. marathonensis, another member of the genus that, although nowhere to be found currently, also lives in this environment, being larger and more robust. Their shoaling behavior (made possible in great part due to the lateral line: a structure composed of mechanoreceptor cells that gives each fish a sense of the position of its peers, preventing collisions and promoting cohesive movement), allows them to better survive the onslaught by means of confusing the attackers and of rapidly alerting others of imminent dangers. One of such attackers is a trio of Australopachycormus, also ray-finned fish part of a group known as Pachycormiformes, in which belong a wide variety of forms, including giant filter-feeders like the Jurassic Leedsichthys, capable of acquiring maximum lengths of more than 16 meters. The Australopachycormus, however, differ greatly from their titanic, long-dead “cousins”, being swift hunters sporting a spear-like snout (giving them a fairly substantial resemblance to modern-day swordfish) and serrated fins they utilize to potentially devastating effect on the poor schooling fish they may target, despite normally going after more armored prey.

Other creatures keen on consuming at least a few Pachyrhizodus are Platypterygius, 5 to 7 meters long ichthyosaur reptiles and, thus, bony fish of the tetrapod variety. Possible archosauromorphs, as is the case with plesiosaurians, they differ from the latter by having a remarkably dolphin-like aspect, but are similar in other regards, being endothermic and viviparous too, birthing the young tail first. Maybe originating as far back as the Late Permian, ichthyosaurs diversified greatly during the Triassic and Early Jurassic and have, since then, progressively decreased in diversity, displaying less divergent body plans that, nonetheless, still allow them to be quite abundant animals. Another less visible difference between the two sea-going amniotes is related to a complete secondary palate: while plesiosaurians have such a structure (akin to mammals and crocodilians), ichthyosaurs do not. As a result of this anatomical distinction, plesiosaurians breathe using their noses, while ichthyosaurs do so with their mouths, being furthermore capable of detecting olfactory signals from the water, seeing as their nasal canals completely communicate with their oral cavities. Returning to the Platypterygius themselves, they exhibit a rather interesting feature shared by some other ichthyosaurs: the ability to adjust their skin tones to ensure protection against UV radiation, to camouflage, or to help stabilize their body temperature. Branched melanophores, pigment cells containing melanosomes (pigment-containing organelles), enable color change by moving the melanosomes and altering their distribution using cytoskeletal components. The just described phenomenon can be readily observed in part due to the more intense solar radiation of this time of day, this time of year: the three Platypterygius trying to catch the Pachyrhizodus at the surface are darker than the one individual that has lagged behind in deeper waters.

Speaking of deeper waters, a couple of Cooyoo, another ray-finned fish, hesitantly approach the shoal. They are ichthyodectiforms (a grouping of predatory representatives including the bigger and more well-known Xiphactinus, a genus to appear only in the Late Cretaceous) and, though still fairly sizeable, at around 3 meters from the tip of the snout to the end of the tail, are sometimes fed upon by Platypterygius, which, counting with both numbers and their robust teeth, could perhaps kill even these fully grown individuals, viciously dismembering them. Despite this, the two are slowly but surely approaching. It is likely the ichthyosaurs will be more interested in the much less dangerous and way smaller Pachyrhizodus, allowing the pair to participate in the feast without becoming part of it. Opposite the Cooyoo, but also far from the surface is yet one more ray-finned fish: Richmondichthys, content to be far away from the action higher in the water column. At more than 1.6 meters in length, it is a filter feeder, slowly crossing the sea with its mouth agape to catch tiny lifeforms. Its peaceful demeanor does not mean it is completely vulnerable, however, being shielded with scales covered by ganoine, multilayered enamel, a highly mineralized tissue that also covers teeth.

Retreating to the surface and beyond, to the air above it, four more animals are taking advantage of the cornered shoal: Mythunga, anhanguerid pterosaurs. With a wingspan of around 5 meters, they are decently sized, but only get close to the sea’s surface when sure of securing a meal, quickly dropping their needle-like teeth (apt for grasping slippery prey) into the water and then hurriedly withdrawing them. Such accuracy and swiftness are justified by the presence of the Platypterygius, more than capable of snatching them should they get too close at the wrong time.

Concomitantly to all that has been described, the two Eromangasaurus have been getting closer and closer. Their swimming session has slowly ground to a halt and it appears the male’s advances have been rather fruitful, as both prepare to participate in more intimate matters. However, the female, to much of the male’s confusion, suddenly becomes agitated, starting to twist her flippers in an effort to quickly move away. That is when he notices a large shadow emerging from the depths in their direction, covering in a short time the distance separating it from them. The mother Kronosaurus, hungry not only due to her body’s demands but also due to the ones of her baby’s, proceeds as hurriedly as she can to the surface, especially as she realizes she has been spotted. At the same time, the male Eromangasaurus starts swimming, trying to keep up with his companion, which, having seen the danger before, has had a head start. It is then the Kronosaurus selects her target, closing in on the straggler. In just a few seconds, he feels close movement in the water just before a terrible pain descends upon his tail as the pliosaurid’s teeth dig deep. Moving frantically, he desperately tries to break free, further cutting and ripping his flesh while the larger plesiosaurian keeps stubbornly latched on. Soon enough, the elasmosaurid feels more stabbing pain as fragments of bone move to the blood-stained water. And, out of nowhere, the pressure is relieved as fast as it originally came, though the pain proceeds to new levels. Briskly tilting his long neck to look back, he sees the Kronosaurus fiercely gripping what once was his tail, violently amputated from his body. Using the available moment, the male bursts into speed and manages to escape, the brachauchenine occupied gobbling up the meal she managed to get. In the end, the Eromangasaurus has managed to live another day, but whether the injury will still claim his life remains to be seen and so does if he will still have the same success with females.

Nevertheless, life, for other beings, goes on as usual, indifferent to the tragedies other organisms may endure. A few of these oblivious creatures are the abundant ammonoids, cephalopod mollusks first seen in our voyage to the Late Devonian. Here, they are represented by a multitude of forms. Some, like a few individuals distancing themselves from the fleeing Eromangasaurus, are more “typical” looking, exhibiting a circular, coiled shell, and others, like a species of Myloceras seen in the distance, are quite peculiar in comparison, possessing a longer shell coiled only at its end. The latter are reasonably similar to Richmondichthys in ecological terms, drifting through the water in no rush while consuming plankton. Directly on the seafloor, more mollusks can be found dotting the marine substrate, these being the giant bivalves Inoceramus. They are very common on these low-oxygen bottom waters thanks to their large and unusual gills that not only provide more surface area for gas exchange but are also capable of filtering considerable amounts of water in a short period, aiding in the just cited gas exchange. The impressive gills also help these invertebrates secure larger food particles, including pieces of microbial mats broken off by strong enough currents. Sometimes, undesired materials or even parasites may find themselves lodged between the shell and the soft body of the Inoceramus. As a reaction, the animal will secrete a crystalline form of calcium carbonate (which also lines the innermost part of the shell) around the foreign body, gradually enveloping it in various layers and, with time, forming a pearl. 

Also in the depths, sharing space with the bivalve behemoths, are way tinier invertebrates, 1-centimeter-long isopod crustaceans part of the genus Brunnaega. Isopods constitute a widespread crustacean order: its members generally have a flattened complexion and their body is divided into several segments, with seven pairs of well-developed legs. While many are encountered in marine settings such as this one, a few are freshwater dwellers and many others are terrestrial, though susceptible to dehydration. Being overall detritivorous (though a handful are notable parasites, sucking hemolymph or blood, as is the case of some fairly close relatives of Brunnaega, which parasitize bony fish, occupying their gills, external body surface or even the inside of their mouths, possessing hook-like legs to anchor themselves), this genus specifically is a carnivorous scavenger. 

Like the Inoceramus, it is adapted to the less oxygenated bottom waters and can survive for long times without food. However, if they are to be blessed with fallen meat from the surface, they gather in large numbers, forming crawling clusters on the carcasses and tearing them apart with their mandibles, scenes that have been occurring for hundreds of millions of years and one of which was even observed in the Cambrian. Like the Cambrian arthropods, isopods, as well as other crustaceans, still suffer from bacterial infestations of their exoskeletons. Such bacteria possess a remarkably similar lifestyle to the ones first mentioned as trilobite pathogens, normally acting as environmental degraders of chitin, but sometimes, usually due to a pre-existing lesion on their host's carapace, they give rise to disease. 

Some of the bacteria responsible for this are those known as vibrios, which likely arose earlier in the Mesozoic. They are part of the class Gammaproteobacteria (component of the larger phylum Proteobacteria, with more of its members mentioned here) and are quite unique due to having a subtle curvature that gives them a kidney or bean shape, with it being the result of the presence of filaments in the space between the peptidoglycan wall and the outer membrane. Having a single flagellum, vibrios are quick to move to any food source and, in this case, they attach to the exposed chitin and proceed to break it down, though many trade a free-living lifestyle for a harmonious symbiotic relationship with other water-liviving organisms (the species Vibrio cholerae, in contrast, is actually a parasite of mammals, generating intense and watery diarrhea that can lead to death due to the significant loss of water). Other bacteria that commonly promote such infections are those of the family Flavobacteriaceae (phylum Bacteroidetes), being rod-shaped and strictly aerobic (many bacteria, such as the cited vibrios, are called facultative anaerobes, because they can revert to anaerobic metabolism in the absence of oxygen), requiring high salt concentrations to survive.

Another quite nonchalant animal is Thapunngaka, an additional anhanguerid pterosaur closely related to Mythunga, but spousing more prominent snout crests and a wingspan stretching between 6 and 7 meters. Soaring above the water, it observed the attack of the Kronosaurus with mild curiosity and did not even pay attention to its smaller kin’s fishing activities, already having a full belly from scavenging on a sauropod carcass on the mainland. Though clumsy walkers, these anhanguerid pterosaurs will often look for food in terrestrial environments, but will, either way, spend most of their time on the wing.

Back in the water, Notochelone also “flies”, moving its flippers in a similar way to that of the plesiosaurians. It is a protostegid sea turtle, a family that, despite not surviving into Cenozoic times, perhaps has the extant leatherback sea turtle (Dermochelys coriacea) as its closest relative. Shining away from phylogenetic matters, this individual is about to take a breath, a necessity after having spent quite some time on deeper waters (which, here, are still reasonably shallow), looking for and eating Inoceramus, a major component of its durophagous diet. Like other sea turtles, Notochelone can dive for several minutes without resorting to anaerobic metabolism and quickly refills its oxygen stores once in contact with the air, making it very well adapted to the aquatic lifestyle. Even so, it, being a mother, remains tied to the land in regard to reproductive matters, currently moving to the very area where she was born years ago in order to lay her own eggs. How do sea turtles even manage to accomplish such a navigational feat? They do so by recognizing the differing magnetic signatures of Earth’s magnetic field that exist in distinct locations, plausibly imprinting on the magnetic parameters they detect when young and utilizing them to reach back their natal regions, sometimes committing mistakes if the magnetic characteristics of localities are similar.

Strolling down the beach the just seen Notochelone is aiming to reach is a herd of Muttaburrasaurus, basal iguanodontian ornithopods 8 meters in length. They are, being more specific, likely close relatives to rhabdodontids, a grouping that curiously only contains Late Cretaceous members from Europe. Living in modestly-sized family units such as this one, these dinosaurs are mostly gentle herbivores. The use of the word “mostly” is intentional, as they can, under the appropriate circumstances, get quite feisty. One of those circumstances is during the mating season (quite the recurring theme), when they engage in constant disputes for harems of females using a variety of techniques. Their almost balloon-like noses, colored a bright yellow in males, not only inflate significantly but also intensify their calls, turning them into powerful booms that can be heard from quite far away. During the loud contests, it is not uncommon for particularly aggressive males to wrestle with one another, the fighting ornithischians bound together in a hug of animosity where scratches and bites are exchanged. Despite all of this, the presently observed family has not been a witness to those savage events. The dominant male, father to all of the herd’s young, is quite the strong and fit individual, keeping the majority of possible rivals away, and his oldest son, despite already having developed the vibrantly pigmented nasal sacs, has not yet kicked into reproductive overdrive. It is probable his expulsion will have already taken place before he does so. Like other basal representatives of Iguanodontia, the Muttaburrasaurus are adorned by fuzzy feathers. When young, the feathery coats help them maintain body temperature and function as effective camouflage. When adults, much of the covering is shed, but some filaments remain along their backs, bearing a distant but recognizable resemblance to the manes seen in a few mammals, like the ones of some equines and of the Astrapotherium, seen in a future tale.

Other ornithischians resting close by are Kunbarrasaurus, these two being siblings and, although quite a bit smaller than their bipedal relatives (at only 2.5 meters long), their armor covering grants them a reasonable amount of protection. Such armored bodies are typical of their clade, Thyreophora, of which the previously encountered Kentrosaurus were members. Despite this, the observed herbivorous creatures are not stegosaurids, but rather ankylosaurians, components of a southern radiation that, marked by a peculiar arrangement of tail osteoderms, likely separated from the other representatives of Ankylosauria back in the Middle Jurassic. Also in contrast with the Muttaburrasaurus, they are quite a bit less social, though occasionally gathering in small groups or pairs as just witnessed. The last dinosaurs to be seen in our fairly quick expedition into the mainland are saurischians belonging to the genus Nanantius, rising from the trees in a large flock. Flying forms, they are sea-faring predators part of the Coelurosauria, a grouping inside Theropoda containing a diverse array of dinosaurs, an array that also encompasses birds. Apart from such stated facts, these prolifically feathered hunters are enantiornithines, a theropod group very closely related to the birds proper, its various members thus being, justifiably, very bird-like, though sometimes containing differing traits, like the toothed snouts the Nanantius possess, their most important tool in helping secure daily fishy meals.

On land, much emphasis must also be put on plants. The forest surrounding the beach is notably composed not of gymnosperms, but of angiosperms (fruiting and flowering plants). They have experienced a burst of diversification in the current period, becoming very important, if not dominant members of their ecological spaces. Despite this, angiosperms originated earlier than in the Cretaceous, though how much earlier remains a topic of contention, possibly going back all the way to the late Permian. Even so, the main angiosperm lineages have already emerged, these being the monocots and the eudicots, the members of the two lineages sharing various traits spanning leaf/root configuration, pollen grain apertures (regions of the pollen grain wall that can allow for the exit of the pollen tube), and other less or more visible traits. The trees of the nearby forest, for instance, are eudicots, forming a dense aggregate that provides shelter and nourishment for a wide variety of organisms, many of which have accompanied the angiosperm explosion.

Some of such organisms are actually various yeasts. Such fungi are unicellular, asexually reproducing either by budding (in which the mother cell gets progressively larger until a protrusion, which may go on to become its own cell, develops)  or the more traditional fission, and have thrived on the abundant source of simple sugars provided by the angiosperms’ fruits, developing a rather curious way of monopolizing the carbohydrate supply: the production of ethanol. What may have started merely as a product of anaerobic respiration gradually became a more integral part of the yeast lifestyle (being even formed aerobically), to the point it transitioned from a simple byproduct into an important tool for eliminating fellow microbes. Parasites have also seen significant development: aphids, sap-sucking insects like many others of the order Hemiptera, have diversified greatly and rapidly, largely shifting from gymnosperms to angiosperms. The small, plump critters display complicated life cycles (which may involve asexual and sexual stages, alternation of hosts, and even the development of flying forms) and are almost always very specialized for their respective hosts, displaying tolerating mechanisms to the defenses put up by their victims, a typical example of co-evolution. The briefly mentioned asexual reproductive stages mean that parthenogenic aphids can produce absurd numbers of offspring in quite a short amount of time.

Returning to fungi, they are more members of the great assortment of plant parasites. Some of the most specialized and ancient ones are those known as rust fungi, being completely dependent on their distant, photosynthetic relatives and capable of infecting from ferns to gymnosperms all the way to angiosperms, likely evolving back in the Carboniferous. Most have a complicated reproductive process involving five different spores and potentially alternation between hosts, with they, like aphids, usually having a very high affinity for such hosts, unable to parasitize others. Affected leaves start by exhibiting yellow to orange pustules called uredinia (which are not present in all rust fungi, these possibly exhibiting great differences from the usual, five-spored reproduction), filamentous growths that emerge from the plant's interior and produce many, but truly many urediniospores (it is via such filamentous growths the fungi get their nourishment, forming branches that penetrate into the vegetal's host cells, serving as a mean for the heterotrophs to get sugars and amino acids).These single-celled spores can be dispersed by wind or water, reinfecting the host or going on to colonize other susceptible plants, resulting in more uredinia either way. 

Apart from this, the uredinia also produce teliospores, stalked multicellular spores with pigmented and thick-walled cells that generally stay close to the parent plant, being unable to disperse as far as the urediniospores, but capable of entering a dormant state in not-so-habitable conditions, such as during winter or drought. In more apt times, the teliospores produce more single-celled spores, but they are thin-walled and short-lived, known as basidiospores. When reaching a colonizable phototroph after being dispersed by wind, they germinate and proceed to infection (for rust fungi spores, the routes of infection may be through the plants' stomata or directly through their covering), similarly to what happens with urediniospores. Differently though, they form yellow spotted structures called spermagonia, which, for their part, form more unicellular and thin-walled spores: spermatia. They are covered in a sugary coat and transferred by insects to another spermagonia, where they will encounter receptive cells and fuse. This new form, containing genetic material both from a spermatium (individual spermatia) and from a spermagonian cell, multiplies and, from its filamentous shape, arise aeciospores, which again, are carried on by rain and wind until a new adequate host is found. Colonization proceeds with the formation of the uredinia that started it all.

Aside from the spot-causing rust fungi, there are additional pathogens causing such symptoms, like Tobamovirus, a genus of reasonably large RNA-based viruses that, unenveloped, possesses a rod-shaped capsid made up of a helically arranged protein. It has accompanied the rise of the angiosperms and causes plants (among various other symptoms) to specifically develop spots on their leaves that resemble mosaics, being very stable and easily transmitted by contact, also infecting seeds. Aphids themselves contaminate plants with other mosaic viruses, which are not necessarily related to Tobamovirus (actually fairly close phylogenetically to the human hepatitis E virus). Luckily for their photosynthetic hosts however, the aphids' predators are many, the most prolific ones likely being braconid parasitoid wasps, hymenopterans that utilize the aphids themselves as hosts for their eggs and subsequently larvae. Even against them, the hardy sap-suckers have developed protection, utilizing bacterial endosymbionts located within their bacteriocytes (mentioned for the first time here) to help encapsulate the aforementioned eggs and larvae. 

Even more amazingly, the braconid wasps possess microscopic partners of their own that help the development of their larvae: Bracovirus, enveloped viruses containing a segmented DNA-based genome, enclosed within a cylindrical capsid that varies in length between different viral particles. Initially, the Bracovirus integrates its DNA with the one of its wasp host. It, however, only replicates in cells of the reproductive tract of females, destroying them and acquiring their cellular membrane envelope in the process, coming to fill in exaggerated numbers the liquid with which the eggs are injected into the hymenopterans' selected victims (the Bracovirus are in such quantities the medium turns a blue iridescent color, a result of the extremely large aggregation of viruses). As the tiny parasites are injected into their new host, they infect primarily hemocytes, invertebrate immune cells that circulate in hemolymph, compromising the targeted arthropod's immune response. Having already been vertically transmitted to the braconid wasps' offspring, the viruses will be ready to assume this immune-suppressing role in the next generation, completing the symbiotic cycle. Concomitantly, other insects have not fared so well. The siphon-bearing scorpionflies (seen in the last chapter), once important gymnosperm pollinators, are on their way to extinction, while the proboscis-bearing lepidopterans of the group Glossata are taking up their niches, but as angiosperm, not gymnosperm pollinators (lepidopterans which are themselves targets of braconid parasitism, also being infected with Bracovirus).

Still remaining on land, it is time to explore some very peculiar freshwater inhabitants: lungfish (called dipnoans), these being of the genera Ceratodus and Neoceratodus, the latter still extant in the present day. As the name indicates, they have lungs (Neoceratodus specifically has only one lung) and, though bony fish, are not actinopterygians (ray-finned fish), but rather sarcopterygians (lobe-finned fish), a clade that also encompasses tetrapods. Unlike other lungfish, Neoceratodus, at 1 meter in length and sporting four flippers used for propelling a cylindrical body covered by scales, is only a facultative air breather, having a carnivorous diet composed of animals that it can crush using its tooth plates. To allow for adequate lung function, dipnoans count, like tetrapods, with pulmonary surfactant, a mixture of lipids and proteins (though the exact composition varies between different animals) that, due to its amphiphilic properties, mixes with water and greatly reduces surface tension, a phenomenon that is important for preventing lung collapse in many nonmammalian lungs and, in the case of mammalian lungs, for impeding the closing of the small airways and for lowering the resistance associated with alveolar inflation. Surfactant is also encountered in the swim bladders of other bony fish, which is not especially surprising given such structures likely evolved from lungs, an ancestral trait of the common bony fish ancestor. In this context, it is possible that surfactant itself, originally functioning as an antiadhesive, as an antiseptic, and even as an inhibitor of oxidative damage, allowed for the development of air-breathing in the first place and, thus, predated even the lungs.

Now finally abandoning the mainland, once again it is time to return to the sea, at least temporarily. A little far from the coast lies a small island that has been serving as a Mythunga nesting ground. The place can be described as somewhat chaotic, as the animals, in great numbers, engage in their daily activities. Some are simply walking, others are landing, others are taking off, and some are occupied feeding their offspring, all immersed in an intense cacophony of sounds that can be heard from quite a distance. While some of the infants are hatchlings, a few are about to start their first flights. On the island itself, there is not much space for training due to the crowded conditions and, thus, many must take to the sea, where they will, if fortunate enough, already start their first hunts. The problem lies in inexperience: while adults are already accustomed to sea-faring flights, the same cannot be said for the youngsters, which not rarely fall into the liquid medium and cannot get out.

Since the first of such failures, predators have become keenly aware of the fact and, currently, the waters around the colony have become a hub for all sorts of creatures looking to get an easy meal. The female Kronosaurus is no exception: she has been making regular visits to the site for the past few days, the number of fledgling Mythunga becoming increasingly more expressive as they mature. Her presence is noted by fellow carnivores, which, none close to rivaling her size, keep a reasonable distance.  As she calmly cruises through the water, ray-finned fish like Pachyrhizodus (more specifically the larger P. marathonesis species) and Australopachycormus rapidly swim away. They count on quick movements and agility to swiftly capture struggling Mythunga on the sea’s surface, a hunting strategy also employed by the Platypterygius, which will not hesitate to try and steal the kills of smaller or similarly-sized competitors. She, on the other hand, approaches the pterosaurs patiently and swallows them whole. Some are fortunate enough to be completely oblivious before the huge jaws descend upon them, while others, not so lucky, get a good look at the leviathan emerging from the depths, desperately trying to swim or fly away as their doom gets closer. And so, the flesh of some young serves to nourish another youngling and to grant it flesh, as the Kronosaurus continues on her journey as a first-time mother.

In the span of a few million years, at the start of the Late Cretaceous, marine environments especially will be severely affected by a significant increase in global temperatures, an episode known as the Cenomanian-Turonian Thermal Maximum. Not only would sea levels rise dramatically, but anoxic events would become more common and widespread. Even before such warm spike, the marine biota was already experiencing changes, associated not only with declines but also with the radiation of certain creatures. Be that as it may, some animals would indeed not survive such perturbations: ichthyosaurs, for instance, would go extinct. However, an important group of marine reptiles, the mosasaurs (these being lepidosauromorphs and, more specifically, squamates) would go on to fill not only some of the ichthyosaurian niches but also occupy the ecological roles of apex predators left behind by the pliosaurids (yet more victims of the aforementioned changes), though plesiosaurians as a whole would still continue flourishing. On that note, ends another tale as we finally progress to the end of the Mesozoic.

***

1-Kronosaurus

2-Eromangasaurus

3-"Beudanticeras"

4-Inoceramus

5-Richmondichthys

6-Myloceras

7-Thapunngaka

8-Notochelone

9-Australopachycormus

10-Platypterygius

11-Pachyrhizodus

12-Cooyoo

13-Mythunga

14-Eudicot trees

15-Muttaburrasaurus

16-Kunbarrasaurus

17-Nanantius