Four-Month Darkness

At about 69 million years ago, this is the Late Cretaceous Epoch. A location that will eventually belong to the American state of Alaska is now a coastal plain located in the Arctic, in an extremely northward position. Consequently, temperatures are indeed quite cool, but ice only establishes itself at higher elevations. As has been the case for the Mesozoic as a whole, the planet still finds itself under a greenhouse effect, though temperatures have generally dropped since the Cenomanian-Turonian Thermal Maximum. The continental arrangement has not changed significantly from the one observed in the last tale: North America’s inland sea has greatly receded, bridging the western and eastern landmasses it formerly divided, and Eurasia as a whole has moved a little closer to the Equator. South America and Africa continue to drift apart, with India, now separated from Madagascar, continuing its trek north. Australasia as well, though still connected to Antarctica, has moved farther away from the south pole.

On these boreal expanses, a period of four-month darkness has set in. Here, the incredibly high latitudes modify day length by an absurd amount, a phenomenon observed in habitats seen before (such as the one from our preceding stop and from our Permian voyage), but not to such a degree. The lack of sunlight and the drop in temperatures pose several hardships both for animal and plant life, these two eukaryote groupings having thus developed many traits to help them withstand such conditions, traits to be cited and elaborated upon throughout this environment’s observation. One of the key elements for survival is to acquire food and that struggle is the first we witness, as a herd of Pachyrhinosaurus is harassed by a pack of Nanuqsaurus, composed of a mother and her two young.

The former are ceratopsian ornithischians about 5 meters in length, being centrosaurine ceratopsids in particular, typically characterized by shorter frills than the ones of the chasmosaurine ceratopsids. Despite this difference, both subfamilies of Ceratopsidae share ornamented heads, spousing the aforementioned bony frills and various horns or bosses of all manners in shapes and sizes. The genus Pachyrhinosaurus, counting with three species, differentiated in part by their assortment of cranial accessories (the earlier, more southern ranging P. canadensis and P. lakustai and this one, P. perotorum), is rather unique among ceratopsids as a whole due to the sizeable nasal bosses, but, in overall body plan, members of the family are fairly similar.

The latter are coelurosaurian theropods (as will be all theropods observed in this stop) and, more specifically, tyrannosaurids, achieving dimensions of roughly 8 meters long. Inside Tyrannosauridae, Nanuqsaurus is part of the Tyrannosaurinae subfamily, which contrasts with the Albertosaurinae subfamily in having some of its members (though not all) be quite more robust, while albertosaurines are more gracile. Either way, as tyrannosaurids, these saurischians possess strong bites and are formidable predators, constituting a worthy threat even to the armored ceratopsians. Hunting in groups, they are even more particularly dangerous. Young tyrannosaurids have quite different body proportions when compared to adults, being lankier and delicate, allowing them not only to occupy a different ecological niche, going after smaller and more cursorial prey, but also, under some circumstances, help fully-grown individuals using their distinct anatomical perks.

This is what they are doing now. The two juveniles are completely incapable of handling an adult Pachyrhinosaurus, but their main objective is to distract and distress the herd, hopefully allowing for a sickly, old, or young individual to be singled out and then killed by their mother, which strides close to them, always attentive to any possible opportunity. The hunt has been ongoing for some minutes already, the young Nanuqsaurus circling the ornithischians and snapping at them, while occasionally rapidly running away in order to avoid a blow from a less indulgent centrosaurine. Protecting a good number of babies, the mature Pachryhinosaurus stay closely knit together, determined to impede any possible attack on their youngsters or any individual to be separated from the others. The theropods, though, are also resolute and, as such, the standoff is sure to last some time longer.

Eventually, the mother tyrannosaurid, progressively hungrier, tries engaging directly with the defending herbivores. Her larger size is more adequate at unnerving the ceratopsians, which, even so, maintain their positions. She bellows and snaps at the Pachyrhinosaurus, responding by producing guttural sounds of their own, inflating their nose flaps, and aggressively moving their heads. The Nanuqsaurus, though, is unphased and she remains close, the ceratopsians not quite so confident in trying to attack her as they did her young, which still dart around the herd, growing more agitated by the minute. These conditions pose an ever-greater risk for the adult theropod, which, even so, stays put, also knowing the ornithischians are closer to panicking. A few more minutes pass and, for one instant, the herd’s defensive formation breaks, as a particularly bothered Pachyrhinosaurus ends up giving chase to one of the young coelurosaurs for a little too far, a little too long. It is enough of an opening for the fully grown Nanuqsaurus to try and seize a victim. Ideally, she would go after bigger fodder, but the risk is too high since the ceratopsians greatly outnumber her. As such, she plunges her head and grabs a baby Pachyrhinosaurus, which shrieks loudly as it gets yanked off by the predator, immediately retreating to a safer distance. Her offspring quickly join her side, trying to keep the tense quadrupeds at bay while their progenitor moves away. The baby ornithischian might constitute enough of a meal for her, but the two juveniles will have to look elsewhere so as to fill their bellies.

And elsewhere many other creatures are going about their lives, shrouded in the darkness only occasionally broken up by moonlight and the ethereal luminous rays of the polar aurora, an interesting phenomenon associated with the interaction of Earth’s magnetic field and the solar wind, a stream of charged particles emanating from the Sun that disperses through the Solar System and even beyond (note that fluctuating electric fields, such as the ones generated by charged particles in motion, like the ones of the solar wind, create magnetic fields and vice-versa). In certain cases, our planet’s magnetic field, if sufficiently affected by the solar wind, can suffer magnetic reconnection, an event in which magnetic field lines break only to then reconnect. As a result, ions and electrons trapped by our world’s magnetic field (such particles form various layers around the Earth) can be accelerated into the poles, exciting atmospheric gases, thus promoting the luminous displays characteristic of the auroras.

Other planets also experience this phenomenon, though not necessarily in the same way. Mercury, for example, having quite the rarified atmosphere, has X-ray auroras (not in the visible spectrum like ours), resulting from the collision of electrons with its surface. And while it is expected for magnetized worlds (Mercury, Earth, Jupiter, Saturn, and Uranus for instance) to have auroras, even planets at least partially lacking magnetic fields can possess them, as happens with Mars, in which they are likely caused by remnants of a former field.

Returning to literally more down-to-Earth matters, another hunt has occurred simultaneously and also quite close to the one of the Nanuqsaurus family, though at a smaller scale. A Saurornitholestes, an arguably tiny 1.3-meter-long dromaeosaurid theropod, has captured, after a stalk followed by a successful ambush, an Unnuakomys, an even tinier creature, more specifically a mammal belonging to the clade Metatheria (a broader group that has marsupials as its extant representatives, with mammal classification also being tackled in our stay in the Jurassic). While having more diversity elsewhere in western North America, the metatherians, in this so northern region, are represented practically by this genus only. The distribution of its predator also varies: Saurornitholestes, while very common in lower latitudes, is quite rare here, seldomly seen, not only due to its lesser numbers, but also thanks to its skittish nature, a reptile always bobbing its head and displaying jerky movements. This individual, though, is constituting an exception, pinning down the agonizing synapsid right next to the juvenile tyrannosaurids with a perhaps irrational calm. However, its stoic demeanor does not last and, before the victim under its talons finally succumbs to fate, it returns to more expected behavior, suddenly grabbing the soon-to-be meal with a decisive movement of its head, then scurrying away to a safer and quieter place.

An even more diminutive mammal is Sikuomys, leaping down from a nearby Parataxodium conifer (such gymnosperm, once thought to almost monopolize the forest, actually shares it with other Cupressaceae conifers, a Ginkgo species, and eudicot angiosperms), trying to go unnoticed by the dromaeosaurid, which now has, fortunately for it, moved elsewhere. It is a leptictidan, part of a superorder of stem eutherian mammals (Eutheria, having placentals as its extant representatives, constitutes a sister group to Metatheria, both inside the more inclusive Theria) that may be paraphyletic, some members of Leptictida closer to Placentalia than to other leptictidans themselves. Sikuomys is partially insectivorous, sporting a trunked snout responsible for giving it quite a unique appearance. Such structure is constantly moving, trying to detect the scent of arthropods and, if successful, frantically scrolling through the undergrowth until the unfortunate invertebrate is captured.

Opposite where the Saurornitholestes was located is a resting troodontid, some 3.5 meters from snout to the tip of the tail, not yet fully grown. In spite of the size difference, they are quite similar, both counting with wings, long tails, and iconic sickle-shaped claws on their feet. Such alikeness is not the result of convergent evolution, as both are very phylogenetically close, likely part of a clade known as Deinonychosauria, which is the most closely related to Avialae, an assembly that includes birds proper and others, like the enantiornithines, seen and mentioned also in our last journey (even though it is possible troodontids are actually paraphyletic regarding avialans, with some of their members closer to modern birds than to other troodontids). Even so, dromaeosaurids and troodontids, in general, occupy reasonably disparate ecological roles. Not only are the representatives of Troodontidae omnivorous, in contrast with the fully carnivorous Dromaeosauridae, but there is also a contrasting preference for prey items: the former are more apt at handling smaller, softer, and less struggling victims, while the latter are better capable of handling larger quarry and consuming bone. In regards to prey capture, troodontids exhibit more cursorial adaptations whereas dromaeosaurids are less speedy but stronger graspers, correlating with their increased capabilities of dealing with resisting targets. Overall, such differences mean that these deinonychosaurs coexist in most habitats without significant competition.

Here, nevertheless, the typical distribution of such coelurosaurs found in more southern expanses is a bit changed. The observed troodontid is not only the most abundant theropod, but also quite bigger when compared to less boreal forms. The very large eyes the saurischian has is one possible explanation for its dominance, being very well attuned to this low light environment, which, even when not in the winter darkness, is cloudy and misty, apart from counting with expressive forest cover. Good vision makes this individual troodontid quite comfortable immersed in its surroundings and, thus, it is quite content sitting in a fairly exposed position, calmly preening its feathers. Preening is quite an important process, one still very much undertaken by the extant dinosaurs, essential for removing parasites and maintaining feather alignment. Showcasing the significance of such behavior, Saurornitholestes, for instance, counts with teeth specialized for it. Feathers are not only inhabited by parasites, but also by commensals, which may even be beneficial for their harborers. This is the case of some feather mites, minuscule arachnids that live attached to the feathery plumes, using their scraping mouthparts to ingest pollen, fungi, and shed skin, food habits that potentially inhibit the growth of pathogens. Other feather-dwelling mites, however, are indeed parasitic: some live inside the part of the feather shaft that anchors to the dinosaur's skin, eating it away. Both, though, are part of Acariformes, a group of mites that extends deep in the past, with fossils all the way from the Early Devonian (likely originating earlier even, during the Cambrian-Ordovician boundary). Mites themselves, however, actually do not constitute a monophyletic group. Arachnids from another other, known as Parasitiformes, are also considered mites, some of their representatives being ticks (seen in the next chapter).

In order to better survive in this chilly location, especially as snow falls, like it is doing now, the two members of Deinonychosauria, like birds, also exhibit thermogenic activity in skeletal muscle. The mitochondria of those muscle fibers contain an uncoupling protein (avUCP), which creates, under appropriate stimuli, an alternate path for the protons pumped out from the mitochondrial matrix by the respiratory chain. Via this alternate path, the protons, instead of generating ATP by going through ATP synthase, actually increase turbulence, leading to more motion and, thus, higher temperatures. Mammals also have a very similar mechanism for non-shivering thermogenesis, but it occurs in a type of fat known as brown adipose tissue. Though functioning quite alike, the uncoupling protein is not the same (in mammals and other vertebrates it is UCP1), likely lost in the reptile lineage.

Nearby the troodontid, but protected by the height of a Parataxodium are two more mammals: Cimolodon, some 20 centimeters long. They are not part of Theria, as the others before, but are actually members of Multituberculata, comprised of omnivorous to herbivorous creatures. More specifically, these are members of Cimolodonta, which has undergone significant diversification since earlier in the Late Cretaceous, becoming the most numerous clade of Mammalia in the Northern Hemisphere. Curiously, multituberculates, despite being fairly distant from therians, actually exhibit reproductive strategies quite similar to that of placentals, displaying more prolonged gestation and less time dedicated to lactation. Wether such features are independently evolved or signify a more ancestral condition remains an unanswered question. Either way, this pair is not a breeding one. Rather, the two individuals have come up the same tree in order to escape from dangers below. Mating will only occur later in the winter, when soon after there will be way more available food.

Also not far lies yet another coelurosaur theropod: a therizinosaur, slowly walking through the forest in search of a suitable meal during these trying times, a somewhat hard task seeing as the Parataxodium, most of their also Cupressaceae relatives, the ginkgo, and the eudicots are all deciduous, losing their leaves with the advent of winter. A herbivorous form roughly 5 meters in length, it has a large gut for processing plant material, something which gives it a rather rotund shape. Unlike other theropods, derived therizinosaurs like this one, member of the family Therizinosauridae, have four toes in contact with the ground, better able to support their weight. Other traits are their long necks, apt to reach foliage, and their large arms, handy at manipulating their food. Talking about the arms, they end in sizeable claws, which, despite their significant dimensions, are more a result of the saurischians’ overall increase in size rather than due to a specific selective pressure (in the case of the Asian genus Therizinosaurus, the claws are especially non-functional, rendering its neck as the main mean to access foodstuffs). In regards to phylogeny, Therizinosauria is possibly part of a clade known as Maniraptora, which includes, among many additional groups, both Deinonychosauria and Avialae. Even so, the therizinosaurs occupy a placement a lot more distant from birds than the aforementioned deinonychosaurs, sitting in a fairly basal position of the Maniraptora clade. The tyrannosaurids and relatives, for instance, are even more distant, not being maniraptorans, branching off closer to the root of the coelurosaurian tree.

Abandoning the dinosaurs for a bit, an important question must be made: how do the plants, such as the deciduous trees, even "know" when winter approaches? For some, it is the drop in temperatures that serves as the main sign, but, for most, it is the ever-decreasing day length, detected by means of photoreceptors, which integrate their signals with the inherent circadian clock of plants, a natural oscillator of the expression of various genes along a practically 24 hour period (it is valid to remember that many organisms, including plants themselves, not only have an inherent circadian clock, but also an inherent annual clock, which can also modulate the typical seasonal responses in more ways). Via this interaction, the regulation of the photosynthesizer's genetic material is changed in a way that promotes the classical macroscopic signs, such as the cessation of growth and the shedding of leaves, which, with photosynthesis compromised due to lack of light, constitute merely a way for water to escape, apart from being possibly damaged by the harsher conditions. Only with a long exposure to chillier temperatures do the plants get eventually released from their dormant state, but they only resume their growth when the temperatures climb back up to allow for such process. But why do colder temperatures, which not even allow for vegetal growth, bring the plant out of its dormancy? The reason such a phenomenon was selected among these autotrophic eukaryotes over their natural histories (and an old phenomemon for that matter, already present in the Paleozoic, such as in Glossopteris) is that brief fluctuations in temperature, perhaps making it high enough to allow for the plant to grow, could serve as "deceivers", bringing the plant prematurely out of its latent stage only for the temperatures to soon become unwelcoming or for the Sun to be still out, in its long months of absence.

Watching the therizinosaurid go by and being watched by it is a thescelosaurid, focused on a burrow, the surroundings of which it will fiercely defend from “trespassers”. It is an approximately 4-meter-long ornithischian and, despite bearing some resemblance to ornithopods, it actually is quite more basal, with ceratopsians and ornithopods sharing a more recent common ancestor. As said for Cimolodon, this reptile’s burrowing habits mean it can afford to reproduce before the end of the darkness. Even so, this female is engaged in the burrow’s maintenance only for herself currently. There still is some time to go before breeding and her main attention is to now survive the season, something she is quite adapted for. Not only does she count with an endothermic metabolism (like the other dinosaurs of this habitat and the mammals, ectothermic tetrapods being absent) and a coat of fuzz-like feathers, but her great smell and digging abilities make her a specialist in locating and then unearthing food, like the rhizomes of ferns and horsetails, plants that lay dormant during winter through such buried structures. And if temperatures drop too much, she is more than capable of retreating to her burrow and waiting until it is less frosty outside.

More ornithischians are represented by a band of Alaskacephale, pachycephalosaurians, a group of fairly small herbivores with reinforced skulls, forming, together with Ceratopsia, Marginocephalia. Like their much larger Pachyrhinosaurus relatives, they form social units of mixed sexes, but, unlike what occurs in Pachyrhinosaurus (in which an alpha male monopolizes mating rights, driving away other mature males), the Alaskacephale are polyandrous. During the warmer intervals of the year, when it is time to reproduce, females become extremely territorial and the bands dissociate. It is under such circumstances they dispute between themselves access to as many males as possible. As such, fights involving their domed heads are common and they will aggressively hit each other, shoving one another with determination until one backs down. As can be expected, injuries are frequent. Most females usually sport some damage to their combat tools. Males also possess the structure despite not engaging in intraspecific competition, since it is useful to ward off and fight similarly-sized potential predators, like Saurornitholestes. Young individuals, in contrast, have flat-heads, the dome only developing later in life. 

The difference in integument between the Alaskacephale and the thescelosaurid is quite remarkable, since both are of fairly similar size and occupy the same seasonally frigid location. Though not very apparent, the pachycephalosaurians have their own adaptations to deal with the cold. Similarly to what occurs in the feet of birds, which have considerably less blood flow than that in other parts of their bodies, the extremities of these dinosaurs show a similar physiology, a feature that hampers their heat loss quite a bit, making those areas very similar to ambient temperature while ensuring their interior is significantly warmer. Not only this, but their more amicable atitude during winter makes it possible for them to snuggle, forming very tight agglomerates that help trap in heat during the most unbearably cold times.

Deeper into the Parataxodium forest are located ornithomimosaurs, herbivorous coelurosaurians reaching close to 4 meters in length. They, part of the more restricted Ornithomimidae, are agile and delicate animals, covered, like the theropods seen before, by a rich plumage, here especially important by serving as thermal insulation. In phylogenetic matters, Ornithomimosauria sits just outside Maniraptora, the previously mentioned grouping that includes, among many others, Therizinosauria (though therizinosaurs potentially have ornithomimosaurs as a sister grouping, in which case both would share a more recent common ancestor between themselves than to any other coelurosaur), Deinonychosauria, and birds themselves (inside Avialae together with closer relatives). Quite gregarious, these are somewhat nervous animals and at least one individual will always be on the lookout while others search for vegetation or gobble down gastroliths, pebbles used to help process plant matter as seen before in Elaphrosaurus, from our second Mesozoic expedition. Two of them are currently watching for dangers, one just a little more focused on trying to find any remaining leaves on a Parataxodium branch, while the other is more attentive, farther away from the rest of the flock.

This constant vigilance soon proves to be justified, since the juvenile Nanuqsaurus from before, not managing to get a meal during the hunt of their mother, are trying to satisfy their appetite. In order to be more or less concealed from the sight of the ornithomimids, the siblings take a rather long path through the forest. Not yet very experienced, they do not form a particularly great team and both move side by side, planning to simply charge at the fellow theropods as soon as they get close enough. Fortunately for them, the reticulated pattern of their feathery coats hides them rather well among the Parataxodium trunks, enabling them to approach considerably. However, one of the siblings bumps the other, which responds with a low growl and snapping motion. Enough of a disturbance for the naturally agitated ornithomimosaurs to spot them and leave in a stampede. The immature tyrannosaurids also burst in speed, but their targets, having gotten the head start, are way too far. In the end, another failed hunt.

Moving away from the so far observed area and to the littoral is a herd of Edmontosaurus, the largest herbivores of this environment, growing to some 8 meters in length, a considerable size but smaller than the more southern species E. annectens (able to achieve lengths from 11 to 12 meters), which is also bigger than the earlier, but also more southern, E. regalis. Ornithischian ornithopods, they are derived iguanodontians, hadrosaurids to be more exact. And while E. annectens lives in age-segregated herds, here the juveniles are usually intermingled with the adults, not a coincidence generated by their common destination towards the shore. More upland habitats such as this one are preferred by the likes of Pachyrhinosaurus, the hadrosaurids usually sticking closer to the coast, having only ventured this far in look for food, much harder to find with the advent of winter. In regards to eating habits, Edmontosaurus, as well as Hadrosauridae as a whole, are capable chewers, unlike some plant-eaters seen previously, such as the therizinosaurid, owner of a weak bite and restricting itself to crop and strip away leaves, instead of actively processing them, made possible by the rapidly replaced tooth batteries of the cited iguanodontians. Despite their chewing specializations, these ornithischians are having to contend with less optimal feeding items, eating the bark of trees, dry twigs, the occasional rhizomes, and, if lucky, herbaceous angiosperms still retaining some foliage. Primarily quadrupedal, but able to shift into a bipedal posture, they, also due to their considerable dimensions, can access more food sources than most ornithischian relatives. Even so, most will have to count on their fat reserves built up during more fruitful months.

Following the herd is quite an unusual reptile, an azhdarchid pterosaur, part of a family of mostly terrestrial hunters, foraging their living spaces in search of victims small enough to be eaten whole, of course also taking in carcasses if fortunate enough to find such easy edibles. Unlike other pterosaurs, the azhdarchids, specialized for spending more time on the ground, have an erect posture, while their counterparts adopt a more sprawling gait. Still capable fliers, they can cross very extensive distances on the wing, however. This individual could thus have escaped the winter if it had so chosen, but the year-round residency of most inhabitants means food is plenty for a carnivore. Besides, its endothermy and shaggy, feathery coat (traits of Pterosauria as a whole, ancestrally shared with dinosaurs through Ornithodira, first mentioned here) means that it is well equipped to deal with the cold. As the hadrosaurids follow their path, scared critters are occasionally flushed out from their hiding or resting spots and so the azhdarchid, an incredible 10 meters in wingspan, is ever vigilant, alert to the smallest movements. For now, it has not had much luck, but should that not change, it will likely soon try nourishing itself somewhere else.

A few weeks deeper into the four-month darkness, many carnivores are drawn into the beach by a pungent smell. In a forest surrounding the sandy shore a herd of Edmontosaurus is eating pieces of rotten Parataxodium logs. As they do so, some insects are consumed as well, mostly grubs hiding within their woody abodes, as are several fungi. Though sometimes accidental, this consumption provides the hadrosaurids with welcome proteins and, as they chow through the trunks, young troodontids, much smaller than the almost fully grown individual from before, take their chance to eat any of the larvae exposed by the herbivorous giants while being careful to not being stepped on, quickly scurrying between the ornithischian’s limbs and giving quite impressive jumps if so needed. In the beach itself lays a mosasaur carcass, the origin of the aforementioned odor. The fairly serpentine, four-flippered body of the marine lizard is largely unrecognizable, since its remains drifted along the coast for several days before arriving in this location. Despite being endothermic, just like all tetrapods inhabiting this region, most of these aquatic reptiles migrate to lower latitudes with the advent of darkness, not being very competent to hunt in such gloomy conditions, though some, like Phosphorosaurus (known from modern-day Japan), sporting nocturnal lifestyles, accommodate just fine during sunlight’s protracted absence.

Following the scent, the Nanuqsaurus family seen previously arrives on the scene. Many scavengers have gotten here before, but none mighty enough to pose the tyrannosaurids a challenge. As the three approach, some azhdarchids, much smaller than the one seen tracking the Edmontosaurus, launch from the dead body, stripping away as much meat as possible during their last seconds. Several Saurornitholestes also disperse and, as they move sufficiently away, start bickering between themselves for any scraps. A few long-billed birds, normally probers of the sandy soil in search of invertebrates, stay put. They, birds proper (belonging to the class Aves and part of the extant order Charadriiformes, containing species usually associated with bodies of water, from gulls to plovers, to auks) are way too small to constitute an annoyance to their way more massive coelurosaurian relatives and agile enough to escape from the jaws of the excitable juveniles, constantly trying to catch the little flying critters, frantically chirping and flapping their wings.

Their mother is having some difficulty eating, finding it very painful to do so. This results from several lesions, necrotic ulcerations developing inside her mouth and in her throat due to a protozoan infection. In more advanced stages, the mobile parasite, counting with four anterior flagella, an undulating membrane, and an arguably pear-like format, (though it has a plastic body shape, enabling it to nibble away at the host's cells, swarmed by many of the feeding microorganisms), can go on to affect bone, also generating erosive damage there. Thriving in anaerobic conditions, it curiously lacks mitochondria (possessing mitochondria-derived organelles called hydrogenosomes) and, even to this day, in the form of Thrichomonas gallinae it affects theropods, being quite widespread among some birds. Another species, Trichomonas vaginalis, is even responsible for venereal disease among humans, in which the unicellular eukaryote usually leads to recurrent illness, in part due to its great immune evasion capabilities, thanks to mechanisms like antibody degradation or exposition of proteins homologous to human ones on its cellular membrane. How she acquired such parasite was most likely through an antagonistic interaction with another adult Nanuqsaurus, possibly in a dispute for some food item or territory: face biting is quite common in tyrannosaurids and constitutes the main method of transmission for this malaise among these animals.

Eventually, the four months of darkness cede away to light once more and, as this occurs, the landscape is profoundly changed. Parataxodium conifers, in conjunction with several other trees, regain their leaves and horsetails along with ferns sprout from their underground rhizomes while several herbaceous angiosperms become lush and vibrant. Many herbivores, having become emaciated during the winter due to the lack of appropriate nutrition, gorge themselves on the new plant growth. In the sea, there is also much change, as phytoplankton blooms and brings along with it the rest of the food chain, taking full advantage of the deep nutrients brought to the surface by the cold waters. Despite the protracted period of unforgiving dark, seas like these are the most productive and it is spring that makes this fact most noticeable. 

Marine reptiles like plesiosaurians lavishly consume the abundant fish and, not getting behind the banquet, the largest of mosasaurs consume the well-fed plesiosaurians in turn. Other marine reptiles also make their presence quite known, turning the comparatively silent winter beaches into grounds of almost intolerable noise as they start nesting. The nesters are hesperornithids, a family of sea-going birds more closely related to modern birds proper than even the enantiornithines. They are flightless, counting with puny wings, but make up for it in terms of size, since the northernmost of these birds are the largest (the slightly earlier genus Canadaga, for instance, reached lengths of 2.2 meters), an example of the great food opportunities these waters provide, at least during this time of year. During the gloomy scarcity, the fat reserves they accumulate during the seasons of plenty usually prove to be more than enough to get them through. Like others of their family, these theropods possess moderately long, serpentine necks, extended beaks with tiny recurved teeth, and lobed feet. It is through their feet they propel themselves underwater, their true environment: it is where they pass the great majority of their day, going far offshore in search of fish, which they may pursue into considerable depths.

The stark contrast between the spring and the winter months serves as an illustration of the stark contrast also existent between climates and how this affects biodiversity. Until this tale, all the habitats we visited were quite warm and it is a very well established fact that, as one moves to increasingly lower latitudes, there is a striking increase in the number of species. This is in part due to the harshness of the place (shared by other high latitude areas, though not necessarily as extreme as the one of this exceedingly polar location) we are in right now: a great part of our visit was dedicated to describing the various adaptations organisms have developed to successfully occupy this environment. In lower latitudes, climatic conditions are considerably more constant along the year, being overall more amicable and this allows for a greater variety of organisms to establish themselves there. 

Apart from this, the higher temperatures are more conducive to metabolisms. As said earlier, there is an equivalence between motion and temperature: the higher the motion, the more chemical reactions can occur, since they are dependent on the collision between particles and also on the energy level of said particles (which is also increased by the higher temperatures). Consequently, there is a quite literal acceleration of life as whole and this speeds up various different processes, from development, to interespecific interactions and so on. The interespecific interactions also serve to considerably augment biodiversity: the existence of many predators, parasites, and competitors not only limits the range of lifeforms, allowing for different organisms to occupy spaces which, under other conditions, would be occupied by a single set of species, but also fosters a very dynamic myriad of selective pressures that may be opportune to the origin of novel traits and behaviors that can, over time, culminate in the formation of even more species. However, there are still many unresolved points regarding this phenomenon and there are many variables at play, but, even so, the ones mentioned here likely take on some important role in shaping what is observed.

Returning to the north pole, the rise in temperatures promotes adequate conditions for egg-laying for the majority of dinosaurs (as already could be seen in the case of the hesperornithids). Troodontids, for instance, come together to form communal nests. Starting during the end of winter, the breeding season is brief, as females pair with males in not very elaborate courtships and soon proceed to exclusively reproductive matters. Taking turns incubating the eggs, the brooding process usually is not too complicated. However, one troodontid male lost his mate, a victim of a hunting accident, and he has had to care for their offspring alone. Earlier today, he tried his luck raiding a thescelosaurid nest. Fortunately for him, the mother was nowhere to be seen and he had unrestricted acess to the helpless younglings that had been already born by the end of winter. Too large to reach the terrified, chirping chicks at the end of the burrow, it rapidly found a way to circumvent the problem by grabbing a stick close by, firmly gripping it in its mouth, allowing it to try and fish the young using it. Though the process was complicated, he could do it, slowly pulling a youngster ever closer to his mouth, from where he would be able to seize it with his toothy jaws. 

His attempt was most unceremoniously interrupted when he was suddenly thrown aside by quite a significant force. It was the mother thescelosaurid. Though much smaller than him, she was robust and unexpectedly strong. Apart from this, the out-of-the-blue blow took him off guard and greatly compromised his balance. Once again back on his legs, the troodontid stood erect, with his fanned tail almost touching the ground, and hissed loudly, opening his wings wide and making a kicking motion: his rival could be sturdy, but his sickle-shaped talon could easily take out one of her eyes. The standoff ensued for some time, as the thescelosaurid stood there, unwavering in its determination to protect its offspring, only producing a strange, deep gurgling-like sound that traveled far. For the troodontid, it was a crossroads: accessing the young was now basically impossible and his only option for food would be tackling the thescelosaurid itself. Was he hungry enough to risk such a large, but equally troublesome meal? No, not quite. And, as such, he moved away, with the thescelosaurid quickly entering the burrow, taking one last look at him from the safety of her den.

Now returning to the nest from his failed foraging trip, he quickens his pace after hearing a commotion. Arriving, he notices part of the nest has been trampled, its sediment walls undone and with no sign of the eggs once sheltered there. The perplexed father then hears a cracking noise and, rapidly bobbing its head to the origin of the sound, focalizes on a lone Pachyrhinosaurus, munching on what soon were to become his chicks. A mixture of egg yolks, whites, and blood drips from the sharp beak of the ceratopsid, as the still-unborn young theropods and egg shells are pulverized by the tooth batteries inside their predator’s mouth, usually shredders of woody plants. The parent lowers its head down and sniffs where the eggs had been, eggs he had, all alone, carefully brooded: now all gone. The other troodontids had managed to defend their part of the nest, but his had been the one closest from where the centrosaurine came, his young were the first and only victims. Plentiful other deaths come with spring as well. The defrosting of ice covers at higher elevations generates floods that constitute a bane to animals caught off guard. Huge herds of Edmontosaurus and Pachyrhinosaurus have already succumbed to such events, tens of individuals having gathered to eat and reproduce only to be buried in watery graves. The mother Nanuqsaurus, accompanied since the start of this tale, also met her fate that way, together with adult peers to try and take down the concentrated and abundant prey. As a result of her infection, she had already lost quite a bit of body mass, but the call of hunger was yet too strong.

And it is hunger that will partake in the destruction of much of Dinosauria, as, in just 3 million years, the whole planet will be immersed in a far more devastating darkness. After an asteroid impact at what is now the Yucatan Peninsula (and a smaller impact off the African west coast), a ginormous amount of debris will be ejected into the atmosphere, generating first a frigid global winter. However, the crash will likely stimulate previously ongoing volcanism in India which is to lead, later, to a period of more warming. This event, the fifth mass extinction, would disrupt food webs in severe ways, first killing off the producers and then the consumers. Surely, a stark reminder of how Earthly life, which possibly first appeared in the abyssal hydrothermal vents, has become so incredibly dependent on the Sun’s energy. As a result, several organisms were wiped out completely: non-avian dinosaurs, pterosaurs, plesiosaurians, mosasaurs, and ammonoids (some would possibly survive past the event, but die shortly after). Others, like fellow reptiles, mammals, insects, plants, and many others would be hit, but several representatives of creatures seen during this chapter would survive, like birds, leptictidans, fellow eutherians, metatherians, and multituberculates. After the cataclysm, living beings would once more rebound, bringing forth a new era: the Cenozoic.

***

1-Nanuqsaurus

2-Pachyrhinosaurus

3-Sikuomys

4-Saurornitholestes

5-Unnuakomys

6-Cimolodon

7-Troodontid

8-Hollickia

9-Parataxodium

10-Therizinosaurid

11-Thescelosaurid

12-Ornithomimids

13-Alaskacephale

14-Azhdarchid

15-Edmontosaurus

16-Cupressaceae