Middle of a Golden Age

At about 150 million years ago, this is the Late Jurassic Epoch. What will eventually be the Tanzanian Lindi Region is now a coastal location of semi-arid, seasonal climate, albeit generally wetter than more inland portions. Pangaea has broken up into a few continents since the Triassic: Gondwana lies to the south, North America stands close to the European Archipelago in the north, and, sharing the boreal hemisphere with the latter both, is a conglomerate that, in the future, will turn into part of Asia. Recently, temperatures have entered a cooling trend, which will continue into the Early Cretaceous, despite the fact that, in general, Earth is quite warm, in a greenhouse state, just like in the prior period. 

On one of Gondwana's eastern shores, this tropical to subtropical area is home to many different creatures, especially dinosaurs, which have diversified tremendously following our last stop. On a mudflat bordering the coast, some examples of this diversity can already be spotted: Elaphrosaurus. These curious theropods are members of a group known as Ceratosauroidea, containing formidable carnivores that contrast greatly with these gracious and elegant animals, part of the more limited family Noasauridae. Still with their teeth, they are a flock of excitable, jumpy juveniles, banding together for increased security and, sometimes, just for plain company. As adults, Elaphrosaurus have only beaks and switch from an omnivorous to an herbivorous diet. For now, though, meaty snacks can be enjoyed and this is why the group has gathered here, on these muddy expanses. They are on the lookout for worms, probing the sludge with their snouts in hopes of catching slimy victims. Soon enough, they find some. First some ribbon worms, usually little invertebrates bearing impressive proboscises utilized to catch prey, some being venomous (the aforementioned proboscises containing a barb used to inject toxins into their victims) and others being simply poisonous, the toxins being used solely for defensive purposes against predators like these Elaphrosaurus. Then some polychaetes (bristled annelid worms) of the family Eunicidae, marked by complex mouthparts used for an array of dietary habits. Foraging will continue until the tide rises, submerging the majority of this mucky field. 

In the coastal waters proper, multiple organisms live among reefs and others actively create them. Corals have, since the Triassic, re-emerged in the form of scleractinians (still extant, constituting the stony corals), initially non-reef building, being descended not from their rugose and tabulate counterparts but from anemone-like ancestors. Though suffering significantly from the Late Triassic extinctions, they have rebounded and most associate with algae (something seen and better discussed in our expedition to the Devonian), a symbiosis that possibly extends to the previous period in an example of coevolution. Mollusks like bivalves and gastropods are present, as are more polychaetes: the serpulids, sedentary filter-feeding worms that construct and live within calcareous tubes, which, when densely packed, can form reef-like structures on their own, but this is not the case here, where they are not so commonly found. Crustaceans are also represented in the form of the familiar crabs and ostracods, minuscule, very widespread double-shelled creatures found in all manners of aquatic habitats, even some on the verge of terrestrial (a few have peculiar, giant sperm cells that may be longer than themselves). Besides the invertebrates, hybodonts, shark-like cartilaginous fishes (an example of which was seen in our voyage to the Early Carboniferous), patrol these waters, acting as larger-sized predators.

Farther from the littoral, a mature female Elaphrosaurus, about 6 meters long, can be seen. Right now, she is inspecting the nests made by several males, which congregate around specific localities in an attempt to woo visiting females with their building skills, an example of lekking behavior. As she goes through, the eager-to-be fathers make elaborate dances, stomping their feet, raising their necks high, splaying their small arms repeatedly and even showing off their cloacas, heavily pigmented and equipped with scent glands. Very picky, the female takes a long time examining each pretender and their construction sites, a circular depression containing leaf litter and, occasionally, other ornaments, like shells (acquired in treks to the beach). After touring a bit more, she still has not set on any mate and walks into more open space, with fewer trees. 

From there, the view is almost panoramic. Close to her, a Giraffatitan is consuming the needles of a Cheirolepidiaceae conifer, well adapted, like other members of its family, to drier conditions as well as higher salinity, making them very "at home" in this coastal and climatic setting. The animal, a sauropod (member of the larger Sauropodomorpha), is simply huge, with a height of some 13 meters and a length of 25 meters, dimensions even more astounding when compared to earlier forms like Saturnalia. An active reptile like its ancestors, it needs to consume a large amount of food every day and, as such, is constantly on the move. In the distance, the rest of this individual’s herd (having stripped much of the nearby forest of the more easily accessible foliage) is already venturing into new grounds, even though this one has decided to prolong its visit a little longer. Swarming close to it are many scorpionflies. These insects of the Mecoptera order are specialized gymnosperm pollinators and count with siphonate proboscises utilized to ingest the plants' ovulate secretions, promoting their fertilization along the way, with some Cheirolepidiaceae, such as this one, having specialized cones that aid in such process. Nearby to the Giraffatitan too is a small insectivorous mammal called Brancatherulum, attracted to the top of the tree by the large concentration of the cited arthropods in hopes of catching a few. It is thus more closely related to modern marsupials and placentals (the two form the grouping Theria) than any other synapsid seen so far, even though the extent of such relation is up for debate, being either a basal dryolestidan (more distant from therians) or a basal zatherian (a grouping that includes therians, thus making it closer to them). 

Dwelling into mammalian phylogeny, two main groupings are still alive: the monotremes (basal, egg-laying forms) and the aforementioned therians (marsupials and placentals), which, despite both having placentae, differ in how exactly the structure is used during gestation, with marsupials having a shorter pregnancy and placentals a longer one. It is probable, though not necessarily true, that the common ancestor of all Mammalia was oviparous, with the adoption of viviparity occurring later on. In this regard, it is worth mentioning that, apparently, the placenta, in a more primordial form, was acquired around the time of this tale through a retrovirus (RNA viruses that integrate themselves into their host’s genome, HIV being one such virus), which provided a syncytin gene, necessary to encode proteins responsible for the formation of the organ. Subsequently, many independent retroviral infections involved with the capture of syncytin genes took place in eutherian lineages, replacing the ancestral gene and giving the placentae of each species particular characteristics. Intriguingly, the lizards of the genus Mabuya (evolved roughly 25 million years ago, during the Cenozoic), animals that also have a placenta, show signs of a past retroviral infection associated with a syncytin gene too. Overall, this constitutes a great example of the intense and significant interchange happening between cellular and acellular lifeforms since the latter's formation (a topic discussed in more detail here).

Going back to the dinosaur, it, in contrast to other sauropods, processes plant material to some degree instead of simply swallowing it. However, in consonance with other sauropods, the Giraffatitan, to deal with such food, counts with an expressive gut and increased retention time, allowing for somewhat more efficient digestion coupled with a very important microbiota. Accompanying the long-necked giants are the small Dysalotosaurus, basal iguanodontian ornithopods (part of the more inclusive Ornithischia) 2.5 meters from snout to tail. Partially covered by fuzz-like feathers, they stick to the behemoths for a couple of reasons. First, because of safety. Few if any predators dare to approach adult Giraffatitan, so, if they can help avoid being trampled, it is very advantageous to stay by their side. Second, there are more food opportunities, since the large saurischians, when browsing, knock down several leaves and other meals the iguanodontians normally do not have access to. 

Additionally, the tiny ornithischians sporadically consume the dung of their “protectors”, harnessing possible nutrients that might otherwise go to the decomposers. One of them, currently resting in a sitting position, is, at 18 years old, an elder and suffers, on one of its vertebrae, from Paget's disease of bone, a condition characterized by an "unbalance" in the reabsorption and growth of bony tissue, carried forth by osteoclasts (multinucleated cells responsible, among other duties, for degrading the bony matrix) and osteoblasts (cells responsible for the secretion of the bony matrix, divided into both organic and inorganic, mineralized, portions) respectively. In the aftermath, bones become disorganized, deformed, enlarged, and weak, the causes of such disorder being likely associated with genetic and environmental factors. 

Anyhow, despite the phylogenetic differences, the Dysalotosaurus are rather similar to the Elaphrosaurus in terms of behavior, both being gracile and speedy creatures, while also being closer to the bottom than to the top of the food chain. Apart from this, the two share similarities in ontogeny, since the Dysalotosaurus transition from omnivorous when young to herbivorous when mature. This may not only be an ecological shift but one case in a larger evolutionary pattern accountable for the development of full herbivory further within Iguanodontia, since the skulls of younger Dysalotosaurus individuals resemble the ones of more "primitive" ornithopods while the adults' are closer to that of more derived iguanodontians. 

Speaking of the food chain, the Veterupristisaurus occupies the top position. At some 8 meters long (but possibly acquiring more substantial dimensions), it is a carnivorous theropod, an allosauroid and, more specifically, a carcharodontosaurid, one of the oldest members of this family as a matter of fact. A family that, in the Cretaceous, will be widespread until later in the period, promoting a radiation of apex predators. This male, in the present moment, is not quite a good symbol of these dinosaurs’ ferocity, content to rest and scratch an itchy spot on his head. Also pleased with the meat-eater’s current disposition is a trio of Kentrosaurus, armored herbivores. They are stegosaurids, members of the more extensive Thyreophora, an ornithischian clade that includes an incredible array of other toughened reptiles, sporting spikes, plates, and clubs. Extending to 4 meters, Kentrosaurus are fairly small, even in comparison with other members of their family. Regardless of their size, they are not to be messed with. Displaying sufficiently flexible necks and tails, these reptiles can, swinging their back ends, strike enemies with reasonable accuracy, their long hindquarters spikes capable of dealing lots of damage and possibly even killing particularly unfortunate assailants. The front plates provide some extra protection and are, equivalently to the back spikes, utilized as display structures, with different applications, such as intimidation and sexual selection. 

Besides all of this, these herbivores are, similarly to the scorpionflies, important for plants by acting as seed dispersers, allowing the autotrophs to spread far and wide, colonizing new lands they, on their own, might be unable to. The Taxaceae conifers are one of the beneficiaries of this relationship, producing seeds covered by sweet, colorful, and fleshy envelopes (known as arils and probably derived from very swollen leaves) which the Kentrosaurus access by rearing on their hind legs. However, such treats are not completely what they seem. Like the rest of the plants' parts (except the just cited arils), the seeds are coated with poisonous alkaloids: "offerings" harmless to some, but deadly to others, those that do not aid in the offspring's diaspora. Since the reproductive structures go unscathed in the stegosaurids' stomachs, they are not affected by the hiding toxins and so can eat as many arils as they wish without concern. 

Opposite the thyreophorans, another theropod, albeit quite distinct from the Veterupristisaurus, is situated. Wading in a lake, it is an Ostafrikasaurus, a saurischian thought to be an early spinosaurid, a family that, like the carcharodontosaurids, will undergo significant diversification in the next period with the differential of many of its representatives becoming increasingly tied to aquatic environments. With a somewhat long snout and arms adorned by big claws, this reptile of 8.4 meters in length already displays some of the iconic traits of its group and, right now, is attentively looking for some fish, which comprise a reasonable part of its diet. It analyses the water calmly and waits for the perfect moment to strike. With meticulous precision, it catches a Lepidotes, an abundant and slow-swimming ray-finned fish (a type of bony fish) that feeds in the following manner: first, it grasps its prey (invertebrates either with soft shells or lacking them altogether), then opens its mouth in a tube-like configuration to suck them in and, finally, the unfortunate critters are crushed by the predator's large teeth.

Circling the lake are three rhamphorhynchid pterosaurs. Modest-sized fliers with a wingspan of around 1.8 meters, they, though being nocturnal, have already become active as the Sun sets. With sets of needle-like, conical teeth, these are piscivores and, rather than catching fish on the wing, they actually prefer to float on the water’s surface, paddling and diving from time to time to capture their victims, though this, in more sizeable bodies of water, sometimes results in they themselves becoming the victims of hunters lurking beneath the surface. Interestingly, hatchlings are able to fly not long after emerging from eggs and, while being provided parental care for some time, take on a very different lifestyle from the adults after starting to live independently, preying mainly upon invertebrates and being more maneuverable than grown-ups yet incapable of flying as fast as they do. Regardless, at present, these rhamphorhynchids are just waiting for the Ostafrikasaurus to go somewhere else, allowing them to land safely and not get snatched by the possibly still-hungry theropod. Flying above them there are other, more common pterosaurs: Tendaguripterus. These are quite small, with their wings stretching to roughly 1 meter. Constituting an example of sexual dimorphism, the males of this genus have larger, more circular crests, whilst the ones of females are smaller and straighter. 

There is one more genus of sauropod in the plains: the 15-meter-long Dicraeosaurus. With a horizontal posture, a much shorter neck, and a significantly lower stature, they diverge a lot from the Giraffatitan, specializing in consuming vegetation closer to or at ground level, being mid-height browsers. This divergence can be observed also in their classification, since Dicraeosaurus are diplodocoids, which are slenderer organisms, commonly with long, whip-like tails, and Giraffatitan are macronarians, which are more vertical, with the predominant clade of sauropods during the Cretaceous, the titanosaurs, emerging from this group. Due to the size difference, Dicraeosaurus are more vulnerable to predation than their larger and taller cousins, forming herds as well and using their tails to defend themselves, despite these not having the same destructive capabilities as the ones of Kentrosaurus (not even being able to break skin, but, even so, still effective in warding off attackers). 

Both also have in common, as do all sauropods, a pneumatic postcranial skeleton and air sacs. The latter structures, perhaps ancestral to archosaurs but lost in representatives like ornithischians (as mentioned in the previous chapter), allow for more efficient respiration, generate cooling mechanisms, and make the animals lighter, facilitating the achievement of various feats, such as growing to gigantic proportions. Moreover, the two communicate mainly through low-frequency sounds, able to travel ample distances and being reverberated via their large, fleshy noses. In regards to reproductive strategies, Dicraeosaurus engage in minimal parental care. In compensation, younglings grow quite fast, minimizing the stretch of time they are in higher danger due to smaller sizes. Like the adults, the youth form groups, normally inhabiting woodlands (like macronarians), while older individuals are found in more open environments such as the one we are observing now. 

Other diplodocoids are Tornieria, significantly longer than their low-stature relatives, but with similar, lacking parental care and feeding preferences. They are currently moving, like the Giraffatitan, to greener pastures but going in the opposite direction, a clear reflection of their contrasting dietary habits. Gastroliths, pebbles consumed by both the Dicraeosaurus and Tornieria and usually used for food processing in other beings, are, in these dinosaurs, a way to uptake lacking minerals or, sometimes, just a product of accidental ingestion when eating plants near the soil, explaining their rarity among Sauropoda as a whole. Contrarily, Elaphrosaurus, for instance, employ gastroliths specifically to help grind vegetable matter in their gizzards, especially in adulthood due to the aforementioned shift in diet.

The contrasting preferences between age groups, as seen in the rhamphorhynchids as well, minimize competition between individuals of the same genus or species and, thus, may prove to be quite favorable. A few of the creatures that have taken this to a whole other level are holometabolan insects, which undergo a radical metamorphosis, from larva to adult and, tracing their origins to the Carboniferous (when they may have arisen via the suppression of embryonic development programs that eventually culminated in the formation of a larval form), are absurdly diverse, including many previously mentioned groups, such as the scorpionflies, hymenopterans (like bees, ants, and wasps), and beetles, though also extending to lepidopterans (butterflies and moths), dipterans (flies and mosquitoes), and more. 

Finally returning to the female Elaphrosaurus, she decides to resume her mate inspection. After some minutes of contemplation, a male in particular catches her attention. Displaying a colorful throat pouch, an equally remarkable cloaca accompanied by a rather pungent smell, an elaborate dance, and a “state-of-the-art” nest, he is the chosen one. As he finishes his presentation, she leans closer, and both nuzzle, signaling the start of an ephemeral pair bond. Soon, they get to business. The father will now go on to impress others of the opposite sex, while the female will build a nest of her own and lay her eggs alone, though the displaying males nearby offer some protection by catching the attention of most predators. Either way, she will be the one to care for the offspring and will do so with incredible dedication, defending her chicks fiercely (with uninterrupted pecks and powerful kicks), even against much larger foes or those as so perceived until they disperse once old enough. 

On the horizon, a thunderstorm is approaching. The rainy season is appreciated in this locality, especially when considering the long and unforgiving dry season. However, it is not without its fatalities, caused, in part, by lightning. With static electricity building up between cloud bottoms and the Earth's surface (it also occurs inside clouds and, consequently, so thus lightning), a massive potential difference and a strong electric field arise. Consequently, the surrounding air ionizes, meaning the electrons are separated from the atoms’ nuclei. While this happens, a current is established in the air, with negative charges usually moving to the ground (positively charged). These are called leaders and exhibit a branching pattern, progressively getting lower and lower, attracting positive charges. Eventually, the leaders get low enough to the point that more air ionizes and currents of positive charges emanating from the ground (or from objects on the ground) establish themselves, these being called streamers. When both connect, the most luminous part of the lightning takes place, as electrons (the negative charges are the main constituents of the current) proceed rapidly downwards. 

This whole process releases a fair amount of electromagnetic radiation originating from various sources within the lightning: charged particles emit photons if accelerated (this is also the reason matter above 0K, known as absolute zero, is constantly releasing radiation, since the atoms are continuously under acceleration, be it slowing down, going faster or changing direction) and electrons release photons if excited (they do so when receiving determined amounts of energy, either through collisions or via photon absorption, which make them occupy new arrangements in the atom and then go back to their original configuration, emitting photons). Part of the released radiation heats up significantly the adjacent air, making it quickly expand and then contract as it cools down, promoting vibrations in the air column that result in the familiar event known as thunder. 

As valuable as the understanding of such an interesting and complicated phenomenon may be, it is of no help to the Giraffatitan from earlier, which lagging behind his group, was caught right in the storm, being zapped to oblivion in the middle of the night. The smell of charred meat does not take long to attract carnivores. Several Veterupristisaurus arrive and immediately dive their heads into the body of the sauropod. They, though, are not keen on sharing, hissing, growling, and exchanging bites, these delivered mostly on the head. Subadults and juveniles, enveloped to some degree in a downy layer absent in mature individuals, are at the most disadvantage, sticking to the periphery of the carcass. Any that attempt to interrupt the adults in order to secure some niblets are lucky to come out only with superficial bruises and scratches. The unlucky ones get turned into part of the meal themselves. A bold Ostafrikasaurus, after getting on the scene, tries clearing its way to the corpse via intimidating, booming calls and quick movements of its clawed hands. It is to no avail however as, after getting mobbed by two or more Veterupristisaurus, the theropod decides it is better to leave alive even if that means going away with an empty belly. And so, the warzone-like feast progresses into the gloom as the rain continues to pour. 

Here, we finish our fifth tale. In the Cretaceous, the current golden age will continue, with dinosaurs still diverse and abundant. However, other organisms will not be left out of these developments in a truly flourishing world, where our next stop will take us for further and more detailed examination.

***

1-Giraffatitan

2-Scorpionflies

3-Brancatherulum

4-Cheirolepidiaceae

5-Lepidotes

6-Ostafrikasaurus

7-Rhamphorhynchids

8-Dysalotosaurus

9-Tendaguripterus

10-Tornieria

11-Dicraeosaurus

12-Veterupristisaurus

13-Elaphrosaurus

14-Kentrosaurus

15-Podocarpaceae

16-Taxaceae

17-Ginkgophytes

18-Ferns

19-Cycads

20-Cupressaceae

21-Araucariaceae