This alien planet is brought to you by
Campfire Technology. If you’re a fan of the Alien Biospheres series and looking to make
your own alien planet, or any other kind of worldbuilding or writing project, Campfire Blaze
can be a big help in keeping your work organized. Blaze comes with pages to help you chart out the
maps and specific locations within your world, while the species page lets you delineate your
world’s native clades of flora and fauna, as well as individual facts, statistics, and miscellaneous
details for every individual species. There are also tools for fleshing out the human aspects of
your world, like cultures, politics, languages, religions, and magic, along with pages for
narrative elements like characters, relationships, timelines, and story arcs. Blaze’s free version
already includes most of these features, but you can also individually purchase new modules to
let you catalogue your world in even more detail. To sign up for Blaze or to learn more,
check out the link in the description. Over the last few episodes, we’ve explored
each of the major biomes of our alien planet and seen how their respective
ranges of temperature, precipitation, and resource availability have shaped the
evolution of their resident species. But in the enormous spans of time over which evolution
occurs, conditions rarely remain stable for long. As the eastern and western tectonic plates
gradually drift away from each other, the supercontinent that has existed since life
first moved onto dry land will finally break apart into two separate landmasses. This will have
several immediate effects on the climate. What was once the arid continental interior will now form
the new coastlines, and the proximity to the sea and the resultant increase in precipitation will
turn the plains and deserts into tracts of forest. Meanwhile, the mountain ranges along the exterior
coasts will continue to rise, which will lower atmospheric carbon dioxide levels through silicate
weathering, and the snow and ice that accumulates on their slopes will increase the planet’s albedo,
leading to a gradual decrease in average global temperatures. And if these mountains, particularly
those in the northern plateau, see the formation of ice caps and glaciers, then for the first time
in the history of terrestrial life, the planet will have entered an ice age. Up until now, the
planet has been going through a greenhouse period, in which there were no polar ice caps or
continental ice sheets, but as the climate cools, a cycle of glacial periods may begin, wherein
for intervals spanning roughly 100,000 years, the global temperature will drop by several
degrees, sea levels will fall by as much as 100 meters, and ice sheets will advance from
the poles and intrude into the temperate zones. Due to subtle changes in the planet’s orbit,
these glacial periods will be punctuated by relatively warm interglacial periods, each of
which may last several tens of thousands of years. These drastic shifts in climate will have
numerous ramifications for many ecosystems across the planet, but the breakup of the
continents has another important consequence: with the exception of particularly mobile species,
the respective inhabitants of the two continents are now cut off from each other, and thus
evolutionary developments that occur on one continent won’t affect the other, meaning they’ll
have entirely separate evolutionary trajectories. Such a situation is called allopatry, and occurs
whenever a population becomes separated by a physical or geographical barrier, whether
it be a mountain range, a body of water, or anything else that might prevent the two
halves of the population from interbreeding. Since gene flow can no longer occur, a frequent
consequence of allopatry is that the separated subpopulations evolve into different species,
which on a global scale will result in the clades and ecosystems on the eastern and western
continents gradually diverging from each other. In light of this, for the first time in this series,
we’re going to need to keep track of biogeography, or the distribution and range of each of our
clades, particularly whether they exist on the eastern or western continent, which will be
essential for determining how ecosystems form. To begin with, the overall landscape of the
eastern continent will remain much the same as it was before the continents split apart, since
it will remain largely within the tropics as it drifts eastward, and even during an ice age the
equator will still experience high temperatures. While stretches of tropical forest will occur
along the coast, the inland areas will stay comparatively sparse, especially within
the rain shadow of the eastern mountains. During the glacial periods, the forests
may diminish with the decrease in rainfall, mainly surviving as patches of fragmented gallery
forest separated by stretches of open plains. Environments like these allow plains-dwelling
animals to live alongside forest-dwelling ones, a perfect habitat for large browsers like the
procerapods, as they’ll be able to continuously feed on the tree tops while still having enough
space to move freely between areas of vegetation. When it comes to browsing herbivores, the larger
the animal is, the better its ability to reach for food, and the more food the animal can
obtain, the larger it can afford to grow, and in especially lush areas, like the periphery
of the equatorial forests, this feedback loop may result in these already massive procerapods
growing even bigger. To help them reach upward, their legs and pedipalps may elongate, and their
foreguts may expand to contain and process the enormous volumes of nutrient-poor foliage
they feed on throughout the day. To support such a huge cephalothorax, they may develop
stronger muscles and hardened tendons in their back, which will attach to enlarged projections
along the dorsal surface of their limb girdles, functioning much like the neural spines of many
large animals on earth. Being big also acts as a deterrent to predators, and as the largest animals
on the eastern continent, these procerapods will be virtually immune to attack once fully grown.
We’ll call these gigantic beasts stylopods. Scurrying among the feet of these giants will
be a menagerie of smaller herbivores like the leptopods. Most leptopods are adapted to live in
open habitats, but in this patchwork of plains and fragmented forest, some clades may evolve to
occasionally wander among the stands of trees in search of food. These clades may benefit from
more generalist diets than most other leptopods, feeding not only on brachyphytes but also mixed
caulophyte species, necrophytes, and low tree branches. The trees will also provide a degree
of shelter from large steppe-dwelling predators, though they can still rely on their speed
to avoid attack when out in the open, but many leptopods and other herbivores may
settle on another common form of defense. We’ve brought up the concept of competition many
times in this series, but whenever we’ve mentioned it before, we’ve been referring to competition
between different species, or interspecific competition. However, intraspecific competition,
or competition between members of the same species is an equally important factor in evolution. The
fewer resources are present in a species’ habitat, the more intense the intraspecific
competition will be to gather those resources. In environments where intraspecific competition is
especially high, a species may evolve territorial behaviours, aggressively repelling other
individuals from their home range to keep as much of the local resources to themselves as possible.
On the other hand, though, the presence of other individuals does come with some benefits: the more
animals are around, the easier it will be for them to collectively keep an eye out for danger,
and the less vulnerable each of them will be to attack, since predators will be forced to pick
out and go after only one target. Because of this, many species exhibit collective behaviours whereby
individuals instinctively congregate in groups, which, depending on the clade and group structure,
can be described as herding, flocking, shoaling, or various other terms. Along the periphery of the
coastal gallery forests of the eastern continent, food will be abundant enough to keep
intraspecific competition to a minimum, and so the resident leptopods can afford to
evolve collective behaviours for greater safety, forming herds comprising anywhere from several
dozen to over a hundred members, giving these leptopods, which we’ll call camptopods, a further
degree of protection from the many predators they share their habitat with. The largest predators
on the eastern continent will be the onychodonts, most of which will be steppe-dwelling
hypercarnivores preying on a variety of big game. However, along the edges of the coastal forests,
many of the local titanopods like the stylopods may be too large and formidable to risk
attacking, so many onychodont species may specialize for hunting smaller, faster herbivores
like the camptopods and other leptopods. To catch such nimble prey, they may adapt to
improve their already impressive weaponry. The onychdonts’ greatest asset lies in their
centaurism, which lets them run faster and more efficiently than other predators and frees
up their forelimbs to use for manipulation. As they play an increasingly prominent
role in catching prey, these limbs may evolve into specialized raptorial appendages,
developing pointed serrations along their undersides to close over the prey like a vice
and minimizing the chance of it struggling free. As these limbs take over the role of securing
prey, the pedipalps may reduce into a pair of retractable fangs that can be jabbed into the
preys’ hide to kill it quickly and efficiently. These eastern onychodonts,
which we’ll call cryptodonts, will be adept pursuit hunters in open areas,
but in this landscape, the trees will be plentiful enough to provide cover for ambush
predators like the deinognathans. The rivers and watering holes may be ideal habitats for the
semi-aquatic trachygnathans and dolichognathans, and the fully terrestrial ancestral forms
may thrive along the forest periphery. Going after plains-dwelling herbivores may bring
them into competition with the onychodonts, and thus the two clades may need to undergo
a degree of niche partitioning if they’re to coexist in the same habitats. The deinognathans’
signature mandibles are so large and robust that they can’t be used to chew or process food, this
job instead being relegated to the pedipalps, which break up the food and pass it into the
throat, the exact opposite condition to that of the onychodonts. Whereas the onychodonts
use their pedipalps to inflict precise, puncturing bites to kill their prey quickly,
the deinognathans rely on their tremendously powerful bite force to crush their prey, which may
serve as a preadaptation for new modes of feeding. Durophagy is a feeding behaviour characterized by
specializations for cracking open bones, shells, or other hard structures, for which having a
high bite strength is a fundamental prerequisite. As such, while the onychodonts may only be able to
feed on the softer, fleshier parts of their prey, some species of deinognathans may adapt feed on
the bones and other carrion they leave behind. To most other carnivores, these parts of the
carcass would be inedible, and thus these deinognathans will face very little competition
as they evolve into durophagous scavengers. As briefly mentioned in part 6, scavenging
doesn’t yield as much food as active predation, so these bone-eating deinognathans will likely be
smaller than their macropredatory cousins and will still actively hunt prey when carrion is scarce.
Durophages often have blunt, rounded teeth, an optimal shape for breaking
through hard materials like bone, along with extra chambers within their digestive
tract and especially powerful stomach acid. Among these deinognathans, these adaptations
may also let them crunch through the shells of placostracans and other diplostomes, or
even to subsist on tough woody vegetation. On account of having such powerful, crushing
jaws, let’s call these animals malleognathans. On the far side of the continent lies the coastal
rainforest that we covered in the last episode, which fortunately, will be largely
unaffected by the onset of the ice age, as it will still receive plenty of rainfall
thanks to the warm ocean currents along the coast. It is, however, blocked off from the
rest of the continent by the mountains, meaning many of the local rainforest species will
exist nowhere else in the world. But a select few clades will still be able to pass over the
mountains, most notably the flying clades like the opisthopterans and pleuropterans, and since these
clades have co-evolved with and help disperse the chromatophytes, they’ll carry these plants
with them as they expand their range westward, effectively bringing their own food source into
their new habitats. The earliest pleuropterans will be small, generalist frugivores, but the
forms that spread over the mountains and colonise the rest of the continent may grow to larger sizes
and adapt for long-distance flight. The same rules of wing design that applied to the opisthopterans
will also apply to the pleuropterans; the bigger they get, the larger their wings will need to be
in proportion to their body to keep them airborne. Many of these forms may retain a similar
diet to the ancestral pleuropterans, feeding mainly on fruit and other mixed plant
matter as well as the occasional malacoform. This radiation, which we’ll call platypterans,
will comprise the majority of pleuropteran diversity outside the eastern rainforest. But
some species may spread into habitats where the chromatophytes, their principle source of
food, may not be able to grow in any abundance, forcing them to adapt to new food sources.
We mentioned last time how the pleuropterans and opisthopterans would avoid competition with
each other by specializing for different diets. However, while the opisthopterans only
have their elastic spines and simple struts extending into the wing membrane, the
pleuropterans have a fully-developed skeleton, which lends them enough internal support to
grow much larger than the opisthopterans can. This means that once the pleuropterans reach a
certain size, they won’t face any competition if they exploit carnivorous niches, which may result
in some of the more generalist clades evolving predatory behaviours. Their wings may become more
pointed to give them bursts of speed when hunting, and their teeth and pedipalps may sharpen
to snatch up and butcher their kills. These forms, which we’ll call theropterans, will
feed on prey too big for any of the opisthopterans to catch, such as large diplostomes, small
osteopods, or even other pleuropterans. Many of the pleuropterans may also manage to
fly across the sea and find habitats on the western continent as well, where they’ll find
a very different landscape to that of the east. The dry ice age climate will turn the majority
of the continent into deserts and arid steppe, with the newly created coastline being the
only area to support any sizable forests. In the inland regions, the retreating woodlands
will give way to plains of tylophytes, pagophytes, and any other brachyphytes that
can manage to withstand the dry conditions, with the drought-resistant nodophytes forming
isolated pockets of forest around lasting sources of freshwater. Such a huge change in
climate and vegetation and the oscillations in temperature between glacial cycles will radically
restructure many ecosystems across the continent. The most vulnerable animals will be ectotherms
like the ancestral platydonts, dromaeopods, and deinognathans, as they’re unable to generate
their own heat to combat the effects of the cold, and so many species may die off outside the
tropics. Even megafauna like the titanopods may be forced into decline, as even though
they’re able to maintain a more stable internal temperature than other ectotherms thanks to their
gigantothermy, this still may not be enough to let them contend with the constant oppressive
cold and the lack of food in the inland deserts. Only the pachypods may have any notable presence
in the temperate zone, as they’re better equipped to deal with the cold than other titanopods due
to their thick layers of fat and stocky build, but even they may struggle to survive beyond a
certain latitude. On the other hand, endotherms, like the eurycheirids, leptopods, and onychodonts,
will have a huge advantage in the new climate, and begin taking over the vacant niches of
the ectotherms in temperate areas. The bulky, cold-adapted hadrodonts may remain the dominant
superpredators in the northern regions, primarily hunting large, slow prey like the
pachypods, but other varieties of onychodonts, especially those that are more gracile and
lightly-built, may have the advantage in warmer areas. The cryptodonts would likely
thrive in these regions, but since they only exist on the eastern continent, a separate clade
may evolve to exploit similar niches in the west, though they may do so in a completely different
way. While the cryptodonts, along with most other onychodonts, have vertically-aligned pedipalps
to bite down through their prey’s hide, in this western clade, the pedipalps may instead
evolve to oppose each other and close laterally, acting like a pincer to securely grab prey
while simultaneously inflicting lethal wounds. This means that unlike their eastern relatives,
these forms won’t need to use their forelimbs in restraining prey, so these limbs may shrink
to allow the animal to support larger, stronger pedipalps. Let’s call these forms ensidonts. But
the animals that are best-suited for the ice age will be the thylacopods and thecopods, both of
which have specialized integument to complement their endothermy, and with such efficient
thermoregulation, they’ll thrive as the climate cools and other clades are driven from the
temperate zone. But as they proliferate and spread across the continent, they’ll need to face new
challenges: while the tundra where the thylacopods first evolved was too cold and desolate
for any predators other than the thecopods, as they move further south they may come into
contact with much larger predators like the hadrodonts and ensidonts, which may incentivize
them to invest in some defensive adaptations. Since the majority of the western continent
consists of open landscape, they can’t rely on the cover of vegetation like their camptopod cousins,
so their speed and stamina will be their primary means of evading attack, and in the arms race
against the swift onychodonts, they may adapt to become even faster. The eight-legged gait of most
osteopods provides a stable base, but if an animal is going to invest in speed, any superfluous legs
may get in the way and present a waste of energy. Long ago, centaurism evolved in the ancestors of
the onychodonts, granting them increased speed and coordination, and now, perhaps these thylacopods
may convergently undergo the same development for a similar reason. Even among tetrapods on earth,
centaurism has independently evolved many times, so it’s likely to be widespread on this
alien planet as well. With only six legs, they’ll be able to run more efficiently, and
their newly freed-up forelimbs can now be used for manipulation and gathering food along
with the pedipalps. With this development, along with their unrivalled ability to tolerate
the cold, this clade of thylacopods, which we’ll give the name allobrachids, may become some of
the most successful megafauna within the temperate latitudes. Where there’s a relative abundance of
food, like the nodophyte forests along the north eastern coast, some species may exploit the
niches left vacant by the titanopods, growing considerably larger and using their forelimbs and
lengthened cephalothorax to reach for foliage. Their size will not only increase their feeding
envelope and discourage predators, but will also increase the amount of body heat they generate,
making them even more resistant to the cold. These animals, which we’ll call teleobrachids,
will be the largest of all leptopods, browsing on nodophytes and other trees within the
northern forests. Other allobrachids may retain more generalist grazing behaviours, taking over
the niches of some of the basal leptopods. Like their camptopod relatives in the east, species
living in areas with enough food to support a large number of animals may evolve herding
behaviours as a further defense against predators, but the structure of and interactions within these
herds may be influenced by the thylacopods’ unique physiology. Back in part 5, we introduced the
dichotomy between R-selection, maximizing the quantity of offspring with minimal parental
investment, and K-selection, producing smaller numbers of offspring but investing more heavily in
them to increase their chances of survival. Where a species falls on the spectrum between R and K
selection is a key aspect of its life history, or the pattern of survival and reproduction
throughout its lifecycle, which will be influenced by a variety of factors. Typically, clades
will transition from R-selection to K-selection as they evolve larger body sizes and as their
populations reach carrying capacity, so by this point in history, most species of megafaunal
osteopods will likely tend towards k-selection. The thylacopods will be more k-selected than most,
since the female protects her young from the cold while they’re within the ootheca by cradling them
with her gonopods, which gives them a greater chance of surviving through their development,
at the expense of encumbering the mother. Note, however, that only the female bears this burden,
which ties into another aspect of life history. One of the only universally applicable differences
between males and females across all clades is that, by definition, female gametes are larger and
require more energy to produce than male gametes, and therefore, males usually have
a higher fecundity than females. This is especially true in species with internal
fertilization, since once a female is fertilized, she’ll be unable to mate again while
she carries the developing young, further limiting the potential number of
offspring she can have during her lifetime. This results in what’s known as Bateman’s
principle, the concept that reproduction is a greater investment for females than for
males, and therefore, while males are usually incentivized to mate with as many females as
possible to maximize their reproductive output, females need to make sure that every attempt
at reproduction will be worth the investment. This means that females will often only choose to
mate with males that display an adequate degree of fitness, and who will therefore pass on their
fitness to the offspring, meaning the males will need to compete with each other for the right to
mate, another form of intraspecific competition. However, in the osteopods, there’s the
confounding factor of sequential hermaphoriditism. The earliest ancestors of the osteopods were
capable of changing sex during their lifecycle, which many modern clades may retain, but exactly
how their hermaphroditism operates will vary based on their life history strategy. Sequential
hermaphrodites can be either protandrous, capable of changing from male to female, or
protogynous, changing from female to male. However, this change in sex will normally only
occur if it presents a potential increase the individual’s reproductive success. For example,
in a population with a biased sex ratio, switching to the less common sex would present
more opportunities for reproduction, and therefore a high degree of competition between males for
access to females will favour protandry. Keeping all of this in mind, among the thylacopods, the
burden of carrying the ootheca while the young develop increases the cost of reproduction
for the female, which will incentivize her to be more selective when it comes to mate choice,
which will increase competition among males, and ultimately lead to protandry being the dominant
form of hermaphroditism among these species. The difference in reproductive priorities between
the sexes may also lead to differences in their physiology as well. Sexual selection occurs when
traits evolve based on their role in competing for and acquiring mates, often arising from a
preference for the trait by the opposite sex. Sexual selection frequently operates differently
between the two sexes, sometimes resulting in sexual dimorphism, wherein males and females of a
species exhibit different characteristics beyond their reproductive system. The thylacopods already
exhibit sexual dimorphism in the shape of their gonopods, as only the female has the distinct,
pouch-like gonopods to help her carry the ootheca, while the males have more typical gonopods
to help them fertilize the females, but sexual selection may end up driving further
dimorphism. One very common dimorphic trait is body size, which is also highly correlated with
mate competition; typically, the more intense the competition between males, the larger the
males will be in comparison to the females, which may therefore also be the case in these
allobrachids. If competition is particularly fierce, males may also evolve horns, antlers,
or other structures with which to spar with each other over access to females, which can also
double as a defence against predators. In these allobrachids, the equivalent of these structures
may evolve from their recently freed-up forelimbs. While the females and the juveniles may simply use
these limbs for manipulation and gathering food, in the mature males these limbs may strengthen
and grow hardened club-like ends to turn them into powerful punching arms, with which the
males may batter each other into submission. To withstand the blows from these limbs, they may
also develop reinforced bones within their skull and further armor around their cephalothorax.
Due to this distinctive armament, we’ll give them the name ceratobrachids. However, even if
a male ceratobrachid wins a contest for a mate, he may still sustain injuries during the
battle, which may reduce his chances of winning further contests, or, in extreme cases,
may even result in him dying shortly thereafter, so these fights may end up limiting the males’
long-term reproductive success. Because of this, another clade of allobrachids may settle for a
less violent way of competing for mates. Another common sexually dimorphic trait is the presence of
frills, crests, or vibrant coloration, which males use to advertise their fitness without needing to
engage in physical confrontations. One clade of allobrachids may opt for this strategy, with males
evolving a large, colorful crests on their arms and cephalothorax to use in courtship displays.
Structures like these usually have no function outside of attracting mates, and therefore
come about purely through sexual selection, providing no direct benefit, or sometimes, even
posing an active disadvantage to the individual. In a process called Fisherian runaway, a
sexually-selected trait may become so exaggerated that it reduces the individual’s overall fitness,
such as how the enormous tails of peacocks and widowbirds demand a lot of energy to grow and
maintain and hamper their ability to escape predators. At first, this seems to contradict the
fundamental premise of natural selection; we’d expect males that are hindered by overly large or
costly ornamentation to be less likely to survive and reproduce, and to be outcompeted by males
with more reasonable, less encumbering displays, so how is it that such exuberant and costly traits
can persist? The answer lies in the fact that, if a male has survived to maturity despite
having such a huge and obvious handicap, that, in and of itself, serves as proof of his
fitness, and so females’ may maintain a preference for the trait even if the males that possess it
have lower survival rates than those without it, creating a feedback loop that selects for
increasingly exaggerated ornamentation. Among these allobrachids, fisherian runaway
may select for larger and larger crests, which, despite the fact that they’ll get in the way
while feeding and make the male more visible and vulnerable to predators, will be maintained purely
because of the females’ preference for them. In these animals, which we’ll call
corythobrachids, a male’s reproductive success will be primarily determined by the size
of his crests, while less successful males may become female as a way of easing the intensity
of competition, shedding their crests as they do so. Along with these clades of thylacopods,
the thecopods will also come to thrive in the temperate latitudes, though they’ll still live
in the shadow of much larger onychodonts like the hadrodonts and ensidonts. Most species may
avoid competition with these clades by remaining comparatively small and inhabiting areas with
too little prey to support larger predators. Where prey is especially scarce, like the cold
deserts and barren scrubland that forms the majority of the continental interior, the smaller,
more generalist species will have the upper hand. To help them handle a wider variety of food, their
mandibles, which in other onychodonts are simply designed for cutting meat into pieces small enough
to be swallowed, may acquire additional grinding surfaces to help them chew vegetation as well,
while the role of shearing off chunks of flesh may be increasingly played by the pedipalps, which
may therefore acquire additional cusps and cutting edges, an innovation broadly equivalent
to heterodonty in earth’s vertebrates. But beyond these physical adaptations, the
limited availability of resources in their habitat may also have implications for their life
history. Once again, Bateman’s principle posits that the difference in investment between the
two sexes during reproduction will incentivize females to be selective in mate choice and
males to mate with as many females as possible. However, the lower a species’ population density,
the fewer opportunities there’ll be for mating, so at some point, instead of wasting time and
energy searching for multiple females, a male may ultimately achieve greater reproductive success
by investing more heavily in a single mate. Possibly the most extreme case of this
phenomenon occurs in ceratioid angler fish, which live at such low population densities
that when a male encounters a female, he may never get another chance at reproduction,
and so he permanently attaches himself to her, providing her with a steady input
of sperm to fertilize her eggs. While the thecopods are unlikely to be driven
to such an extreme, the scarcity of resources in their habitat will inevitably result in
low population densities, which may select for increased reproductive investment in the males and
ultimately culminate in the evolution of monogamy, staying with only a single mate at a time.
Monogamy is quite rare as a reproductive strategy, but may evolve when females are widely scattered
or if the offspring are likely to die without investment from both parents. When a male
fertilizes a female, he may remain with her while she’s pregnant, assisting in hunting and
helping to care for the young once they’re born. He may only leave once the young are
old enough to survive on their own, at which point he may begin looking for another
female, a behaviour called serial monogamy. Another consequence of resource scarcity will
be increased intraspecific competition, so to protect their access to local food sources, these
thecopods might exhibit territorial behaviours. However, in the search for a mate, the
male will be forced to wander far and wide, and thus it may not be feasible for him to
defend a territory of his own, instead sharing a territory with the females he mates with. But
the female may not accept his presence lightly; having a male within her territory will mean
she’ll have to share her resources with him, which she may only tolerate if she judges
him as a decent mate, violently attacking him and chasing him off if he doesn’t meet her
standards. Because of this heightened interest in territorial defence, the females may be
larger and more aggressive than the males, and since the process of acquiring a mate is
a lot more hazardous for the male, find his own territory and becoming female may present
a potential increase in reproductive success, and therefore protandry will once again
be the dominant form of hermaphroditism among these animals. With this life
history strategy, these thecopods, which we’ll call amphidonts, will be some of the
most highly k-selected animals yet to evolve, letting them maintain stable populations
as the ice age encroaches. But while the cold-adapted thecopods and thylacopods
will have an advantage in the new climate, so will the clades that once inhabited the
central desert, whose ability to survive on very little food and water will let them considerably
expand their range as the continent dries out. The amblypods may begin outcompeting other small
herbivores in arid areas within the tropics, while the lystrocheirids, being adaptable enough
to eat just about anything, may be able to spread into the temperate plains and scrublands. As
they arrive in these more fertile areas, some lystrocheirid species may increase the proportion
of plant matter in their diet, with their broad, multipurpose teeth enlarging to help gnaw
through brachyphyte bulbs and tree roots. As they rely less and less on scavenging, their
pedipalps, previously adapted for reaching inside carcasses to probe for fresh meat, may lose their
role in feeding and assume an entirely sensory function, essentially becoming a pair of antennae.
Both their fossoriality and their nocturnality will be major assets, having originally evolved as
a way of escaping the desert heat, but now serving to help them avoid the higher levels of predation
they’ll encounter as they colonize new ecosystems. But one of the most exceptional aspects
of their biology is in their reproduction: While the offspring of most other osteopods are
precocial, more or less fully-developed at birth, the lystrocheirids are highly altricial,
undergoing a significant portion of their development after they’re born, with their young
being blind, boneless, and largely immobile. For these young to have any chance of survival,
they’ll need at least some help from the parents, which in the basal desert-dwelling species
came in the form of birthing the offspring within a carcass to serve as a first meal, but
in these new species, this may evolve into a more dedicated form of parental care, such as
keeping the young safe within a deep burrow and providing them with food as they develop.
Once again, however, Bateman’s principle means that a male will see greater reproductive
success from mating with multiple females, and spending time and energy caring for the
young will limit his ability to look for mates, so unlike the amphidonts, whose females are widely
scattered enough to justify the male assisting in a parental care, the role of nurturing the
young may fall solely on the female lystrocheirid. However, while multiple matings will maximize the
male’s fitness, searching for mates will expose them to many of the steppe-dwelling predators,
and so an individual’s reproductive potential may be cut short by an early death. If a species
has an especially high mortality rate, then rather than relying on a decently long lifespan to
provide multiple opportunities for mating, an animal may benefit from investing in only a
single clutch of offspring early in life while it still can. Therefore, while most osteopods
are iteroparous, producing multiple clutches of offspring over the course of their lives, these
lystrocheirids may evolve to become semelparous, investing all their energy in exactly one
reproductive event and dying soon afterward. Semelparity is most commonly seen in R-selected
species, and is correlated with small sizes, short lifespans, low survival rates, and large clutch
sizes. Semelparity is quite rare in clades with internal fertilization, especially tetrapods, but
it does happen occasionally. Likewise, iteroparity is likely to be the norm for most osteopods, but
at least some of these lystrocheirids, especially smaller species with high growth rates, may be
some of the few osteopods to evolve semelparity. Like many animals, the lystrocheirids may time
their reproduction to occur during the part of the year when food is most abundant. During this
time, the males may become extremely aggressive and compete furiously to find and
fertilize as many females as possible, with the less successful males taking advantage
of protandry to bypass the competition. The fertilized females may then dig a
burrow in which to incubate the offspring, of whom they’ll be fiercely protective until
they’re developed enough to survive on their own. Both the males and females will begin to die
off once the breeding season ends and winter approaches. The death of the adults may also
help the young survive by limiting intraspecific competition between adults and juveniles. If
older individuals are able to more effectively gather resources than younger ones, which will
almost certainly be the case for altricial species like the lystrocheirids, the juveniles may end up
being outcompeted before they can reach adulthood. Having the older generation die after the
juveniles leave their mothers’ protection will help to relieve this competition, but some
species, especially those that retain iteroparity, may further avoid competition by undergoing
a form of age-dependent niche partitioning. These lystrocheirids are generalist omnivores,
but the ability to digest vegetation requires a large foregut, which the young won’t have
developed yet by the time they’re born. Because of this, the younger animals may be more
inclined to feed on meat and other food sources that are relatively easy to digest, growing
expanded foreguts and thickened teeth as they age, gradually transitioning from carnivory to
omnivory. This unusual life history will give these animals, which we’ll call trypanocheirids,
a bizarre mix of r-selected and k-selected traits, since they simultaneously invest in large
clutch sizes and high growth rates while also exhibiting parental care. As mentioned earlier,
K-selection tends to become increasingly favoured as ecosystems become more heavily populated
and as clades reach larger body sizes, but there are some niches where R-selection is
quite likely to be maintained: Parasitism is a form of symbiosis in which one species benefits
at the expense of another, usually characterized by the parasite living on or within a host from
which it derives part or all of its nourishment. On earth, parasitism is seen in a huge number of
entirely unrelated clades, although curiously, it’s virtually non-existent among vertebrates,
possibly because their physiology and life history strategies are often the opposite of what
would be optimal for parasitism. Parasites tend toward small body sizes, which let them attach
to hosts more easily and less conspicuously, and are usually strongly r-selected, as the
biggest obstacle to their reproductive success is usually the challenge of dispersing to new
hosts, which may only be overcome by producing offspring in vast numbers. Contrary to this,
most clades of vertebrates are biased toward relatively large sizes, which usually goes
hand in hand with k-selection, putting them at a disadvantage in competing for parasitic
niches. Likewise, the osteopods share similar specializations to vertebrates, so they too
may take up parasitism less readily than other, smaller clades. In particular, the malacoformes
are some of the smallest animals on the planet, and have largely maintained the r-selection of
the ancestral lophostomes, making them prime candidates to evolve parasitism. The transition
to parasitism may begin with phoresis, or phoresy, using another organism simply as a means of
transport, usually without any negative impact on the host. This may be ubiquitous among the tiny
malacoforms, as hitching a ride on a larger animal will let them travel much further than they could
possibly crawl on their own. But perhaps some of these malacoforms adapt to use their host not just
for dispersal, but also for feeding, subsisting on the organic debris, glandular secretions, or even
dead skin that builds up on the animal’s hide. If these malacoforms are especially vigorous in their
feeding, they may even open wounds on the host’s body, which may result in some species evolving
hematophagy, or blood-drinking behaviours. Blood has a very high nutrient content, making it
an enticing food source for many animals. While in facultative hematophages, blood
forms only part of the animal’s diet, obligate hematophages are specialized for feeding on blood
exclusively. These specializations may involve optimizing the mouthparts for blood-sucking and
the production of chemical agents to stop the blood from clotting while the animal drinks it,
maintaining a constant flow of blood as they feed. This may be accomplished in these malacoforms with
sharp, toothy tentacles to bite through the host’s skin and simultaneously inject anticoagulants
into the wound. Hematophagy doesn’t automatically qualify as parasitism, as the interaction between
a hematophage and its host is often quite brief, while parasitism typically
entails a long-term association. To be considered a full-fledged parasite, the
animal must be adapted to remain attached to the host for a significant part of its lifecycle,
which these malacoforms may achieve by evolving powerful, spine-covered tentacles to
securely anchor themselves to the host, and a long proboscis to bore into
the skin and reach the bloodstream. The specializations a parasite undergoes will be
contingent on the exact physiology of the host, resulting in co-evolution between the two. As
such, many parasites are extremely host-specific, only adapted for parasitizing a single or a
small number of closely related host species, so this clade of parasitic malacoforms, which
we’ll call myzognathans, may contain many hundreds or even thousands of different species,
each one adapted to its own unique host species. But another clade of malacoforms may evolve
parasitism in an entirely different way. While ectoparasites like the myzognathans
attach themselves to the outside of the host, endoparasites live inside their host’s body,
where they’re protected from the environment and are less likely to be removed. This requires
a way of entering the host, which very small, especially microbial parasites may achieve
by infiltrating the respiratory tract, burrowing through the skin, or even by using
another parasite or hematophage as a vector, but one of the most common methods is
by allowing themselves to be eaten. Among the malacoforms, many species may lay
their eggs along or under leaves, where they may end up being unwittingly swallowed by foraging
herbivores. If one clade evolves to produce eggs that are durable enough to survive the digestion
process, some of the larvae may hatch out while still inside the herbivore, where they’ll be
able to feed on the contents of the animal’s gut. Offering ample nutrients and devoid of predators,
the herbivore will serve as an ideal incubator, prompting some malacoforms to spend
their entire lives within their host. However, one drawback to this lifestyle is the
difficulty it presents when it comes to dispersal; being unable to survive outside the host
means that the only way to transmit their offspring to a new host will be to disperse
their eggs in the host’s excrement, in the hope that they land somewhere where they have a
chance of being ingested by another herbivore. To maximize the chances of this happening, most
endoparasites once again tend towards R-selection, producing enormous quantities of eggs
of which the vast majority will die. While their reproductive systems expand, many
endoparasites also show a severe reduction or simplification in other body systems, as
passively feeding on the host’s gut doesn’t require a great deal of bodily complexity, to the
point that many endoparasites, like tape worms, thorny-headed worms, and horsehair worms, have
no functioning digestive or respiratory tracts, simply absorbing oxygen and nutrients
through their outer surface. Similarly, in these malacoforms, the eyestalks and
other sensory organs, being essentially useless inside a host, may atrophy completely, along with
the limbs and the majority of the internal organs, while the oviduct may grow so large that its
extends into the spine-covered proboscis, with which they anchor themselves to
their wall of their host’s intestines. This lifestyle is likely to have first evolved
very early on in the history of terrestrial life, with the parasitic clades diversifying in
tandem with their hosts. Like in ectoparasites, co-evolution with the host results in most
endoparasites being highly host-specific, but with one major caveat: being utterly reliant
on a single host means that if the host dies, then so does the parasite, which is an especially
big problem if the host species experiences a high rate of predation. Because of this, unlike
ectoparasites, which are often monoxenous, completing their entire lifecycle on a single
host species, many endoparasites are heteroxenous, developing inside one or more intermediate
hosts before being transmitted to the final, or definitive host, only then maturing into
their adult form and commencing reproduction. On the barren scrublands of the western continent,
one clade of these malacoforms may coevolve with a family of xerostracans, which will serve
as prey for some species of theropterans. When a theropteran kills and feeds on one
these xerostracans, it may unwittingly ingest any malacoforms infesting in, which, if they
survive, may adapt to use the theropteran as their definitive host. This will have the added
benefit of massively increasing the range of their dispersal, since the theropterans
can travel much further than the slow, ponderous placostracans can. This heteroxeny means
that to complete their lifecycle, endoparasites like these often rely on their intermediate hosts
being eaten, so to maximize the chances of this happening, they may produce chemical agents
that alter the morphology or behaviour of the host to make it more susceptible to predation,
such as how the nematode myrmeconema causes the abdomen of the ants it infests to turn bright
red, making them more noticeable to predators, while lancet liver flukes manipulate their
ant hosts into climbing blades of grass, where they’re more likely to be inadvertently
swallowed by cows, which serve as the flukes’ definitive host. In these malacoforms, when
the larvae reach maturity, they may secrete chemicals that cause their xerostracan hosts to
lose their fear of predators, and to forage for food during the time of day when the theropterans
are most active. Only once the larvae pass into the theropteran’s gut will they develop their
expanded oviducts and begin producing eggs. This may be only one of hundreds of different
lifecycles that develop among this clade, with every species adapting for its own sequence
of hosts within its local ecosystem. We’ll call them echinostomes. While it would be impossible
to account for all the changes that occur as the continents break apart, the clades we’ve discussed
here will constitute some of the major groups that will evolve over the course of about 30 million
years as life in the east and west diverges. But beyond the two continents, there are some
habitats on this planet that present opportunities for far more extreme forms of allopatry to
occur, and whose biogeography demands a more thorough examination. In the next episode, we’ll
explore how life adapts to colonize the many islands across the sea, and explore the unique and
highly unusual ecosystems that arise as a result. Thanks to all the artists over on discord who
contributed artwork for this episode, and who save me an enormous amount of time and effort when
making these videos while simultaneously making them look a hell of a lot better in the process.
Links to the main server and the alien biospheres fan server in the description. And once again, a
massive thanks to all the patrons, whose continued support makes videos like this possible. Thanks
for watching, and I’ll see you in the next video.
Every single episode of this has me thinking 2 things:
1: "What's gonna become the dominant species on the planet?"
&, 2. "When's he gonna throw a rock at it?"
Polypodia gang
Mass extinction episode when?
I’m just a junkie waiting for his next fix